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Literature survey
Iiterature survey I
resutvev
Prepared by M . E . Adams, R.S. Bailey, L. Boniface and d . W . Rock
CARBON FIBRES
absorbed by the composite at static loading rates can be one or two orders of magnitude higher than that of the unreinforced matrix.
CEMENT MA TRICES
Repair and protection of reinforced concrete in high-rise buildings (Part 1) Ovens, A. Concrete Vol 18 No 5 (May 1984) p 21 The article discusses the current approach in repairing reinforced concrete in situ, using state of the art information. This first part of the article is concerned with the structural repairs of those buildings in which an inadequate quality or thickness of concrete cover over the steel reinforcement has been used. This results in loss of passivating alkalinity, active corrosion of the steel and eventual spalling of the concrete. The relationship between balancing the economics and the individual aspects of survey, diagnosis, repair and protection and the use and life expectancy of the structure is realized. The second part of this report is concerned with the coating recommended.
Durability of cement sheets reinforced with fibrillated polypropylene networks Hannant, D.J.Magazine of Concrete Research Vol 35 No 125 (December 1983) pp 197-204 The durability of cement sheets reinforced with fibrillated polyproylene networks is examined in terms of the effects upon the tensile stress/ strain curve of three years of natural weathering or of a similar period of indoor storage. It is shown that three years of exposure has little effect upon the strengths of the networks, but that increases in the strength and elastic modulus of the matrix in the naturally weathered specimens alter some features of the stress/strain curve in comparison with similar material stored indoors. (Author's abstract)
Steel fibre reinforced concrete beams in torsion Narayanan, R. and Kareem-Palanjian International Journal of Cement Composites and Lightweight Concrete Vol 5 No 4 (November 1983) pp 235-246 The results of 28 torsion tests on steel fibrereinforced beams are presented and discussed. The parameters varied were the type, aspect ratio and the volume fraction of the fibres used, as well as the cross-sectional dimensions of the beam. Methods of predicting the cracking torque and the ultimate torque are proposed; the predictions obtained by the suggested methods are shown to be satisfactory by comparing them with the torques observed in the tests carried out by the authors and previous investigators. (Authors' abstract)
XPS studies by use of digital difference spectrum technique of functional groups on the surface of carbon fibres Takahagi, T. and Ishitani, A. Carbon Vo122No 1 (1984) p 43 A technique is described which has proved effective for the analysis of the functional groups introduced by surface oxidation treatment of carbon fibres. The major functional groups introduced to the carbon fibre surface by surface oxidation are hydroxyl and carboxyl groups. Disordering of the crystal lattice is also reported, together with the generation of carboxyl groups in the case of surface oxidation of graphite fibres.
Flax fibres as a reinforcement in cement mortars Coutts, R.S.P. International Journal of Cement Composites and Lightweight Concrete Vol 5 No 4 (November 1983) pp 257-262 The flexural strength and fracture properties of a cement mortar reinforced with New Zealand flax fibres were studied. The effect of refining the fibres before incorporation into the matrix was also investigated. It was found that at a fibre loading of approximately 8-10% by mass flexural strengths in excess of 20 MPa could be obtained, which are comparable to Pinus radiata fibre-reinforced mortars. However, the fracture toughness values were approximately half that of the P radiata composite. (Author's abstract) Impact properties of steel fibre reinforced concrete in bending Naaman. A.E. and Gopalaratnam International Journal of Cement Composites and Lightweight Concrete Vol 5 No 4 (November 1983) pp 225-233 The effects of strain rate of loading including impact, on the bending properties of steel fibrereinforced mortar were studied using an instrumented drop-weight impact machine. The results of the tests indicate that, depending upon the fibre reinforcement, the energy
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CERAMIC MA TRICES Advanced ceramics and composites. Shortfalls surface at CMS Workshop Fisher, G. American Ceramics Society Bulletin Vo163 No 2 (February 1984) p 247 A report is made of the 8th A n n u a l Conference on Advanced Ceramics and Composites. The applicability of ceramic matrix composites, and ceramics is discussed. The proceedings include; a recommendation for a rational programme on ceramics for gas turbines, the reliability and characterization of these materials, modelling, defects and their effect on fracture toughness and fabrication. It was concluded that ceramic matrix composites could fill a gap in gas turbine and high temperature applications, but that there is a place for both these and monolithic ceramic materials for which reliability and progress in development has been proved. Composites, engineering the ultimate material Fisher, G. American Ceramics Society Bulletin Vo163 No 3 (March 1984) p 360 A brief review of currently used composites is made. The limitations of organic, glass and metal matrix composites are discussed and it is
pointed out that higher temperature applications indicate ceramic matrix composites to be of potential. The state of the art of these materials is discussed, and their reliability assessed. It is pointed out that ceramic matrix composites is a growth area in terms of materials development and testing. It remains to be seen whether these toughened materials or the high strength monolithic ceramics will prevail in the advanced gas turbine applications. Optical properties and solar selectivity of coevaporated Co-AI203 composite films Niklasson, G.A, and Granqvist, C.G. Journal of Applied Physics Vo155 No 9 (May 1984) p 3382 These deposits were analysed using a variety of techniques. A uniform separated-grain structure with regular HCP Co particles embedded in A1203 has been developed using Co contents up to 30% by volume. The complex dielectric permeability was evaluated for a range of wavelengths and numerical accuracy and internal consistancy investigated. At large Co contents discrepancies were observed to the derived classical scattering theory and evidence for anisotropy was obtained. Computer optimization was carried out for the system and experimental evidence reinforced the results of varying composition, and improving the solar selectivity.
