- Email: [email protected]
Very stiff fibres woven into history – very personal recollections of some of the British scene
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 65 (2005) 2285–2294 www.elsevier.com/locate/compscitech
Very stiff fibres woven into history – very personal recollections of some of the British scene Anthony Kelly
*
Churchill College, Cambridge CB3 0DS, UK Department of Materials Science, and Metallurgy University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Received 4 May 2005; accepted 4 May 2005 Available online 10 August 2005
Abstract A personal memento is given concerning how the author became interested in the ideas of fibre reinforcement in 1960 and how he watched as a close consultant the invention of very stiff fibres at Farnborough by Watt and Phillips. Some account is also given of the early work at Farnborough both before during and after the second world war and the part played by de Bruyne and then by J.E. Gordon and his team. The early development of the materials science of composites by the author and his colleagues and the discovery and enunciation of the principles of how all brittle materials can show appreciable works of fracture are dealt with. Some short remarks follow on the part played by academics in the development of a new technology. 2005 Elsevier Ltd. All rights reserved. Keywords: Composites; Carbon fibres; Toughness
1. First researches I read physics for my first honours degree and in those days great attention was paid to teaching practical skills – we measured the charge on the electron by MillikanÕs method for instance. The domain structure in a ferromagnetic was a modern development and we examined Bitter patterns – the designs formed by iron filings on the surface of a ferromagnet. This last led me as an undergraduate to an interest in what were called subgrains in metals. I heard of these through reading the wonderful Physics of Metals by Fred Seitz [1]. I applied to enter the Cavendish Laboratory at Cambridge – at that time widely regarded as the foremost academic physics laboratory in the world. Prospective research students then were not molly coddled with suggestions by prospective supervisors eager for graduate
*
Tel.: +44 1223 363691; fax: +44 1223 33456. E-mail address: [email protected].
0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.05.010
students but were expected to indicate a real problem on which they would like to work. I was lucky to have mentioned subgrains in metals since this was a burning interest of the Cavendish Professor Laurence Bragg (of BraggÕs law) and so he was to be my official supervisor when I was accepted by the Cavendish and also by Trinity College. At Cambridge one enters the University through one of its colleges. I worked on detecting subgrains in metals and through that route became a metallurgist. Bragg, during my time at the Cavendish, lost interest in metals and turned passionately to encouraging the elucidation of the structure of biological materials by means of X-ray diffraction. It was that interest which enabled Crick and Watson to meet and to work out in a spectacular and intuitive fashion the structure of DNA – the secret it seems of the genetic code – the physical means by which hereditary features are transmitted from generation to generation [2]. I was in no way involved but enjoyed a spectatorÕs grandstand view of the personalities involved in this wonderful scientific achievement.
2286
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
Having been converted to Metallurgy I fell under the spell of Alan Cottrell; he examined my PhD Thesis and later offered me a job, or suggested that I apply for a job at Cambridge some years later. I left the Cavendish Laboratory in 1953 and worked in the United States with Paul Beck at Illinois – where I met and came greatly to admire Fred Seitz who was in the Physics Department and whose brilliant lectures based on his massive text book Modern theory of Solids [3] I humbly attended. During that period I also met Bardeen, of course who was in both Electrical Engineering and in Physics and earning his second Nobel Prize. Heissenberg passed through on his first (?) visit to the United States after the war and I did some very preliminary work with Koehler, Seitz and Baluffi. The Metallurgy department of Illinois was suddenly modernised by the influx of a group from Columbia under Tom Read who had published the first US papers on dislocations with Seitz and Read [4]. Illinois was a lovely place to work in those days as solid state science had its origins and expanded. I returned to England briefly for the calendar year of 1955, became engaged to be married and then returned to the USA having been recruited by Morrie Fine to teach X-ray crystallography to students of a newly formed department of Metallurgical Engineering in the Technological Institute of Northwestern University in Evanston Illinois – just north of Chicago and the home of the WomenÕs Christian Temperance Union one of the forceful progenitors of the move to prohibit the sale of alcohol within the USA after World War I. Fine was and still is, now well into this ninth decade, a very forward-looking man in his science. Before coming to Northwestern he had taken part in the Manhattan project (the making of the first atomic bomb) at the University of Chicago and subsequently at Los Alamos. Through him and others at Northwestern I met Oppenheimer – the father of the fission bomb – whom I remember as a wise and kindly man who much encouraged me in my researches with well-spoken words of praise. Fine it was who led our department at Northwestern to attach to its name the study of Materials Science in 1957. We could rightly claim to be the first department in the world to adopt this in our title. So our horizons were broadened from the study of metals to include plastics or polymers, at that time the fastest growing of the materials producing industries. Of course, we already knew a little about ceramics – the third major component of materials science – through the use of these as refractories for furnace linings and as tundishes, etc. Cottrell accepted the Goldsmiths Chair of Metallurgy at Cambridge and went there from the Atomic Energy Research Establishment at Harwell in September 1958. In 1956, he had mentioned to me the great likelihood of this occurring and had asked me whether I would join
him there if he could secure a lectureship – a post not lightly created at Cambridge in those years. I treasured this thought in the intervening years at Northwestern but when the opportunity finally arose I was sad to leave Evanston. Cottrell asked me (I knew very little of Metallurgy at the time) to give a set of lectures on ceramics. I came to Metallurgy at Cambridge with the first Instron tensile testing machine imported to Britain and set about preparing these lectures. I did this by reading SlaterÕs Quantum Theory of Matter [5] while crossing the Atlantic by ship. Other people lecturing on ceramics would have scoffed at that but, as it happened, it stood me in very good stead. I discovered in the literature rather little materials science of ceramics. There was a wealth of chemistry but less material relating structure to properties. I persevered and discussed with the undergraduates things like the physical interpretation of Mohs hardness scale, the difficulty of moving dislocations in materials such as sapphire, why silicon carbide sintered slowly and, following Slater, why covalent solids are stiff and of low density, since the atoms are held apart by spatially directed bonds. We discussed a host of other things not usually dealt with in ceramics courses. I was pleased afterwards to discover that the graduates who went into such industries as the steel industry found my lectures very useful, even though I had not dealt in excruciating detail with how furnace linings were rammed into place.
2. Fibre reinforcement It is difficult to appreciate nowadays that in those years some very obvious data were missing from the literature, e.g., the elastic constants of boron, or even of fully dense graphite. At Cambridge I started some work on ceramics, notably, for what comes later, on the investigation of graphite single crystals and on magnesium oxide. My own special interest was in precipitation hardening of metals and I discovered that, in aluminium–copper and copper–beryllium and such like, precipitation hardened alloys in the overaged condition developed very strong rates of work-hardening. Cottrell, at the time, was becoming a very fashionable young scientist and was invited to give lectures such as the Royal SocietyÕs Bakerian, and discourses to the Royal Institution, Royal Society of Arts, etc. He often used to give me the text of these lectures to read before delivering them. In a discourse delivered to the Royal Institution on 15 June 1960 [6] he enunciated the principle of fibre reinforcement. I quote what he said: No, the practical approach is to admit the existence of cracks and notches and to try to render them innocuous.
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
Suppose that we are stretching a rod. . . and. . . that it consists of a bundle of parallel. . . fibres joined together – by some adhesive or solder. If there is a transverse notch in the rod, cutting across a number of these fibres, the forces from the cut fibres can be transmitted to the fibres at the tip only by passing as shearing forces through the layers of adhesive. . . If this adhesive has a fairly low resistance to shear. . . it will then be incapable of focussing the transmitted forces sharply. . . There are tremendous possibilities for developing this principle further, particularly by using fibres of materials with very strong atomic forces, e.g., refractory oxides and carbides. It is all there. It was new to me. And in those days in pounds per square inch! The discourses of the Royal Institution give no references, or rather AHC gave no references, to that particular talk. ÔHad he dreamt the principle up for himself?Õ, was the question I asked, ÔWas there some literature unknown to me?Õ The hint that there was an extensive literature occurred in the list of exhibits in the library, listed at the end of his lecture. These were from the Royal Aircraft Establishment. People there had approached strong materials in a completely different fashion from that of the metallurgist. Cottrell will have learnt of this from his membership of various quasi-confidential Government committees, which abounded in that period and on which most academics interested in physical metallurgy sat. The situation of modern fibrous composite science at that time in Britain, and its subsequent development, paying special attention to the work at the Royal Aircraft Establishment is well described by McMullen [7] which I now follow in part – see also [36] for recent years.
3. Modern composites at Farnborough The beginning of research and development on composites for aircraft structures began in the UK in 1937 when N.A. de Bruyne, an engineering don at Cambridge, read a paper to the Royal Aeronautical Society entitled ‘‘Plastic materials for aircraft construction’’ [8]. It is a massive paper showing in great detail how de Bruyne, with great vision foresaw the many difficulties and also some solutions to these – but most importantly he recognised the great promise. De Bruyne was a keen amateur airman who built his own aircraft the Snark. Doing this he recognised the importance of stressed plywood and the necessity to orientate the material so that the wood fibres ran parallel to the largest loads. Because of his knowledge of plastics he was employed as a consultant by the de Havilland company to advise on the making of propellers. Plastics as materials of construction for propellers had been considered since the 1920s.
