Processing and property relationships for fibre composites

Processing and property relationships for fibre composites J.W. JOHNSON This paper reviews the relationship between raw materials, manufacturing processes and properties for fibre-reinforced composites. The variability in the perceived properties of composite structures is identified as arising from three sources: firstly, from the nature of the raw materials used and their handling; secondly, from the particular manufacturing process in use and its characteristics; and thirdly, from the tests used to assess the composite's properties. Important factors such as fibre volume fraction, voidage, dimensional tolerances, resin matrix parameters and fibre surface treatment are discussed in detail. It is concluded that consistent composite properties can only be achieved by careful attention to detail at every stage in the manufacturing process. Key words: composite materials; pre-impregnated materials; production processes; fibre volume fraction; voidage; resin formulation; tow testing," shear strength; fibre surface treatments

Fibre-reinforced plastics, as seen by the moulder, consist of two main raw materials; the reinforcement in one of many textile forms, and a more or less fluid matrix resin. For thermosetting resins, some chemically triggered process is necessary to convert them to stable solids. All fabrication processes, as seen in Fig. 1, involve manipulation of the fibre m some manner. Each process, in ~ts own way, aims to position the fibre wathin the component so as to carry the load efficiently from one point to another, as determined by the designer. At the same time, it attempts to meet the additional twin goals of economy of labour (manufacture) and space (function). Some techmques require the resin to be introduced to the fibre early in the fabrication process, creating a preimpregnate (prepreg) with easy handling characteristics. In other techniques, such as resin injection, the resin is introduced only at the last moment, in the act of forming the component. In either case the final intentlon is the same; ie, to replace the air in the texture of the reinforcement by resin, and to fill completely the remaining space of the component shape. Finally, to complete the fabrication step, the chemical process is activated, most commonly by thermal methods, but not exclusively so. Many factors control the speed of this reaction, and during solidification of the resin, air (or other gases and vapours) must not be allowed to accumulate or reform at any place within the component. As may be seen from Fig. 1, a multiplicity of methods have been devised for carrying out this process and new variations are constantly being added.

THE EFFECT OF PROCESSES In considenng which factors affect the properties of the material during manufacture, it should be noted that the important properties at risk wall vary from application to application and with intended function. They will not always beexclusively mechamcal properties such as stiffness or strength; dimensional tolerance, weight, etc are also important process variables. In the case of thermosetting materials which are consolidated to some extent during fabrication, there are two basic categories of process: 1)

methods for producing components with closely defined thickness tolerances, possessing at least two opposing 'finished' faces, eg matched-die moulding, close-cavity vacuum or pressure assisted resininjection techniques, injection moulding;

2)

methods for producing components where consistent thickness requirements are not of prtrnary importance (not more than one face is 'finished'), eg autoclaving, press-claving, vacuum-bagging, Fdamentwinding or the like.

Some high pressure moulding methods also fall into the category of having only one defined face, eg elastomeric moulding and its variations. Problems which affect the properties of the composite are generally of the following type: those which are common to most processes; those which are specific to individual processes; or those which arise from the nature of the raw materials used.

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Consider a component from category 1) above, eg a small compressor blade in which there are stiffness and vibration criteria to meet and an overall dimensional tolerance. Although ttus would be moulded in a conventional and apparently straightforward manner from a prepreg in a matched-die process, it presents many problems as a moulding control exercise.

dimension is reached before gelation can take place, and the resulting loss in hydraulic pressure leads to voidage in the component. To correct this, the cure schedule may be altered to bring about gelation before hydraulic pressure is lost; in which case there is a frequent failure to achieve the f'txed stop dimensions, although there will usually be little voidage.

To meet the requirements for a specific fibre volume fraction (Vf) and weight target, it would be usual to mould this component to sorrte constant dimension by use of 'f'txed stops' in the mould (Fig. 2). Unfortunately, to achieve the state depicted in Fig. 2b, in which the component is shown to have reached its exact size simultaneously with gelation of the resin system, Is an almost impossible task. The application of pressure to collapse trapped air in the moulding leads to a rapid loss of excess resin from the mould, the stop

The laminate stack is often 'advanced' outside the mould in a pre-curing step to increase resin viscosity. However, for a laminate assembly with large thickness variations, it is virtually impossible to bring all parts to identical states and the resin may well be found to have been too far advanced for subsequent application of pressure to bring about consolidation. This results in both voidage and an oversize component occuring together, low Vf, incorrect stiffness and frequency response, and weight outside the specification.