2287
In 1936, de Bruyne had noted, following Meyer and Lotmar [9], the theoretical properties of some cellulosic fibres in terms of their high stiffness and/or strength coupled with a low specific gravity. To reinforce and stiffen the plastic a flax roving was used teased out into flat bands as a continuous reinforcement De Bruyne introduced a material which he called Aerolite which was phenol–formaldehyde (Bakelite) containing aligned fibres of cotton ramie and other textiles. He also experimented with urea formaldehyde resins. De Bruyne is principally remembered now for his wonderful inventions of adhesives such as Redux, which successfully bonded wood and metal for aircraft construction. In 1938, J.E. Gordon a young man just trained as a marine engineer was hired at Farnborough and asked to evaluate new non-metallic materials. Among these was Gordon-Aerolite (flax fibre-reinforced phenolic resin) invented by M.F. (Malcolm) Gordon, J.E.Õs namesake – but no relation – working at Aero Research Ltd., a firm founded by de Bruyne. It was Malcolm Gordon one of de BruyneÕs students who, because he had a connection with a Belfast linen mill, (in the charge of his father) suggested the use of flax as reinforcement. A supply of unbleached flax thread was impregnated with phenolic resin, wound into a skein and hot-press moulded to form what was probably the first example of a high performance composite. M.F. Gordon went on to become managing director of a spinning company and an honoured Engineer/Industrialist in Northern Ireland. We know that M.F. and J.E. met but I know no more of any interaction between them. Gordon [10] as the first to point out that, if composites were to be used for structures, they had to be used as efficiently as possible in order to be competitive with metals. To this end, he drew attention again to the need for the highest possible degree of fibre orientation in each layer of material and the requirement then for the layers to be crossed at appropriate angles – to meet the applied loads. This required careful calculation and the early mathematics of fibre orientation date from this time and is attributable to Cox of NPL [11] and Gordon and Bishop of RAE [12]. One of the first composite components of any consequence tested by J.E. Gordon was an experimental fullscale main wing spar made by de BruyneÕs company for the ÔBlenheimÕ bomber. This was of lattice construction using Gordon-Aerolite but was designed like conventional metal structures, and so was a misuse of this novel material. A similar abortive exercise involving a ÔSpitfireÕ fuselage was conducted at the outset of the war (WW-2) when there was a temporary shortage of aluminium alloys in Britain. A Gordon-Aerolite replica of the real thing was made but the structure was hopelessly overweight. Throughout World War II, cellulose fibre composites were developed with moderate priority and with some
2288
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
real success. Aircraft drop tanks saw service, while wing tip tubes and ducting were in production and a composite seat was made for the ÔSpitfireÕ. Work at the RAE on cellulose fibre-reinforced plastics ceased in 1946 and, although only a partial success, enough had been demonstrated and learned to justify further composites R&D. It was realised, for example, that the reinforcing fibre had to be inherently inert to climatic changes, it had to be possible to give it a high degree of orientation and it had to be possible to achieve a good bond between it and phenolic resins – the only types available at that time. The resin/fibre pre-impregnate had to have a good Ôshelf-lifeÕ and it had to be possible to mould it at low pressures in low-cost tooling and the resulting laminates had to develop a high proportion of the properties of the fibre. The work on cellulose fibre composites also led to the beginnings of the understanding of fibre orientation theory, the realization of the importance of strict control of moulding conditions and the appreciation that these new materials would require the development of new methods of test. In short, much of the technology learned in this early work formed the basis of all that followed. At the end of the RAE work on cellulose fibres in 1946, samples of glass fibre obtained from the USA were examined as a possible reinforcement for plastics for primary structures. By comparison with cellulose fibres they are heavy and thus their specific stiffness is about half that of flax – one of the best of the cellulose fibres – but they have an enormous advantage in strength and can of course be highly oriented. Earlier developments of GRP (c 1943) owed much to the invention of radar and the urgent need for radomes these are radar housings and contain E glass which is a glass fibre free from ions which lead to absorption at microwave frequencies (cf. Stealth aircraft today). In view of the very poor bond between glass fibres and the phenolic resins of the time and with no immediate prospect of improving this, glass fibres as reinforcement for major structural plastics were abandoned in 1946 at RAE. The years from 1952 saw, however, an enormous growth in the glass-reinforced plastics (GRP) industry due to the advent of polyester and epoxy resins. Due to their prior developed expertise RAE were well placed to take advantage of this especially when glass fibres, stiffer than E glass became available in the late 1960s [13]. Glass is relatively heavy and no stiffer than aluminium and so its use for aircraft primary structures is very limited. However, for rocket motor cases stiffness is less important and strength paramount [14]; glass then becomes very useful. The filament winding technology developed by RAE in conjunction with Bristol Aerojet Ltd. and Imperial Metal Industries Ltd. led to successful firing trials of
all-plastic rocket motors in 1951/1952 and to equally successful flight trials of guided weapon boost-motors soon afterwards. The Admiralty Research Establishment in Dunfermline pioneered the use of glass fibres in mine hunters under the direction of CS (Charles Stuart) Smith (1936–1991). Of course, fibreglass boats were displacing wooden boats. The Admiralty made the very courageous decision to displace wood with grp in large vessels (ships rather than just boats) with greater than 500 tonnes displacement – see for example [15]. The successful introduction of HMS Wilton, the prototype, led to the Hunter and Sandown class of mine hunter, the largest in the world in the early 1980s. Interestingly, the first vessels were no lighter than the competition because they had to be over-designed owing to the notorious plastics factor – see below. Smith and his team did much to reduce both this and, very effectively, the cost of production. When RAE work on cellulose-reinforced plastics ceased in 1946, a search began for other, stiffer and more inert fibre reinforcements. There was interest in the potential of asbestos as a reinforcing fibre and it was a happy coincidence that in 1947 Turner Brothers Asbestos Company produced Durestos – a chrysotile asbestos fibre-reinforced phenolic resin. In temperature resistance for example, it can readily out-perform GRP and aluminium and can be used for the venturii of short burning time rocket motors. The great disadvantage of this material is the health hazard associated with asbestos. Chrysotile is very stiff and strong, inert to the effects of moisture and is well bonded by phenolic and other resins. The big drawback in 1947 was the absence of technology for highly orientating the fibre. In the years 1948–1954, J.E. GordonÕs section at RAE demonstrated beyond doubt the feasibility of making large high performance composite structures. The main demonstrator structures were delta wings for the Fairey experimental E10/47 aircraft. A description is contained in the proceedings of the Third Anglo-American Aeronautical Conference 1951 [16]. When the demonstrator exercise was terminated in 1955 the wings had been developed to the stage where they could sustain test loads well in excess of the 12 g load required of the metal ones. They had weight in hand – but they had not sustained the loads derived from the required use of the notorious 1.5 Ôplastics factorÕ, i.e., 12 · 1.5 = 18 g. Because of relative ignorance of the behaviour of the materials compared to the knowledge base concerning the properties of aluminium, a plastics factor of 1.5 was introduced. In other words, the composite had to be 50% better than the stated design figure before its use would be considered. This situation is reminiscent of that regarding performance under fatigue conditions today, when lack of a predictive capa-
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
bility comparable to that found with metallic structure inhibits the use of composites in some applications. Nevertheless these developments during the 1950s were a significant milestone in the development of aircraft composite structures and showed beyond any doubt the potential advantages to be gained. Above all they demonstrated that composites for aircraft primary structures would have to have specific stiffness properties substantially greater than aluminium alloys if worthwhile structure weight savings were to be achieved. At that time (c 1954) this meant only one thing: new, stiff, lightweight fibres had to be invented. This was the approximate position when I first heard of the field in 1960. The principles of fibre composite science/engineering were known but not widely and the fibres being considered were whisker crystals, asbestos fibres, glass and possibly metal wires.
4. The field as I entered it In the late 1950s and very early 1960s the ideas for controlling cracks following the ideas of fracture mechanics were only slowly being used in practice. There was a great and understandable fear of using all brittle systems in load bearing applications based on the experience of the spectacular Liberty ship failures in World War II [17] and the Comet disaster [18]. Fibre composites appeared to be in the class of such all brittle structures and so even if high strength were to be obtained many people thought that they would not be useful especially under any conditions of impact. My associates and I were very fortunately placed in the early 1960s in that we were elucidating the principles of the strengthening of metals and yet were lucky enough to spot the advantages of non-metallic solids as the major strengthening agents, because I was lecturing on ceramics. In the summer of 1961 Phillip Bowden, Cottrell and others organised a closed conference at Caius College Cambridge, financed by the National Research Development Corporation (a forerunner of the British Technology Group), which discussed fibre reinforcement and we at Cambridge learnt, through our colleagues from Oxford, of work on directional solidification of eutectics, both in the USA and at Oxford. Work started at Cambridge on this topic. I mentioned the high rates of work hardening of alloys in the overaged condition, which appeared to support the idea of fibre reinforcement with a metal matrix. This was because we knew that particles of a strong solid within the metal were being elastically strained as a result of the motion of dislocations in the metal. The dislocations being unable to shear the strong particles, piled up around them and consequently the metal appears to become very much stronger as plastic strain is increased [19].
2289
I immediately asked Bill Tyson (who joined me with an Athlone Fellowship from Canada) to investigate the validity of CottrellÕs idea with a metallic matrix as the adhesive or solder. We tried to produce aligned rods of a very hard phase in a metal by cold working and by directional solidification of a eutectic and other means, since whisker crystals were difficult to use experimentally. This Bill did but without exciting results. What really got us going was a review by Gene Machlin from Columbia [20], and through it, coming across the work of McDanels, Jech and Weeton [20]. This latter gave us an excellent model system to work on. They had introduced aligned very strong tungsten wires into soft ductile copper, noting that strong wires of many refractory metals are insoluble in the noble metals. Machlin had already recognised the importance of the fibre reinforcement of metals. Tyson went to work with a will and we produced in a relatively short time a set of principles, which could be, called the colligative mechanical properties of an aligned fibre-reinforced system. We could explain adequately the strength of the system in terms of the strengths of the two components and how it depended upon volume fraction and on the relative elongation of the two components; on the length of the fibres and more important, the ratio of length to diameter. We explored the idea of minimum and critical volume fraction. I remember with surprise the work being described as being elegant, but I thought it was rather straightforward, but necessary to establish the principles. In the course of doing it we made one of the fundamental discoveries needed. We found a new means of dissipating energy. The fibrous composite, of course, broke. Since we were working with metals and were aware of the emerging concepts of fracture mechanics, we were able to recognise immediately the significance of pull-out at a fracture surface. Tyson found that when a fibrous composite breaks all the fibres in the fracture plane do not break at once. This is particularly pronounced with brittle fibres. The fracture surface, therefore, had a jagged appearance and the sliding apart of the two faces or the pull-out of the fibres from holes in the matrix, dissipates a lot of energy and provides a quasi-ductile form of behaviour. That was the discovery and that this would dissipate energy was the natural suggestion. Cottrell and I (I do not remember whether Tyson was present) discussed the finding before the Royal Society meeting on New Materials in 1963. With his wonderful facility for doing the physics immediately with simple algebra Alan Cottrell produced the first formula for estimating the work of fracture due to pull-out. He altered his original draft of the introductory paper to the Conference in order to include it. His simple formula was much quoted for a few of the succeeding years.
2290
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
This behaviour is not confined to metals and, from TysonÕs work, sitting on the numerous quasi-confidential committees I have mentioned, I remember saying that this would probably happen with forms of fibrous composites made from all-brittle systems and would provide an energy absorbing mechanism. A little later, Leslie Phillips (see below) said, Ôand it broke, just like Dr. Kelly said it wouldÕ. This may seem all very obvious to people nowadays, but at the time it was something genuinely new. Other people had described types of pull-out but no one recognised its importance as a source of dissipating energy around a crack. We announced our results at the Royal Society meeting in 1963 to which I refer again below. J.E. Gordon left RAE in 1954 and went to TI Research Laboratories, where I met him for the first time. He worked principally on whiskers with C.C. Evans and N.J. Parratt. He returned to what is now the DERA fold in 1962 at ERDE Waltham Abbey. There, he and his colleagues developed methods of growing and handling large quantities of whiskers, principally of Si3N4 but also of SiC, as well as short fibres of glass carbon and various varieties of asbestos. SiC whiskers in alumina now provide a very effective cutting tool. The methods developed at ERDE are very important for the development of composites containing short fibres, since composites with only long fibres may lack mouldability; they are what is called boardy. A summary of much of the work at TI and subsequently at Waltham Abbey is contained in the book by Parratt [21].
5. Carbon fibres Remember that at this stage we are still looking for a fibre stiffer than steel (200 GPa) and of much lower density. Talley, working in the research laboratory of the Texaco Company for the Office of Naval research invented a method of making boron fibre – depositing boron from the vapour phase onto a tungsten wire in 1958. The stiffness was 460 GPa and the strength 2.3 GPa [22]. The invention was described by a general of the US Airforce as the biggest breakthrough in materials since the Stone Age! This fibre is not easily handleable due to a relatively large diameter, however, and the really modern fibres (the various carbons and aramids) are more closely related to textiles. I became a member of the graphite community for a few years in the early 1960s due to my interest in ceramics. High stiffness carbon fibres were invented almost simultaneously in the UK, USA and Japan at about that time and I was familiar with much of the relevant work in those three countries. My early work on ceramics stemmed from CottrellÕs obtaining funds from the Atomic Energy Authority for
work in support of the high temperature atomic reactor – the Dragon project. And three of my research students worked directly on graphite. Earl J. Freise who followed me from Northwestern was the first of these – a most dedicated man who obtained a PhD at Cambridge in record time. He went on to become a University administrator finally in a high position at Caltech. Clive Baker – the twin brother of Colin, who later did seminal work at Alcan on metal matrix composites, studied radiation damage in graphite and Murray Gillin who came to Cambridge in 1962 and investigated the mechanical properties of graphite at very high temperatures. Gillin came from the Australian government research centre at FishermanÕs Bend near Melbourne and when he finished his PhD in early 1965 he had seen as an avid spectator, as had I, the invention of high performance carbon fibres at Farnborough. This knowledge he took back to Australia and being a government employee was able to influence and initiate Australian research in composites. Gillin has cited this experience in a number of publications [23]. My three students worked with me on single crystals of graphite – large naturally occurring single crystals. Freise and I were able to obtain these because I had worked at Northwestern on a US Airforce contract (AP 1438 if I remember correctly) and so had contacts in that organisation. Through these, and they gave ready help, we obtained natural crystals from the pyroxene bearing rocks at the Lead Hill Lead mine in Ticonderoga New York. Freise published the definitive work on twinning [24] of crystals of graphite and carried out deformation studies on kish crystals and on pyrolytic graphite and on commercial grades of polycrystalline graphite. BakerÕs studies of the change of stiffness of graphite crystals when irradiated led me to urge him to carry out and for me to interpret the results of one of the pieces of work of which I am most proud [25]. We determined directly for the first time the elastic modulus C44 of graphite. This is the modulus governing shear on the basal plane which has an extremely low value: we found a value of <100 MPa. And to do this we had to amplify Lord RayleighÕs equations for the bending of a cantilever beam; one should include both shear and rotary inertia, both of which Rayleigh had neglected. Gillin constructed a unique apparatus for carrying out tensile tests at temperature as high as 3000 C. The detailed results still remain unpublished, though an account of the construction of the machine has appeared [26]. He deformed specimens of natural graphite and of commercial grades of polycrystalline graphite and of an exciting form known as pyrolytic graphite. This last is obtained by the decomposition of methane gas at a temperature of say 2100 C onto a commercial graphite substrate. The material consists of individual graphite sheets which are usually wrinkled but of density 2.14 – hence close to that
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
of pure graphite, 2.26. The wrinkling arises due to the very low thermal conductivity normal to the layer planes so that the individual layers are deposited in a very strong thermal gradient. As a result of these researches I got to know Willie Watt at Farnborough who was working on pyrolytic graphite. Gillin and I obtained specimens of the material from him. The Atomic Energy Authority also supported Watt and our association lasted until his death in 1985. I became a consultant at RAE. Work on graphite at that time involved naturally the Union Carbide Corporation and on a visit to the USA in 1962 I met Roger Bacon. There were clues to the possibility of very stiff carbon fibres in the late 1950s and early 1960s, which were emphasized by some of us at the time. These were, Roger BaconÕs graphite whiskers (now fashionable again following the terrestrial manufacture of fullerenes). Bacon made these in a carbon arc under pressure in 1960 [27] some specimens of which he later presented to me and I in turn to a young scientist working on carbon nanotubes in 2000. The whiskers had moduli of 700 GPa and strengths of 20 GPa. They are scrolls of graphite layer planes. Kotlensky Titus and Martens [28] in 1962 hot stretched pyrolytic graphite and attained a modulus of 560 GPa. The graphitization of polymers such as polyacrylonitrile can lead to a preferred orientation and Shindo in 1961 [29] had produced a fibre with a modulus of 120 GPa. Akio Shindo visited me in Cambridge in 1965 and presented me with a print of a woodcut by Kiyoshi Saito – a well-known Japanese printmaker. Hence, I was fortunate enough to know personally all three inventors of carbon fibre of high modulus. In June of 1963 the Royal Society held a discussion meeting on New Materials [30]. It was organised by J.D. Bernal (1901–1971), Alan Cottrell (b. 1919), Charles Frank (1911–1998), and Cecil Bawn (1908– 2001?) – what a panoply of talent! Both Jim Gordon and I spoke at the meeting, he on designing with brittle materials (when he produced the very well-known dramatic diagram of an aircraft wing illustrating the need for stiffness). I, on the principles of determining the strength of continuous and discontinuous aligned composites of fibre reinforcement. And, as I have said emphasised that all brittle systems could be tough. During the discussion, the lack of a stiff fibre of carbon was mentioned. Watt (1912–85) was present at the meeting and told me later that he immediately determined to see if he could make stiff graphite fibres. Cottrell and I returned to Cambridge having discussed stiff graphite on the train, and on that evening I put some cotton wool in our graphite tube furnace overnight. The resulting carbonised cotton wool was not stiff – one had to be much more sophisticated. Watt, Phillips and Johnson at the RAE succeeded. Three groups almost simultaneously invented stiff carbon fibres in
2291
1963-64: Bacon in the USA, Shindo in Japan and Watt, Phillips, and Johnson. Of these three original processes that invented by the last three named became dominant [31]. The first experiments on the production of carbon fibres by Watt were according to Watt, stimulated by the Royal Society Discussion Meeting. Phillips always vigorously denied this. He said Willie Watt and Bill Johnson asked for a meeting with him in order to get his views on the polymer chemistry connected with making silicon carbide. Anyway, it was Leslie who suggested trying black Orlon and referred Watt to the observations of Houtz who had shown that, on heating a proprietary fibre based on PAN, viz. Orlon, in air for up to 20 h, it underwent a colour change to yellow, then brown, then black and became insoluble in PAN solvents. The key step in the process developed at RAE is the oxidation of the PAN fibre while under longitudinal constraint. The possibility of shrinkage was pointed out by Bill Johnson and according to his own account Leslie Phillips banged the table and said Ôwe must not let itÕ. PhillipsÕ recollection at the time was that it would have been relatively easy to get a high elastic modulus carbon fibre – it depended (cf. Kotlensky) on the coking temperature. The difficulty was to obtain a useable high strength, which depended on clean fibre, optimum time and temperature of heat treatment so as to decompose the PAN. The initial batch process in which PAN fibres were wound on frames were made of graphite and ÔMeccanoÕ and later of glass. This was to provide the tension during the oxidative stabilising process, which was the key to the invention so as to stop the fibres shrinking. In effect they were stretched during the oxidation stage. It was this step which had eluded the Japanese. The RAE fibres were some three or four times stiffer than those produced in Japan. Watt and Johnson actually made the first fibre and Phillips showed these fibres to be compatible with epoxy and polyester resins. After the invention carbon fibres were made on a pilot scale at AERE Harwell. The RAE patents covering the production of carbon fibre from PAN were assigned to NRDC (later BTG) and three licensees established: Courtaulds, Rolls-Royce and Morganite. Details of the invention were given in an exceptionally complete and detailed form. All three licensees used Courtaulds PAN (a variant of their Courtelle) as a precursor. Rolls-Royce set up production facilities at Hucknall to produce material for the RB211 jet engine and excellent carbon fibre prepreg was produced under the name of Hyfil [32], although the material was not used in the engine. The business passed to Bristol Composite Materials. Production ceased around 1980. Morganite made carbon fibre under the name of Modmor, but ceased
2292
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
production in the early 1970s. Courtaulds continued to produce until 1991 when they ceased production in the UK and sold their American interests to a Japanese company. They continue strongly in Advanced Composite Structures. A later comer, RK-Textiles, maintain a carbon fibre manufacturing capability in the UK. During the establishment of the processes in industry, Phillips played a leading part in advising the licensees, drawing attention to the dangers (avoided) of sodium cyanide and HCN evolution, to the need for controlled flow rates of all the gases and to the removal of tars [33]. In the early days Phillips was also concerned with the treatment by hydrogen peroxide to essentially make pores on the surface of the fibre so as to vary the adhesion to the resin. It was he, I believe, who made the crucial decision that, since carbon fibres were likely to be expensive, they should be married with the expensive resins and it was I think the development of carbon fibres which led very much to the development of improved properties of epoxy resins. WattÕs main interest was in the chemistry of making high performance carbon fibre and he continued to work on these until his death. I felt privileged to have been able to attract him to work at Surrey University where I was then the Vice Chancellor when he retired from the RAE. Watt, through his position as a Fellow of the Royal Society – he was elected in 1976 – had an interest in composites of course and organised several successful meetings. The aerospace industry saw the potential of carbon fibres immediately but it was Phillips of the three British inventors who undertook the prosecution of the widespread application of these things not only into aerospace but also into much more fashionable and exciting artefacts, such as kayaks, golf clubs, fishing rods (he was a keen fisherman), motor car bodies, furniture, textile machinery and many other forms. For many years the body of the Maclaren racing car – a sweeping Grand Prix winner – embodied the largest piece of carbon fibre moulding ever made. Phillips was also the man who realised, with others, that the most important product form would be fabrics and fibre hybrids and he recognised the need for the invention of simple manufacturing processes. He would be delighted today with the prospect of non-woven, two-dimensional arrays of fibres, which are becoming available. He was always concerned with finding widespread uses for carbon fibres and avoiding solely esoteric ones like, as he said Ôblack piano keys and false teeth for soldiers on night operationsÕ. He will be particularly remembered for his efforts to introduce carbon fibres into thermoplastics, first by compounding and then by the invention of the film stacking method and also of course of various types of vacuum moulding. He said:
One should be careful to choose possible applications where high initial costs can be tolerated and where anisotropic and highly orientated composites are an advantage rather that the reverse. These conditions apply in the construction of helicopter blades, hovercraft propellers and in a variety of industrial fans and compressors, subjected primarily to centrifugal forces. A somewhat similar reasoning of cost/efficiency applies to satellite rocket bodies and other space hardware because it costs a great deal to put a pound of weight into orbit. After the invention of carbon fibres the scientists and engineers at RAE pioneered the proper evaluation and use of carbon fibre-reinforced plastics – principally in epoxy resins. They developed manufacturing methods, validated test methods, and carried out design studies. They showed how to design in the presence of fillets and notches and understood that most important property and one so elusive of definition, toughness. This property-toughness under impact conditions – defeated the brave attempt by Rolls-Royce to introduce carbon fibre fan blades for the RB 211 in 1969–1970. The names of Ham, Ewins, Potter, Dorey, Sidey, Cook, Bradshaw, Moreton, Wadsworth, Parratt and lately Bishop and Curtis have become very well known either for work related to advanced composites at RAE or at other centres of British Defence research.