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Furthermore, the presence of voidage is not, in general, an acceptable price to pay for achieving correct dimensions and Vf. Voidage (as illustrated by Fig. 3) has been shown to lead to a fall in interlaminar shear strength, single overlap shear strength, to cause delamination of composites under stress, and to interact with moisture adsorption. Thus to escape from this impasse, the decision was made to define an acceptable tolerance range on dimensions and Vf in moulding small rotor blades and other components in GRP for the RB162 lift jet. These criteria were met by strict control of the prepreg quality and component cure schedule; in fact moulding without using stops. 1 The acceptance specificatmn box for a particular component is shown in Fig. 4. To land within this box, certain critical values need to be maintained in the prepreg specification, and the target area to 'make' a component is shown in Fig. 5.

COMPOSITES. A P R I L 1983

Many of the observations concerning the origin of voidage in components manufactured by matched-die mouldings, apply directly to autoclaving, press-claving and similar methods; indeed some studies indicate these methods to be even more complicated. Childs 2, for example, believes that the l,'f achieved in autoclaved structures is determined by the inherent fibre packing developed during manufacture of the prepreg, and is not a parameter subject to significant variation during moulding.

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Fig. 5 Basic specification for Kerimid 601 glass fibre prepreg. Note that moulding to 0.15 mm per ply produces Vf variations from 58-63% and requires precure alterations

Fig. 4 Variat=ons in Vf and rotor blade root flat thickness for RB162-86 rotor blades. Glass Vf determined by burn out. Dimensional variations experimentally shown to produce only small changes m blade vibration frequency

The penalties incurred for accepting this procedure are complex, tight, laboratory supervision of production as a routine practice, varying precure according to the position of the material in its specification box, residual solvent, and time out. In this case, the matrix was a polyimide resin (Kerimid 601, Rhone Poulenc) which, in its uncured state, is a solid with a relatively high melting point. To create a viable prepreg system, a solvent had to be used which reacted with the matrix in a complex manner, affecting storage life, handling characteristics, flow during moulding and the retention of high temperature properties. Fig. 6 shows the effect of solvent levels on the high temperature properties of the laminate. It demonstrates that the control of residual solvent levels is crucial, not only for successful processing, but also for subsequent performance.

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temperature and to increased rates of cure after a period of latency. The family of thermoplastic-modified resin systems developed by Ciba-Geigy (as typified by BSL914) are good examples of this type. For other resin systems however, small changes in chemistry can lead not only to noticeable processing changes, but also to changes m mechanical properties.

For example, a thermoplastic component is present in the BSL914 system to control flow during gelation, thereby limiting void formation during moulding. The latent hardener needed for rapid cure and stable shelf-life is provided by a dispersion of crystalline dicyandiamide. The thermoplastic viscosity-modifier is thrown out of solution during the cross-linking phase of the cure of the epoxide fraction and appears as a fine second phase network. The morphology of this second phase was a useful pointer to formulation problems m the early development stages of the system. Fig. 9 shows how the local chemical concentration gradient around an oversized hardener particle affects the morphology, which may in turn influence the apparent cure rate. Fig. 10 shows that these particles could be detected in composite microsections, where they were also responsible for blistering problems during post cure• This behawour was caused by the presence of large individual particles in the hardener, and was quickly corrected by the manufacturer once their importance was realized. Fibres

Apart from obvious cases of malpractice, or instances where materials are out of specification, itis clear that to achieve the expected composite properties, the fibre should be present in the desired orientation, m the correct volume,

The system MY750 (DGEBA, Ciba-Geigy) and DDS hardener illustrates the type of behavioural changes which can occur. Fig. 7 shows the effect of hardener content variation on the mechanical properties of the unrelnforced resin. In general, composites having matrices with low moduh will show lower values m many of the standard flexural mechanical property tests• This is usually due to the failure of the matrtx to sustain the compressive loads being generated in it. At first sight therefore, it might appear to be an advantage to decrease the hardener proportions, leading to high resin modulus and improved compression behavaour. The variation of glass transntion temperature (T o with hardener ratio, however, shows that the benefit would be rapidly lost as the temperature of application rose. In addition, the hydrolyc stability of the resin system would be affected and this would also adversely influence the resin modulus at elevated temperature. Fig. 7 also illustrates the importance of complete mixing of the reactants and of achieving resin homogeneity on a microscale• Calvert a et al have recently shown, by ultra-violet microscopy, that large areas of rmcro-inhomogeneity exist in amine cured epoxy resins. Fig. 8 illustrates the effect for Epon 828/triethyltetramine; the effect was not eliminated by warming at 50°C and stirring with a laboratory stirrer• Only blending in a high speed shear mixer or dissolving the reactants in a common solvent produced a uniform structure. Even if these non-uniformities did not directly affect the primary mechanical properties, it would be reasonable to expect some detenoration in the longer term environmental performance. This highlights the fact that the composite materials scientist is somewhat at a disadvantage compared with his metallurgical colleague, since the thermo-mechanical history of an alloy can usually be deterrruned from an examination of its crystalline fabric and fine structure• In the main, no such convenient clues are available In dealing with crosslinked glassy resin systems and thus It ~s difficult to ascnbe varmtions in material properties to particular practices in formulation and fabrication. Where such structural clues do

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COMPOSITES.