6. Toughness again In the early 1960s engineers were fearful (as indeed we still are to some extent) of putting brittle materials under tensile load. The recognition of pullout as a source of dissipating energy around a crack went some way to assuaging this worry for fibre composites and particularly for those with discontinuous fibres. I noted above that we announced our results on this at the Royal Society meeting in 1963. J.D. Bernal summing up at the end of the conference, said the contributions on fibre reinforcement were all obvious; he had seen it all done during the war! As, indeed, he could claim: though I would argue that the mode of conferring toughness he was referring to is of a different type. He knew the story of ÔPykreteÕ. ÔPykreteÕ was ice reinforced with wood fibres, named after Pyke who proposed to make a floating aircraft carrier out of it for emplacement in the mid-Atlantic during World War II [34]. I became really familiar with the details because the idea was mooted again to us at NPL in the early 1970s to assist the exploitation of oil from the North Sea. However, Pykrete was more concerned with reinforcement of brittle matrices than with ductile ones and hence with
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
the second mode of dissipating energy which I now describe. G.A. Cooper was the second of my research students to work on fibre reinforcement. Since we had discovered pull-out, it was his job to professionalise that and explore it as a means of dissipating energy; a task that he did very well. He also demonstrated, most elegantly, that a crack could not extend if faced with an interface which yielded easily in shear. George was (is?) a superb experimentalist, somewhat difficult to restrain, and he discovered while at Cambridge, though we did not investigate it fully until we had both gone to the NPL, the principle of multiple fracture. The actual experiment performed by George was as follows. He strained in tension a resin, of elongation to failure, 6% at room temperature and 1% at 77 K, containing silica fibres coated with carbon of strain to failure 4% at both temperatures. At room temperature, then, the strain to failure of the resin exceeded that of the fibre; at 77 K the reverse was the case. At room temperature the system was quite brittle but at 77 K a large extension occurred. The load extension curve is striking [35]. One of the components breaks in a series of parallel cracks while the other remains and bears the load. The high elongation phase need not be continuous. We applied the results to reinforced cement, showing that it could be quite tough. Fibre-reinforced cement had again been stimulated by the development of a fibre, namely Cemfil glass which was alkali resistant. Here, indeed, was a very striking phenomenon; we had produced ductility. You could produce it with the metal matrix; that is commonplace, but to produce it in an all-brittle system was magic. We had made a fully brittle system show the sort of load extension curve which metallurgists were always willing to find acceptable because the material appeared to be ductile. Of course, it had been done before, as Bernal would have recognised, but we were the first to name it clearly and to analyse it. The whole subject is now covered under the sobriquet of internal cracking in laminates. So Tyson and Cooper had each separately discovered a totally new method of dissipating energy in a material so that it could resist fracture: pull-out is one; what is now called microcracking or multiple fracture another. Both will work in all-brittle systems though, as I have said, we discovered pull-out first in a metallic matrix system. They are both vital for understanding the materials science of composites and form contributions from the materials scientist to composite science, which is otherwise a very engineering discipline. Pull-out, as I said, represented a new mode of dissipating energy and gave people confidence that all brittle fibre systems would not be fragile. It has been used to produce energy-absorbing structures and the idea developed using helically conformed fibres. It is one of the principal means by which ceramics may be toughened.
2293
Multiple fracture, whether of laminates, or of aligned fibre composites, or of planar matted composites, or of oxide layers on a metal, or of protective coatings, is of the utmost importance. Under various other names, like microcracking, or given the initials MC, its control is vital in all fibre-composite systems of brittle constituents. Control of this phenomenon determines how, for example glasses or ceramics are, or will be, used in gas turbine engines [37]. Both those two new methods of dissipating energy in or around a crack were discovered; they were not predicted. I was lucky enough to be able to recognise their significance and to partly and imperfectly analyse them. They illustrate to my mind together with carbon fibres and whiskers, ceramic superconductors and now carbon nanotubes the central dogma that new materials are always discovered experimentally. They are not predicted.
7. The academicÕs contribution When I had written this account I fell to wondering what had been the contribution to the development of composite materials by an academic such as my self. I, and my associates, through our work on reinforced metals, namely copper reinforced with tungsten wires, introduced the idea of fibre reinforcement to metallurgists who were the dominant practitioners of structural materials science at the time. Being academics we aimed to enunciate principles-in this case the principles of the colligative rules governing strength (stiffness had already been done). Secondly, we attracted bright new persons into a new field through our teaching and researches at a well-known university. Students, those from overseas in particular, carry the ideas with them and effectively disseminate the knowledge – see the example of Gillin cited above. Thirdly, we publicise the subject; and this role continues today in collaboration with younger colleagues who are at the forefront of current research [38]. And we act as consultants, and being freer than most, provided stimulus and contribution to the networking of ideas. Acknowledgements I am grateful to Professor A.J. Kinloch, Trinity College and Michael Gordon for tracing the personality M.F. Gordon.
References [1] Seitz F. Physics of metals. New York: McGraw-Hill; 1943. [2] Watson JD, Crick FHC. Nature 1953;171:737–8. [3] Seitz F. Modern theory of solids. New York: McGraw-Hill; 1940. [4] Seitz F, Read TA. J Appl Phys 1941;12:100. 170, 470, 538.
2294
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
[5] Slater JC. Quantum theory of matter. New York: McGraw-Hill; 1959. [6] Cottrell AH. The strength of solids. Proc R Inst Gt Br 1960;38:346–56. [7] McMullen P. Fibre/resin composites for aircraft. Primary structures: a short history 1936–1984. Composites 1984;15:222–30. [8] de Bruyne NA. Plastic materials for aircraft construction. J Roy Aeronaut Soc 1937;41:523–90; . see also obituary of de BruyneKinloch AJ. Biograph Memoirs Roy Soc 2000;46:127–43. [9] Mayer KH, Lotmar W. Sur lÕelasticite´ de la cellulose. Helv Chim Acta 1936;19:68–72. [10] Gordon JE. On the present and potential efficiency of structural plastics RAE Report No. Chem 469; 1949. [11] Cox HL. The elasticity and strength of paper and other fibrous materials. Brit J Appl Phys 1952;3:72–9. [12] Bishop PHH. The effect of fibre orientation on the mechanical properties of glass reinforced materials. RAE Report No. Chem 493; 1953. [13] Loewenstein KL. Phys Chem Glasses 1961;2:69. 169–184. [14] Bernstein M, Kies JA. The fibreglass motor case in the Polaris Program. In: Proceedings of the filament winding conference. Azuza, CA: Society of Aerospace Materials and Process Engineers; 1961. p. 273–81. [15] Smith CS. Design of marine structures in composite materials. Amsterdam: Elsevier Applied Science; 1990. [16] Gordon JE. Plastics and plastic structures. In: Proc. 3rd AngloAmerican aeronautical conf. aeronautical research council No. 14738; 1951. p. 177–98. [17] Tipper CF. The brittle fracture story. Cambridge: Cambridge University Press; 1962. [18] HMSO (1955). Civil aircraft accident. Report of the court of enquiry into the accidents to two comet aircraft. Heywood RB. Designing against fatigue. London: Chapman & Hall; 1962. [19] Kelly A. Strong solids. Oxford University Press; 1966. [20] Machlin ES. Status report on non-metallic fibrous reinforced metal composites. Materials Research Corporation Contract. NOW 61-0209-c pp; 1961; See also McDanels DL, Jech RW, Weeton JW. Metal Progr 1960;78:118. [21] Parratt NJ. Fibre-reinforced materials technology. van Nostrand Reinhold; 1972. [22] Talley CP. J Appl Phys 1959;30:1114–5. Mechanical properties of glassy boron.
[23] Jensen JE, Gillin LM, Beckett RC, Long G. Hawker de Havilland – a case study. In: Corporate entrepreneurship and innovation conference 27–29 August. Australian Graduate School of Entrepreneurship; 2003. [24] Freise EJ, Kelly A. Twinning in graphite. Proc Roy Soc A 1961;264:269–76. [25] Baker C, Kelly A. The effect of neutron irradiation on the elastic moduli of graphite single crystals. Phil Mag 1964;9: 927–51. [26] Gillin LM. A multipurpose high-temperature tensile-testing machine using electron beam heating. In: Proceedings first conference electron and ion beam science and technology, Toronto; 1964. p. 567–82. [27] Bacon R. Growth structure and properties of graphite whiskers. J Appl Phys 1960;31:283–90. [28] Kotlensky WV, Titus Jr KH, Martens HE. Youngs modulus of hot worked pyrolytic graphite. Nature 1962;193:1066–7. [29] Shindo A. Rep. Gov. Ind. Res. Inst. Osaka No. 317, Osaka, Japan; 1961. [30] Bernal JD, Bawn CEH, Cottrell AH, Frank FC. A discussion on new materials. Proc Roy Soc London A 1964;282:1–154. [31] Watt W, Phillips LN, Johnson W. British Patent 1 110 794. [32] Standage AE, Prescott R. High elastic modulus carbon fibre. Nature 1966;197:169. [33] Kelly A. Leslie Nathan Phillips OBE 1922–1991. Plast Rub Compos Proces Appl 1992;18:197–200. [34] Phillips CE, Thurston RCA. Some mechanical properties of pykrete below 0 C National Physical Laboratory Report H/2509; 1943. [35] Aveston JA, Cooper GA, Kelly A. Single and multiple fracture. In: Conference Proceedings. National Physical Laboratory, IPC Science and Technology Press; 1971. p. 15–25. [36] Kelly A. Fibre composites; the weave of history. Interdiscipl Sci Rev 2000;25:34–44. [37] For example: Miller RJ. Design approaches for high temperature composite aeroengine components. In: Kelly A, Zweben C, editors. Comprehensive composite materials, vol. 6. Amsterdam: Elsevier; 2000. p. 181–570 [chapter 6.10]; Richerson DW. Industrial applications of ceramic Ômatrix compositesÕ. In: Kelly A, Zweben C, editors. Comprehensive composite materials, vol. 6. Amsterdam: Elsevier; 2000. p. 549–70 [chapter 6.28]. [38] Kelly T, Clyne B. Composite materials-reflections on the first half century. Phys Today 1999:37–41.
Very stiff fibres woven into history – very personal recollections of some of the British scene Anthony Kelly
*
Churchill College, Cambridge CB3 0DS, UK Department of Materials Science, and Metallurgy University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Received 4 May 2005; accepted 4 May 2005 Available online 10 August 2005
Abstract A personal memento is given concerning how the author became interested in the ideas of fibre reinforcement in 1960 and how he watched as a close consultant the invention of very stiff fibres at Farnborough by Watt and Phillips. Some account is also given of the early work at Farnborough both before during and after the second world war and the part played by de Bruyne and then by J.E. Gordon and his team. The early development of the materials science of composites by the author and his colleagues and the discovery and enunciation of the principles of how all brittle materials can show appreciable works of fracture are dealt with. Some short remarks follow on the part played by academics in the development of a new technology. 2005 Elsevier Ltd. All rights reserved. Keywords: Composites; Carbon fibres; Toughness
1. First researches I read physics for my first honours degree and in those days great attention was paid to teaching practical skills – we measured the charge on the electron by MillikanÕs method for instance. The domain structure in a ferromagnetic was a modern development and we examined Bitter patterns – the designs formed by iron filings on the surface of a ferromagnet. This last led me as an undergraduate to an interest in what were called subgrains in metals. I heard of these through reading the wonderful Physics of Metals by Fred Seitz [1]. I applied to enter the Cavendish Laboratory at Cambridge – at that time widely regarded as the foremost academic physics laboratory in the world. Prospective research students then were not molly coddled with suggestions by prospective supervisors eager for graduate
*
Tel.: +44 1223 363691; fax: +44 1223 33456. E-mail address: [email protected].
0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.05.010
students but were expected to indicate a real problem on which they would like to work. I was lucky to have mentioned subgrains in metals since this was a burning interest of the Cavendish Professor Laurence Bragg (of BraggÕs law) and so he was to be my official supervisor when I was accepted by the Cavendish and also by Trinity College. At Cambridge one enters the University through one of its colleges. I worked on detecting subgrains in metals and through that route became a metallurgist. Bragg, during my time at the Cavendish, lost interest in metals and turned passionately to encouraging the elucidation of the structure of biological materials by means of X-ray diffraction. It was that interest which enabled Crick and Watson to meet and to work out in a spectacular and intuitive fashion the structure of DNA – the secret it seems of the genetic code – the physical means by which hereditary features are transmitted from generation to generation [2]. I was in no way involved but enjoyed a spectatorÕs grandstand view of the personalities involved in this wonderful scientific achievement.
2286
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
Having been converted to Metallurgy I fell under the spell of Alan Cottrell; he examined my PhD Thesis and later offered me a job, or suggested that I apply for a job at Cambridge some years later. I left the Cavendish Laboratory in 1953 and worked in the United States with Paul Beck at Illinois – where I met and came greatly to admire Fred Seitz who was in the Physics Department and whose brilliant lectures based on his massive text book Modern theory of Solids [3] I humbly attended. During that period I also met Bardeen, of course who was in both Electrical Engineering and in Physics and earning his second Nobel Prize. Heissenberg passed through on his first (?) visit to the United States after the war and I did some very preliminary work with Koehler, Seitz and Baluffi. The Metallurgy department of Illinois was suddenly modernised by the influx of a group from Columbia under Tom Read who had published the first US papers on dislocations with Seitz and Read [4]. Illinois was a lovely place to work in those days as solid state science had its origins and expanded. I returned to England briefly for the calendar year of 1955, became engaged to be married and then returned to the USA having been recruited by Morrie Fine to teach X-ray crystallography to students of a newly formed department of Metallurgical Engineering in the Technological Institute of Northwestern University in Evanston Illinois – just north of Chicago and the home of the WomenÕs Christian Temperance Union one of the forceful progenitors of the move to prohibit the sale of alcohol within the USA after World War I. Fine was and still is, now well into this ninth decade, a very forward-looking man in his science. Before coming to Northwestern he had taken part in the Manhattan project (the making of the first atomic bomb) at the University of Chicago and subsequently at Los Alamos. Through him and others at Northwestern I met Oppenheimer – the father of the fission bomb – whom I remember as a wise and kindly man who much encouraged me in my researches with well-spoken words of praise. Fine it was who led our department at Northwestern to attach to its name the study of Materials Science in 1957. We could rightly claim to be the first department in the world to adopt this in our title. So our horizons were broadened from the study of metals to include plastics or polymers, at that time the fastest growing of the materials producing industries. Of course, we already knew a little about ceramics – the third major component of materials science – through the use of these as refractories for furnace linings and as tundishes, etc. Cottrell accepted the Goldsmiths Chair of Metallurgy at Cambridge and went there from the Atomic Energy Research Establishment at Harwell in September 1958. In 1956, he had mentioned to me the great likelihood of this occurring and had asked me whether I would join
him there if he could secure a lectureship – a post not lightly created at Cambridge in those years. I treasured this thought in the intervening years at Northwestern but when the opportunity finally arose I was sad to leave Evanston. Cottrell asked me (I knew very little of Metallurgy at the time) to give a set of lectures on ceramics. I came to Metallurgy at Cambridge with the first Instron tensile testing machine imported to Britain and set about preparing these lectures. I did this by reading SlaterÕs Quantum Theory of Matter [5] while crossing the Atlantic by ship. Other people lecturing on ceramics would have scoffed at that but, as it happened, it stood me in very good stead. I discovered in the literature rather little materials science of ceramics. There was a wealth of chemistry but less material relating structure to properties. I persevered and discussed with the undergraduates things like the physical interpretation of Mohs hardness scale, the difficulty of moving dislocations in materials such as sapphire, why silicon carbide sintered slowly and, following Slater, why covalent solids are stiff and of low density, since the atoms are held apart by spatially directed bonds. We discussed a host of other things not usually dealt with in ceramics courses. I was pleased afterwards to discover that the graduates who went into such industries as the steel industry found my lectures very useful, even though I had not dealt in excruciating detail with how furnace linings were rammed into place.
2. Fibre reinforcement It is difficult to appreciate nowadays that in those years some very obvious data were missing from the literature, e.g., the elastic constants of boron, or even of fully dense graphite. At Cambridge I started some work on ceramics, notably, for what comes later, on the investigation of graphite single crystals and on magnesium oxide. My own special interest was in precipitation hardening of metals and I discovered that, in aluminium–copper and copper–beryllium and such like, precipitation hardened alloys in the overaged condition developed very strong rates of work-hardening. Cottrell, at the time, was becoming a very fashionable young scientist and was invited to give lectures such as the Royal SocietyÕs Bakerian, and discourses to the Royal Institution, Royal Society of Arts, etc. He often used to give me the text of these lectures to read before delivering them. In a discourse delivered to the Royal Institution on 15 June 1960 [6] he enunciated the principle of fibre reinforcement. I quote what he said: No, the practical approach is to admit the existence of cracks and notches and to try to render them innocuous.
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
Suppose that we are stretching a rod. . . and. . . that it consists of a bundle of parallel. . . fibres joined together – by some adhesive or solder. If there is a transverse notch in the rod, cutting across a number of these fibres, the forces from the cut fibres can be transmitted to the fibres at the tip only by passing as shearing forces through the layers of adhesive. . . If this adhesive has a fairly low resistance to shear. . . it will then be incapable of focussing the transmitted forces sharply. . . There are tremendous possibilities for developing this principle further, particularly by using fibres of materials with very strong atomic forces, e.g., refractory oxides and carbides. It is all there. It was new to me. And in those days in pounds per square inch! The discourses of the Royal Institution give no references, or rather AHC gave no references, to that particular talk. ÔHad he dreamt the principle up for himself?Õ, was the question I asked, ÔWas there some literature unknown to me?Õ The hint that there was an extensive literature occurred in the list of exhibits in the library, listed at the end of his lecture. These were from the Royal Aircraft Establishment. People there had approached strong materials in a completely different fashion from that of the metallurgist. Cottrell will have learnt of this from his membership of various quasi-confidential Government committees, which abounded in that period and on which most academics interested in physical metallurgy sat. The situation of modern fibrous composite science at that time in Britain, and its subsequent development, paying special attention to the work at the Royal Aircraft Establishment is well described by McMullen [7] which I now follow in part – see also [36] for recent years.
3. Modern composites at Farnborough The beginning of research and development on composites for aircraft structures began in the UK in 1937 when N.A. de Bruyne, an engineering don at Cambridge, read a paper to the Royal Aeronautical Society entitled ‘‘Plastic materials for aircraft construction’’ [8]. It is a massive paper showing in great detail how de Bruyne, with great vision foresaw the many difficulties and also some solutions to these – but most importantly he recognised the great promise. De Bruyne was a keen amateur airman who built his own aircraft the Snark. Doing this he recognised the importance of stressed plywood and the necessity to orientate the material so that the wood fibres ran parallel to the largest loads. Because of his knowledge of plastics he was employed as a consultant by the de Havilland company to advise on the making of propellers. Plastics as materials of construction for propellers had been considered since the 1920s.