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matched-die moulding will punish any such deficiencies severely. Fig. 11 shows an early example of a wedge root in a carbon fibre blade having gone wrong due to incorrect fibre content, mainly as a result of errors in laminate placement, coupled with a resin system of low flow. Clearly this structure has virtually no strength. Less obvious cases may still arise and will result in an insidious falling short of composite properties from the expected value. Fibre surface treatment On a d i f f e r e n t n o t e , a serious and controversial c o n t e m p o r a r y problem is the level of surface treatment needed to

Fig. 9 Particle of dicyanduamide in BSL914 film, showing change in resun morphology close to particle

achieve certain composite properties when using carbon fibre. Although fibre surface treatment is a parameter under the control, and at the discretion, of the fibre manufacturer, whose detailed methods are commercially secret, it can also be regarded as a potential manufacturing variable. The method currently in use for specifying fibre strength embodies an impregnated tow test and some resin formulations are more sensitive to the fibre surface chemistry than others. Thus it is possible for apparent fibre strength to become a function of the resin used in the tow test, and by inference, a change m the type and character of the resin system, (as may be necessary to carry out the manufacturing process, Fig. I) carries with it the possibility of an effective change in the strength of the fibre, even though the latter remains unchanged in an absolute sense.

Fig. 10 Unreacted d0cyandiamide on carbon fibre laminate. (a) m dark ground shows the presence of the crystal; and (b) in bright field reflecuon shows that the particle is entirely subsurface

and with the appropriate degree of surface treatment to effect the correct lnterfacial bond. It is a tribute to the recent improvements m prepreg quality that a serious problem with fibre volume fraction Is less likely to occur today as a direct primary fault; such problems were common in the early days, and may still occur in techniques which seek to produce a high Vf composite by a non-prepreg method. Prepreg methods may still give rise to problems; eg in the manufacture of cross-plied components of complex shape and high Vf, where the ability to place laminates correctly is of paramount importance, and the use of a high pressure method such as

C O M P O S I T E S . APRI L 1983

Fug. 11 Carbon fibre composite blade root; badly distorted laminae result from basnc bulk factor problems, errors in laminate placement and hugh viscosity resin matrix

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Table 1.

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The common DGEBA resin systems, used by some manufacturers for tow testing, are insensitive to the state of the fibre surface and are also little used in current aerospace apphcatlons. Other types of resin systems more commonly used in aerospace are extremely sensitive to the level of surface treatment of the fibre. This sensitivity shows itself m the composite properties in two ways. Firstly, as differences between the expected and observed values of fibre strength, and secondly, as problems in evaluation of the level of fibre surface treatment by the commonly-used technique of measuring the interlaminar shear strength of the composite. Fibre strength is normally estimated by impregnated-tow testing, in preference to more tedious single fibre tests. The field is full of pitfalls and any future change in fibre manufacturing conditions could bring about serious difficulties. Table 1 shows the results of tow tests on an experimental carbon fibre from Toray Industries. The fibre was tested initially following the standard resin cure (30 min at 130°C) and then after a further higher temperature postcure (1 h at 165°C). It is clear that extending the somewhat inadequate initial cure cycle leads to a fall in the tensile strength of the tow. The chemical actiwty of the fibre surface can be estimated by measuring its tendency to adsorb water in a microbalance apparatus. Fibres with high and low activity can be correlated to a large extent wath strength values obtained from tow tests. Fig. 12 shows the water adsorption isotherms for a standard commercial fibre and for an experimental high-stram fibre modified in the Rolls-Royce Laboratories (high adsorption levels represent highly active surfaces). The expected and observed strengths obtained from tow tests on these fibres (resin system CY179/BF3 MEA, Ciba-Geigy), are shown in Table 2. Furthermore, the expected strength levels (based on single fibre tests) were still not reached after an intermediate high temperature treatment in nitrogen, aimed at de-sensitizing the fibre surface. Thus certain problems remain to be solved before fibre strength can be effectively assessed by composite methods. Fibre surface treatment level also affects another parameter by which the general integrity of the composite is judged, ie the interlaminar shear strength (ILSS). ILSS is obtained from a 3-point bend test of a composite beam of short aspect ratio, and its measurement creates difficulties in all the divisions of composite activity; problems arise from the material, the processes, and when carrying out the tests. ILSS assesses the level of fibre surface treatment indirectly by measuring the fibre/resin bond strength. The property is conventionally measured on unidirectional laminates, although applications for purely unidirectmnal laminates are not common. The cross-ply mode is much more repre-