2287
In 1936, de Bruyne had noted, following Meyer and Lotmar [9], the theoretical properties of some cellulosic fibres in terms of their high stiffness and/or strength coupled with a low specific gravity. To reinforce and stiffen the plastic a flax roving was used teased out into flat bands as a continuous reinforcement De Bruyne introduced a material which he called Aerolite which was phenol–formaldehyde (Bakelite) containing aligned fibres of cotton ramie and other textiles. He also experimented with urea formaldehyde resins. De Bruyne is principally remembered now for his wonderful inventions of adhesives such as Redux, which successfully bonded wood and metal for aircraft construction. In 1938, J.E. Gordon a young man just trained as a marine engineer was hired at Farnborough and asked to evaluate new non-metallic materials. Among these was Gordon-Aerolite (flax fibre-reinforced phenolic resin) invented by M.F. (Malcolm) Gordon, J.E.Õs namesake – but no relation – working at Aero Research Ltd., a firm founded by de Bruyne. It was Malcolm Gordon one of de BruyneÕs students who, because he had a connection with a Belfast linen mill, (in the charge of his father) suggested the use of flax as reinforcement. A supply of unbleached flax thread was impregnated with phenolic resin, wound into a skein and hot-press moulded to form what was probably the first example of a high performance composite. M.F. Gordon went on to become managing director of a spinning company and an honoured Engineer/Industrialist in Northern Ireland. We know that M.F. and J.E. met but I know no more of any interaction between them. Gordon [10] as the first to point out that, if composites were to be used for structures, they had to be used as efficiently as possible in order to be competitive with metals. To this end, he drew attention again to the need for the highest possible degree of fibre orientation in each layer of material and the requirement then for the layers to be crossed at appropriate angles – to meet the applied loads. This required careful calculation and the early mathematics of fibre orientation date from this time and is attributable to Cox of NPL [11] and Gordon and Bishop of RAE [12]. One of the first composite components of any consequence tested by J.E. Gordon was an experimental fullscale main wing spar made by de BruyneÕs company for the ÔBlenheimÕ bomber. This was of lattice construction using Gordon-Aerolite but was designed like conventional metal structures, and so was a misuse of this novel material. A similar abortive exercise involving a ÔSpitfireÕ fuselage was conducted at the outset of the war (WW-2) when there was a temporary shortage of aluminium alloys in Britain. A Gordon-Aerolite replica of the real thing was made but the structure was hopelessly overweight. Throughout World War II, cellulose fibre composites were developed with moderate priority and with some
2288
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
real success. Aircraft drop tanks saw service, while wing tip tubes and ducting were in production and a composite seat was made for the ÔSpitfireÕ. Work at the RAE on cellulose fibre-reinforced plastics ceased in 1946 and, although only a partial success, enough had been demonstrated and learned to justify further composites R&D. It was realised, for example, that the reinforcing fibre had to be inherently inert to climatic changes, it had to be possible to give it a high degree of orientation and it had to be possible to achieve a good bond between it and phenolic resins – the only types available at that time. The resin/fibre pre-impregnate had to have a good Ôshelf-lifeÕ and it had to be possible to mould it at low pressures in low-cost tooling and the resulting laminates had to develop a high proportion of the properties of the fibre. The work on cellulose fibre composites also led to the beginnings of the understanding of fibre orientation theory, the realization of the importance of strict control of moulding conditions and the appreciation that these new materials would require the development of new methods of test. In short, much of the technology learned in this early work formed the basis of all that followed. At the end of the RAE work on cellulose fibres in 1946, samples of glass fibre obtained from the USA were examined as a possible reinforcement for plastics for primary structures. By comparison with cellulose fibres they are heavy and thus their specific stiffness is about half that of flax – one of the best of the cellulose fibres – but they have an enormous advantage in strength and can of course be highly oriented. Earlier developments of GRP (c 1943) owed much to the invention of radar and the urgent need for radomes these are radar housings and contain E glass which is a glass fibre free from ions which lead to absorption at microwave frequencies (cf. Stealth aircraft today). In view of the very poor bond between glass fibres and the phenolic resins of the time and with no immediate prospect of improving this, glass fibres as reinforcement for major structural plastics were abandoned in 1946 at RAE. The years from 1952 saw, however, an enormous growth in the glass-reinforced plastics (GRP) industry due to the advent of polyester and epoxy resins. Due to their prior developed expertise RAE were well placed to take advantage of this especially when glass fibres, stiffer than E glass became available in the late 1960s [13]. Glass is relatively heavy and no stiffer than aluminium and so its use for aircraft primary structures is very limited. However, for rocket motor cases stiffness is less important and strength paramount [14]; glass then becomes very useful. The filament winding technology developed by RAE in conjunction with Bristol Aerojet Ltd. and Imperial Metal Industries Ltd. led to successful firing trials of
all-plastic rocket motors in 1951/1952 and to equally successful flight trials of guided weapon boost-motors soon afterwards. The Admiralty Research Establishment in Dunfermline pioneered the use of glass fibres in mine hunters under the direction of CS (Charles Stuart) Smith (1936–1991). Of course, fibreglass boats were displacing wooden boats. The Admiralty made the very courageous decision to displace wood with grp in large vessels (ships rather than just boats) with greater than 500 tonnes displacement – see for example [15]. The successful introduction of HMS Wilton, the prototype, led to the Hunter and Sandown class of mine hunter, the largest in the world in the early 1980s. Interestingly, the first vessels were no lighter than the competition because they had to be over-designed owing to the notorious plastics factor – see below. Smith and his team did much to reduce both this and, very effectively, the cost of production. When RAE work on cellulose-reinforced plastics ceased in 1946, a search began for other, stiffer and more inert fibre reinforcements. There was interest in the potential of asbestos as a reinforcing fibre and it was a happy coincidence that in 1947 Turner Brothers Asbestos Company produced Durestos – a chrysotile asbestos fibre-reinforced phenolic resin. In temperature resistance for example, it can readily out-perform GRP and aluminium and can be used for the venturii of short burning time rocket motors. The great disadvantage of this material is the health hazard associated with asbestos. Chrysotile is very stiff and strong, inert to the effects of moisture and is well bonded by phenolic and other resins. The big drawback in 1947 was the absence of technology for highly orientating the fibre. In the years 1948–1954, J.E. GordonÕs section at RAE demonstrated beyond doubt the feasibility of making large high performance composite structures. The main demonstrator structures were delta wings for the Fairey experimental E10/47 aircraft. A description is contained in the proceedings of the Third Anglo-American Aeronautical Conference 1951 [16]. When the demonstrator exercise was terminated in 1955 the wings had been developed to the stage where they could sustain test loads well in excess of the 12 g load required of the metal ones. They had weight in hand – but they had not sustained the loads derived from the required use of the notorious 1.5 Ôplastics factorÕ, i.e., 12 · 1.5 = 18 g. Because of relative ignorance of the behaviour of the materials compared to the knowledge base concerning the properties of aluminium, a plastics factor of 1.5 was introduced. In other words, the composite had to be 50% better than the stated design figure before its use would be considered. This situation is reminiscent of that regarding performance under fatigue conditions today, when lack of a predictive capa-
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
bility comparable to that found with metallic structure inhibits the use of composites in some applications. Nevertheless these developments during the 1950s were a significant milestone in the development of aircraft composite structures and showed beyond any doubt the potential advantages to be gained. Above all they demonstrated that composites for aircraft primary structures would have to have specific stiffness properties substantially greater than aluminium alloys if worthwhile structure weight savings were to be achieved. At that time (c 1954) this meant only one thing: new, stiff, lightweight fibres had to be invented. This was the approximate position when I first heard of the field in 1960. The principles of fibre composite science/engineering were known but not widely and the fibres being considered were whisker crystals, asbestos fibres, glass and possibly metal wires.
4. The field as I entered it In the late 1950s and very early 1960s the ideas for controlling cracks following the ideas of fracture mechanics were only slowly being used in practice. There was a great and understandable fear of using all brittle systems in load bearing applications based on the experience of the spectacular Liberty ship failures in World War II [17] and the Comet disaster [18]. Fibre composites appeared to be in the class of such all brittle structures and so even if high strength were to be obtained many people thought that they would not be useful especially under any conditions of impact. My associates and I were very fortunately placed in the early 1960s in that we were elucidating the principles of the strengthening of metals and yet were lucky enough to spot the advantages of non-metallic solids as the major strengthening agents, because I was lecturing on ceramics. In the summer of 1961 Phillip Bowden, Cottrell and others organised a closed conference at Caius College Cambridge, financed by the National Research Development Corporation (a forerunner of the British Technology Group), which discussed fibre reinforcement and we at Cambridge learnt, through our colleagues from Oxford, of work on directional solidification of eutectics, both in the USA and at Oxford. Work started at Cambridge on this topic. I mentioned the high rates of work hardening of alloys in the overaged condition, which appeared to support the idea of fibre reinforcement with a metal matrix. This was because we knew that particles of a strong solid within the metal were being elastically strained as a result of the motion of dislocations in the metal. The dislocations being unable to shear the strong particles, piled up around them and consequently the metal appears to become very much stronger as plastic strain is increased [19].
2289
I immediately asked Bill Tyson (who joined me with an Athlone Fellowship from Canada) to investigate the validity of CottrellÕs idea with a metallic matrix as the adhesive or solder. We tried to produce aligned rods of a very hard phase in a metal by cold working and by directional solidification of a eutectic and other means, since whisker crystals were difficult to use experimentally. This Bill did but without exciting results. What really got us going was a review by Gene Machlin from Columbia [20], and through it, coming across the work of McDanels, Jech and Weeton [20]. This latter gave us an excellent model system to work on. They had introduced aligned very strong tungsten wires into soft ductile copper, noting that strong wires of many refractory metals are insoluble in the noble metals. Machlin had already recognised the importance of the fibre reinforcement of metals. Tyson went to work with a will and we produced in a relatively short time a set of principles, which could be, called the colligative mechanical properties of an aligned fibre-reinforced system. We could explain adequately the strength of the system in terms of the strengths of the two components and how it depended upon volume fraction and on the relative elongation of the two components; on the length of the fibres and more important, the ratio of length to diameter. We explored the idea of minimum and critical volume fraction. I remember with surprise the work being described as being elegant, but I thought it was rather straightforward, but necessary to establish the principles. In the course of doing it we made one of the fundamental discoveries needed. We found a new means of dissipating energy. The fibrous composite, of course, broke. Since we were working with metals and were aware of the emerging concepts of fracture mechanics, we were able to recognise immediately the significance of pull-out at a fracture surface. Tyson found that when a fibrous composite breaks all the fibres in the fracture plane do not break at once. This is particularly pronounced with brittle fibres. The fracture surface, therefore, had a jagged appearance and the sliding apart of the two faces or the pull-out of the fibres from holes in the matrix, dissipates a lot of energy and provides a quasi-ductile form of behaviour. That was the discovery and that this would dissipate energy was the natural suggestion. Cottrell and I (I do not remember whether Tyson was present) discussed the finding before the Royal Society meeting on New Materials in 1963. With his wonderful facility for doing the physics immediately with simple algebra Alan Cottrell produced the first formula for estimating the work of fracture due to pull-out. He altered his original draft of the introductory paper to the Conference in order to include it. His simple formula was much quoted for a few of the succeeding years.
2290
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
This behaviour is not confined to metals and, from TysonÕs work, sitting on the numerous quasi-confidential committees I have mentioned, I remember saying that this would probably happen with forms of fibrous composites made from all-brittle systems and would provide an energy absorbing mechanism. A little later, Leslie Phillips (see below) said, Ôand it broke, just like Dr. Kelly said it wouldÕ. This may seem all very obvious to people nowadays, but at the time it was something genuinely new. Other people had described types of pull-out but no one recognised its importance as a source of dissipating energy around a crack. We announced our results at the Royal Society meeting in 1963 to which I refer again below. J.E. Gordon left RAE in 1954 and went to TI Research Laboratories, where I met him for the first time. He worked principally on whiskers with C.C. Evans and N.J. Parratt. He returned to what is now the DERA fold in 1962 at ERDE Waltham Abbey. There, he and his colleagues developed methods of growing and handling large quantities of whiskers, principally of Si3N4 but also of SiC, as well as short fibres of glass carbon and various varieties of asbestos. SiC whiskers in alumina now provide a very effective cutting tool. The methods developed at ERDE are very important for the development of composites containing short fibres, since composites with only long fibres may lack mouldability; they are what is called boardy. A summary of much of the work at TI and subsequently at Waltham Abbey is contained in the book by Parratt [21].
5. Carbon fibres Remember that at this stage we are still looking for a fibre stiffer than steel (200 GPa) and of much lower density. Talley, working in the research laboratory of the Texaco Company for the Office of Naval research invented a method of making boron fibre – depositing boron from the vapour phase onto a tungsten wire in 1958. The stiffness was 460 GPa and the strength 2.3 GPa [22]. The invention was described by a general of the US Airforce as the biggest breakthrough in materials since the Stone Age! This fibre is not easily handleable due to a relatively large diameter, however, and the really modern fibres (the various carbons and aramids) are more closely related to textiles. I became a member of the graphite community for a few years in the early 1960s due to my interest in ceramics. High stiffness carbon fibres were invented almost simultaneously in the UK, USA and Japan at about that time and I was familiar with much of the relevant work in those three countries. My early work on ceramics stemmed from CottrellÕs obtaining funds from the Atomic Energy Authority for
work in support of the high temperature atomic reactor – the Dragon project. And three of my research students worked directly on graphite. Earl J. Freise who followed me from Northwestern was the first of these – a most dedicated man who obtained a PhD at Cambridge in record time. He went on to become a University administrator finally in a high position at Caltech. Clive Baker – the twin brother of Colin, who later did seminal work at Alcan on metal matrix composites, studied radiation damage in graphite and Murray Gillin who came to Cambridge in 1962 and investigated the mechanical properties of graphite at very high temperatures. Gillin came from the Australian government research centre at FishermanÕs Bend near Melbourne and when he finished his PhD in early 1965 he had seen as an avid spectator, as had I, the invention of high performance carbon fibres at Farnborough. This knowledge he took back to Australia and being a government employee was able to influence and initiate Australian research in composites. Gillin has cited this experience in a number of publications [23]. My three students worked with me on single crystals of graphite – large naturally occurring single crystals. Freise and I were able to obtain these because I had worked at Northwestern on a US Airforce contract (AP 1438 if I remember correctly) and so had contacts in that organisation. Through these, and they gave ready help, we obtained natural crystals from the pyroxene bearing rocks at the Lead Hill Lead mine in Ticonderoga New York. Freise published the definitive work on twinning [24] of crystals of graphite and carried out deformation studies on kish crystals and on pyrolytic graphite and on commercial grades of polycrystalline graphite. BakerÕs studies of the change of stiffness of graphite crystals when irradiated led me to urge him to carry out and for me to interpret the results of one of the pieces of work of which I am most proud [25]. We determined directly for the first time the elastic modulus C44 of graphite. This is the modulus governing shear on the basal plane which has an extremely low value: we found a value of <100 MPa. And to do this we had to amplify Lord RayleighÕs equations for the bending of a cantilever beam; one should include both shear and rotary inertia, both of which Rayleigh had neglected. Gillin constructed a unique apparatus for carrying out tensile tests at temperature as high as 3000 C. The detailed results still remain unpublished, though an account of the construction of the machine has appeared [26]. He deformed specimens of natural graphite and of commercial grades of polycrystalline graphite and of an exciting form known as pyrolytic graphite. This last is obtained by the decomposition of methane gas at a temperature of say 2100 C onto a commercial graphite substrate. The material consists of individual graphite sheets which are usually wrinkled but of density 2.14 – hence close to that
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
of pure graphite, 2.26. The wrinkling arises due to the very low thermal conductivity normal to the layer planes so that the individual layers are deposited in a very strong thermal gradient. As a result of these researches I got to know Willie Watt at Farnborough who was working on pyrolytic graphite. Gillin and I obtained specimens of the material from him. The Atomic Energy Authority also supported Watt and our association lasted until his death in 1985. I became a consultant at RAE. Work on graphite at that time involved naturally the Union Carbide Corporation and on a visit to the USA in 1962 I met Roger Bacon. There were clues to the possibility of very stiff carbon fibres in the late 1950s and early 1960s, which were emphasized by some of us at the time. These were, Roger BaconÕs graphite whiskers (now fashionable again following the terrestrial manufacture of fullerenes). Bacon made these in a carbon arc under pressure in 1960 [27] some specimens of which he later presented to me and I in turn to a young scientist working on carbon nanotubes in 2000. The whiskers had moduli of 700 GPa and strengths of 20 GPa. They are scrolls of graphite layer planes. Kotlensky Titus and Martens [28] in 1962 hot stretched pyrolytic graphite and attained a modulus of 560 GPa. The graphitization of polymers such as polyacrylonitrile can lead to a preferred orientation and Shindo in 1961 [29] had produced a fibre with a modulus of 120 GPa. Akio Shindo visited me in Cambridge in 1965 and presented me with a print of a woodcut by Kiyoshi Saito – a well-known Japanese printmaker. Hence, I was fortunate enough to know personally all three inventors of carbon fibre of high modulus. In June of 1963 the Royal Society held a discussion meeting on New Materials [30]. It was organised by J.D. Bernal (1901–1971), Alan Cottrell (b. 1919), Charles Frank (1911–1998), and Cecil Bawn (1908– 2001?) – what a panoply of talent! Both Jim Gordon and I spoke at the meeting, he on designing with brittle materials (when he produced the very well-known dramatic diagram of an aircraft wing illustrating the need for stiffness). I, on the principles of determining the strength of continuous and discontinuous aligned composites of fibre reinforcement. And, as I have said emphasised that all brittle systems could be tough. During the discussion, the lack of a stiff fibre of carbon was mentioned. Watt (1912–85) was present at the meeting and told me later that he immediately determined to see if he could make stiff graphite fibres. Cottrell and I returned to Cambridge having discussed stiff graphite on the train, and on that evening I put some cotton wool in our graphite tube furnace overnight. The resulting carbonised cotton wool was not stiff – one had to be much more sophisticated. Watt, Phillips and Johnson at the RAE succeeded. Three groups almost simultaneously invented stiff carbon fibres in
2291
1963-64: Bacon in the USA, Shindo in Japan and Watt, Phillips, and Johnson. Of these three original processes that invented by the last three named became dominant [31]. The first experiments on the production of carbon fibres by Watt were according to Watt, stimulated by the Royal Society Discussion Meeting. Phillips always vigorously denied this. He said Willie Watt and Bill Johnson asked for a meeting with him in order to get his views on the polymer chemistry connected with making silicon carbide. Anyway, it was Leslie who suggested trying black Orlon and referred Watt to the observations of Houtz who had shown that, on heating a proprietary fibre based on PAN, viz. Orlon, in air for up to 20 h, it underwent a colour change to yellow, then brown, then black and became insoluble in PAN solvents. The key step in the process developed at RAE is the oxidation of the PAN fibre while under longitudinal constraint. The possibility of shrinkage was pointed out by Bill Johnson and according to his own account Leslie Phillips banged the table and said Ôwe must not let itÕ. PhillipsÕ recollection at the time was that it would have been relatively easy to get a high elastic modulus carbon fibre – it depended (cf. Kotlensky) on the coking temperature. The difficulty was to obtain a useable high strength, which depended on clean fibre, optimum time and temperature of heat treatment so as to decompose the PAN. The initial batch process in which PAN fibres were wound on frames were made of graphite and ÔMeccanoÕ and later of glass. This was to provide the tension during the oxidative stabilising process, which was the key to the invention so as to stop the fibres shrinking. In effect they were stretched during the oxidation stage. It was this step which had eluded the Japanese. The RAE fibres were some three or four times stiffer than those produced in Japan. Watt and Johnson actually made the first fibre and Phillips showed these fibres to be compatible with epoxy and polyester resins. After the invention carbon fibres were made on a pilot scale at AERE Harwell. The RAE patents covering the production of carbon fibre from PAN were assigned to NRDC (later BTG) and three licensees established: Courtaulds, Rolls-Royce and Morganite. Details of the invention were given in an exceptionally complete and detailed form. All three licensees used Courtaulds PAN (a variant of their Courtelle) as a precursor. Rolls-Royce set up production facilities at Hucknall to produce material for the RB211 jet engine and excellent carbon fibre prepreg was produced under the name of Hyfil [32], although the material was not used in the engine. The business passed to Bristol Composite Materials. Production ceased around 1980. Morganite made carbon fibre under the name of Modmor, but ceased
2292
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
production in the early 1970s. Courtaulds continued to produce until 1991 when they ceased production in the UK and sold their American interests to a Japanese company. They continue strongly in Advanced Composite Structures. A later comer, RK-Textiles, maintain a carbon fibre manufacturing capability in the UK. During the establishment of the processes in industry, Phillips played a leading part in advising the licensees, drawing attention to the dangers (avoided) of sodium cyanide and HCN evolution, to the need for controlled flow rates of all the gases and to the removal of tars [33]. In the early days Phillips was also concerned with the treatment by hydrogen peroxide to essentially make pores on the surface of the fibre so as to vary the adhesion to the resin. It was he, I believe, who made the crucial decision that, since carbon fibres were likely to be expensive, they should be married with the expensive resins and it was I think the development of carbon fibres which led very much to the development of improved properties of epoxy resins. WattÕs main interest was in the chemistry of making high performance carbon fibre and he continued to work on these until his death. I felt privileged to have been able to attract him to work at Surrey University where I was then the Vice Chancellor when he retired from the RAE. Watt, through his position as a Fellow of the Royal Society – he was elected in 1976 – had an interest in composites of course and organised several successful meetings. The aerospace industry saw the potential of carbon fibres immediately but it was Phillips of the three British inventors who undertook the prosecution of the widespread application of these things not only into aerospace but also into much more fashionable and exciting artefacts, such as kayaks, golf clubs, fishing rods (he was a keen fisherman), motor car bodies, furniture, textile machinery and many other forms. For many years the body of the Maclaren racing car – a sweeping Grand Prix winner – embodied the largest piece of carbon fibre moulding ever made. Phillips was also the man who realised, with others, that the most important product form would be fabrics and fibre hybrids and he recognised the need for the invention of simple manufacturing processes. He would be delighted today with the prospect of non-woven, two-dimensional arrays of fibres, which are becoming available. He was always concerned with finding widespread uses for carbon fibres and avoiding solely esoteric ones like, as he said Ôblack piano keys and false teeth for soldiers on night operationsÕ. He will be particularly remembered for his efforts to introduce carbon fibres into thermoplastics, first by compounding and then by the invention of the film stacking method and also of course of various types of vacuum moulding. He said:
One should be careful to choose possible applications where high initial costs can be tolerated and where anisotropic and highly orientated composites are an advantage rather that the reverse. These conditions apply in the construction of helicopter blades, hovercraft propellers and in a variety of industrial fans and compressors, subjected primarily to centrifugal forces. A somewhat similar reasoning of cost/efficiency applies to satellite rocket bodies and other space hardware because it costs a great deal to put a pound of weight into orbit. After the invention of carbon fibres the scientists and engineers at RAE pioneered the proper evaluation and use of carbon fibre-reinforced plastics – principally in epoxy resins. They developed manufacturing methods, validated test methods, and carried out design studies. They showed how to design in the presence of fillets and notches and understood that most important property and one so elusive of definition, toughness. This property-toughness under impact conditions – defeated the brave attempt by Rolls-Royce to introduce carbon fibre fan blades for the RB 211 in 1969–1970. The names of Ham, Ewins, Potter, Dorey, Sidey, Cook, Bradshaw, Moreton, Wadsworth, Parratt and lately Bishop and Curtis have become very well known either for work related to advanced composites at RAE or at other centres of British Defence research.