112

*Toray experimental carbon fibre (1.8% BS);+Resin system CY179/BF3MEA, cured for 30 mm at 130°C

sentative, and a cross-phed ILSSspecimen yields more useful information. However, even the unidirectional test itself is often misapplied, and it is an illuminating exercise to analyse m detail the failure mode of samples tested in the standard apparatus. It is not unusual to find that less than 50% of the samples have failed in the approved manner. Correcting this state of affairs improves the variability of the test results, with a smaller effect on the observed mean value. Varlablhty in the ILSSresults is a consequence of those same processing problems which lead to voided samples, as shown in Fig. 3 and the use of resin formulations which produce low matrix moduli can be expected to result in compressive failure problems. The observed ILSS value is only occasionally a measure of the bond strength between fibres and resin. It is a true

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COMPOSITES. APRI L 1983

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there is any important interaction between crystallizatmn and chemically active sites on the fibre surface• The role of processing in determining the properties of chopped fibre injection-moulded materials has not been dealt with, since this is an area of spemahst activaty which requires its own separate treatment. The effects of processing on composite properties is not a subject which is easily or conveniently categorized, it permeates the whole field of composite actwlties, and

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measure most often when the bond strength is poor, as in glass fibre composites. For void-free carbon fibre composites, particularly when surface treatments have been applied, the parameter is more a reflection of the tensile strength of the matrtx in use. This is illustrated m Fig. 13, which shows the ILSS values for carbon fibre composltes as a function of surface treatment, the latter being determined by water adsorption and nitrogen surface area measurements in a microbalance. The values can be seen to approach the tensile strength of the resin system at moderate surface treatment levels. The inference that LESSis related to the tensile behaviour of the resin matrix can be drawn from a study of cross-phed laminate composites. Here the tension cracks form within the 90 ° laminates at 45 ° to the direction of the applied load, m response to the resolved tensile component generated by the applied shear stress, Fig. 14. Attempts to generalize this idea, to account for failure of unidirectional samples in short beam shear, have so far been unsuccessful. This is due to the rather interesting fact that no genuinely unidirectional testpieces have so far been observed in both commercially prepared and laboratory samples of prepreg. There has always been clear evidence of stray non-parallel fibres somewhere in the failure surface, which confuse the interpretation of the failure sequence (Fig. 15). These isolated, rmsplaced fibres could be said to represent one kind of ultimate processing/manufacturing defect, and the extent to which they can be eliminated by attention to detail in such manufactunng methods as filament winding, or preimpregnate techniques, may determine the ultimate superiority of one method over another in producing high strength laminates.

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of: (a) C F RP ; and (b) G RP, showing tension gashes m the 90 ° crossply laminates as a result of I LS failure. Cracks form at 90 ° to the resolved tensile component generated by the apphed shear stress, Laminate thickness ~ 2 mm

CONCL USIONS The topics discussed so far have been limited; for example, there has been no-discussion of the role of fibre finishing agents or sizes in deterrmning properties. There are severe problems ahead here m dealing with polyimide and thermoplastic matrices;in most cases the processing temperatures of the matrix are high, degrading the commercial fibre f'mishes on both glass and carbon fibres, resulting in poor composite properties. Also missing is a discussion on thermoplastic matrix-composites, shown in Fig. 1 as an area of potentially rapid growth. In this field there is the added dimension of matrix crystallisation to contend with and work will be necessary to determine whether

COMPOSITES coMr 1~-2 - c

. APRI L 1983

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113

starts much further back in the manufactunng cycle than perhaps many people would be prepared to accept. Also the properties of the component are capable of a much wider interpretation than one based simply on an assessment of its elastic behavmur. There is no simple answer except careful attention to detail at every step of the way from first consideration of the fibre/matrix combination, through processing, to testing.

REFERENCES

1

Johnson, F.C. Symp on Fabrication Techniques for Advanced Reinforced Plasttcs, 1980, Umversity of Salford, England

2 3

(IPC Science and Technology Press) Childs,R. ibid Ghaemy, Calvert, P.D. and Biilingham, N.C. 'Uneven curing in epoxy resins' Polym Letters 20 (1982) pp 439--443

A CKNOWL EDGEMEN TS

The author wishes to thank colleagues in the Plastics & Composite Materials Laboratory, Rolls-Royce Limited, for permission to discuss various aspects of their work. Fig. 8 is reproduced from Reference 3 by kind permission of the authors and John Wiley and Sons Inc. © Rolls-Royce Limited, 1983.

114

AUTHOR

Dr Johnson xs with Rolls-Royce Limited, Non-Metallics Laboratories, Plastics and Composite Department, Alfreton Road, Derby DE2 8B J, England.

COMPOSITES. APRI L 1983