6. Toughness again In the early 1960s engineers were fearful (as indeed we still are to some extent) of putting brittle materials under tensile load. The recognition of pullout as a source of dissipating energy around a crack went some way to assuaging this worry for fibre composites and particularly for those with discontinuous fibres. I noted above that we announced our results on this at the Royal Society meeting in 1963. J.D. Bernal summing up at the end of the conference, said the contributions on fibre reinforcement were all obvious; he had seen it all done during the war! As, indeed, he could claim: though I would argue that the mode of conferring toughness he was referring to is of a different type. He knew the story of ÔPykreteÕ. ÔPykreteÕ was ice reinforced with wood fibres, named after Pyke who proposed to make a floating aircraft carrier out of it for emplacement in the mid-Atlantic during World War II [34]. I became really familiar with the details because the idea was mooted again to us at NPL in the early 1970s to assist the exploitation of oil from the North Sea. However, Pykrete was more concerned with reinforcement of brittle matrices than with ductile ones and hence with
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
the second mode of dissipating energy which I now describe. G.A. Cooper was the second of my research students to work on fibre reinforcement. Since we had discovered pull-out, it was his job to professionalise that and explore it as a means of dissipating energy; a task that he did very well. He also demonstrated, most elegantly, that a crack could not extend if faced with an interface which yielded easily in shear. George was (is?) a superb experimentalist, somewhat difficult to restrain, and he discovered while at Cambridge, though we did not investigate it fully until we had both gone to the NPL, the principle of multiple fracture. The actual experiment performed by George was as follows. He strained in tension a resin, of elongation to failure, 6% at room temperature and 1% at 77 K, containing silica fibres coated with carbon of strain to failure 4% at both temperatures. At room temperature, then, the strain to failure of the resin exceeded that of the fibre; at 77 K the reverse was the case. At room temperature the system was quite brittle but at 77 K a large extension occurred. The load extension curve is striking [35]. One of the components breaks in a series of parallel cracks while the other remains and bears the load. The high elongation phase need not be continuous. We applied the results to reinforced cement, showing that it could be quite tough. Fibre-reinforced cement had again been stimulated by the development of a fibre, namely Cemfil glass which was alkali resistant. Here, indeed, was a very striking phenomenon; we had produced ductility. You could produce it with the metal matrix; that is commonplace, but to produce it in an all-brittle system was magic. We had made a fully brittle system show the sort of load extension curve which metallurgists were always willing to find acceptable because the material appeared to be ductile. Of course, it had been done before, as Bernal would have recognised, but we were the first to name it clearly and to analyse it. The whole subject is now covered under the sobriquet of internal cracking in laminates. So Tyson and Cooper had each separately discovered a totally new method of dissipating energy in a material so that it could resist fracture: pull-out is one; what is now called microcracking or multiple fracture another. Both will work in all-brittle systems though, as I have said, we discovered pull-out first in a metallic matrix system. They are both vital for understanding the materials science of composites and form contributions from the materials scientist to composite science, which is otherwise a very engineering discipline. Pull-out, as I said, represented a new mode of dissipating energy and gave people confidence that all brittle fibre systems would not be fragile. It has been used to produce energy-absorbing structures and the idea developed using helically conformed fibres. It is one of the principal means by which ceramics may be toughened.
2293
Multiple fracture, whether of laminates, or of aligned fibre composites, or of planar matted composites, or of oxide layers on a metal, or of protective coatings, is of the utmost importance. Under various other names, like microcracking, or given the initials MC, its control is vital in all fibre-composite systems of brittle constituents. Control of this phenomenon determines how, for example glasses or ceramics are, or will be, used in gas turbine engines [37]. Both those two new methods of dissipating energy in or around a crack were discovered; they were not predicted. I was lucky enough to be able to recognise their significance and to partly and imperfectly analyse them. They illustrate to my mind together with carbon fibres and whiskers, ceramic superconductors and now carbon nanotubes the central dogma that new materials are always discovered experimentally. They are not predicted.
7. The academicÕs contribution When I had written this account I fell to wondering what had been the contribution to the development of composite materials by an academic such as my self. I, and my associates, through our work on reinforced metals, namely copper reinforced with tungsten wires, introduced the idea of fibre reinforcement to metallurgists who were the dominant practitioners of structural materials science at the time. Being academics we aimed to enunciate principles-in this case the principles of the colligative rules governing strength (stiffness had already been done). Secondly, we attracted bright new persons into a new field through our teaching and researches at a well-known university. Students, those from overseas in particular, carry the ideas with them and effectively disseminate the knowledge – see the example of Gillin cited above. Thirdly, we publicise the subject; and this role continues today in collaboration with younger colleagues who are at the forefront of current research [38]. And we act as consultants, and being freer than most, provided stimulus and contribution to the networking of ideas. Acknowledgements I am grateful to Professor A.J. Kinloch, Trinity College and Michael Gordon for tracing the personality M.F. Gordon.
References [1] Seitz F. Physics of metals. New York: McGraw-Hill; 1943. [2] Watson JD, Crick FHC. Nature 1953;171:737–8. [3] Seitz F. Modern theory of solids. New York: McGraw-Hill; 1940. [4] Seitz F, Read TA. J Appl Phys 1941;12:100. 170, 470, 538.
2294
A. Kelly / Composites Science and Technology 65 (2005) 2285–2294
[5] Slater JC. Quantum theory of matter. New York: McGraw-Hill; 1959. [6] Cottrell AH. The strength of solids. Proc R Inst Gt Br 1960;38:346–56. [7] McMullen P. Fibre/resin composites for aircraft. Primary structures: a short history 1936–1984. Composites 1984;15:222–30. [8] de Bruyne NA. Plastic materials for aircraft construction. J Roy Aeronaut Soc 1937;41:523–90; . see also obituary of de BruyneKinloch AJ. Biograph Memoirs Roy Soc 2000;46:127–43. [9] Mayer KH, Lotmar W. Sur lÕelasticite´ de la cellulose. Helv Chim Acta 1936;19:68–72. [10] Gordon JE. On the present and potential efficiency of structural plastics RAE Report No. Chem 469; 1949. [11] Cox HL. The elasticity and strength of paper and other fibrous materials. Brit J Appl Phys 1952;3:72–9. [12] Bishop PHH. The effect of fibre orientation on the mechanical properties of glass reinforced materials. RAE Report No. Chem 493; 1953. [13] Loewenstein KL. Phys Chem Glasses 1961;2:69. 169–184. [14] Bernstein M, Kies JA. The fibreglass motor case in the Polaris Program. In: Proceedings of the filament winding conference. Azuza, CA: Society of Aerospace Materials and Process Engineers; 1961. p. 273–81. [15] Smith CS. Design of marine structures in composite materials. Amsterdam: Elsevier Applied Science; 1990. [16] Gordon JE. Plastics and plastic structures. In: Proc. 3rd AngloAmerican aeronautical conf. aeronautical research council No. 14738; 1951. p. 177–98. [17] Tipper CF. The brittle fracture story. Cambridge: Cambridge University Press; 1962. [18] HMSO (1955). Civil aircraft accident. Report of the court of enquiry into the accidents to two comet aircraft. Heywood RB. Designing against fatigue. London: Chapman & Hall; 1962. [19] Kelly A. Strong solids. Oxford University Press; 1966. [20] Machlin ES. Status report on non-metallic fibrous reinforced metal composites. Materials Research Corporation Contract. NOW 61-0209-c pp; 1961; See also McDanels DL, Jech RW, Weeton JW. Metal Progr 1960;78:118. [21] Parratt NJ. Fibre-reinforced materials technology. van Nostrand Reinhold; 1972. [22] Talley CP. J Appl Phys 1959;30:1114–5. Mechanical properties of glassy boron.
[23] Jensen JE, Gillin LM, Beckett RC, Long G. Hawker de Havilland – a case study. In: Corporate entrepreneurship and innovation conference 27–29 August. Australian Graduate School of Entrepreneurship; 2003. [24] Freise EJ, Kelly A. Twinning in graphite. Proc Roy Soc A 1961;264:269–76. [25] Baker C, Kelly A. The effect of neutron irradiation on the elastic moduli of graphite single crystals. Phil Mag 1964;9: 927–51. [26] Gillin LM. A multipurpose high-temperature tensile-testing machine using electron beam heating. In: Proceedings first conference electron and ion beam science and technology, Toronto; 1964. p. 567–82. [27] Bacon R. Growth structure and properties of graphite whiskers. J Appl Phys 1960;31:283–90. [28] Kotlensky WV, Titus Jr KH, Martens HE. Youngs modulus of hot worked pyrolytic graphite. Nature 1962;193:1066–7. [29] Shindo A. Rep. Gov. Ind. Res. Inst. Osaka No. 317, Osaka, Japan; 1961. [30] Bernal JD, Bawn CEH, Cottrell AH, Frank FC. A discussion on new materials. Proc Roy Soc London A 1964;282:1–154. [31] Watt W, Phillips LN, Johnson W. British Patent 1 110 794. [32] Standage AE, Prescott R. High elastic modulus carbon fibre. Nature 1966;197:169. [33] Kelly A. Leslie Nathan Phillips OBE 1922–1991. Plast Rub Compos Proces Appl 1992;18:197–200. [34] Phillips CE, Thurston RCA. Some mechanical properties of pykrete below 0 C National Physical Laboratory Report H/2509; 1943. [35] Aveston JA, Cooper GA, Kelly A. Single and multiple fracture. In: Conference Proceedings. National Physical Laboratory, IPC Science and Technology Press; 1971. p. 15–25. [36] Kelly A. Fibre composites; the weave of history. Interdiscipl Sci Rev 2000;25:34–44. [37] For example: Miller RJ. Design approaches for high temperature composite aeroengine components. In: Kelly A, Zweben C, editors. Comprehensive composite materials, vol. 6. Amsterdam: Elsevier; 2000. p. 181–570 [chapter 6.10]; Richerson DW. Industrial applications of ceramic Ômatrix compositesÕ. In: Kelly A, Zweben C, editors. Comprehensive composite materials, vol. 6. Amsterdam: Elsevier; 2000. p. 549–70 [chapter 6.28]. [38] Kelly T, Clyne B. Composite materials-reflections on the first half century. Phys Today 1999:37–41.