GENERAL An advanced theory of the strength of hybrid composites Hiroshi Fukuda Journal of Materials Science Vol 19 (1984) pp 974-982 This paper presents a statistical approach to the strength of a unidirectional hybrid sheet considering a modification of Zweben's theory. The concept of the hybrid effect is first explained with reference to two hybrid systems in terms of enhancement of initial failure strain, ie failure strain of the low elongation fibre. The systems are graphite/E-glass (Bunsell, Harris) and Kevlar/graphite (Zweben). This study uses a 'di-plet' criterion to evaluate failure of the hybrid composite. Zweben's theory is modified by calculating more precise stress concentration factors using a shear lag analysis (appendix). Experimental data for stress/strain behaviour is compared with theoretical predictions of the hybrid effect. Analysis of the effect of embedded fibre length on fibre debonding and pull-out from an elastic matrix (Part 1 review of theories) Gray, R.J. Journal of Materials Science Vol 19 (1984) pp 861-870 Theories to explain the processes involved in pull-out tests on single fibres are reviewed. This gives an estimation of the resistance to fibre/ matrix interfacial debonding in composites with ductile fibres in a brittle or elastic matrix This is a function of the interfacial bond shear strength and interfacial bond area. Good experimental agreement is obtained for ductile/ plastically deforming matrices with uniform interfacial shear strength, where debonding
COMPOSITES
. OCTOBER 1984
load is proportional to embedded fibre length. .For a brittle/elastic matrix, a non-uniform shear stress distribution along the interface gives a non-linear relation between pull-out load and embedded fibre length. Major theories take account of the frictional resistance to slipping over the debonded part of the interface, as well as the effect of embedded fibre length, and give numerical estimates of interfacial shear strength and frictional resistance to slipping. Analysis of the effect of embedded fibre length on fibre debonding and pull-out from an elastic matrix (Part 2 application to a steel fibrecementitious matrix composite system) Gray, R.J. Journal of Materials Science Vol 19 (1984) pp 1680-1691 This paper tests the applicability of the theories discussed in part 1 to explaining interfacial debonding and pull-out in a brass-coated steel wire/cementitious mortar composite. For an embedded fibre length of 100 mm, failure occurred by fibre debonding and pull-out whereas for length of 125 mm, failure was by fibre fracture rather than pull-out. The total resistance to interfacial debonding comprises adhesional bonding and frictional resistance to slipping after debonding (for a sufficiently long embedded fibre length). Those theories which ignore the frictional term cannot explain failure by fibre fracture instead of pull-out but give reasonable values of m a x i m u m shear strength of the fibre/matrix interfacial bond. Design of laminated plates for maximum stiffness Taucherl, T.R. and Adibhatla, S. Journal of Composite Materials Vol 18 (1984) p 58 The design of symmetrical fibre-reinforced laminated plates for m a x i m u m stiffness is discussed. An analysis, based on the criterion of m i n i m u m strain energy, is developed. Deformation of the laminate plate is obtained using the Rayleigh-Ritz method. For optimum laminate design, the strain energy must be minimized and this was achieved using a quasiNewton procedure. Examples of results obtained for a graphite/epoxy (T300/$208) composite system are presented. Experimental and analytical studies on the onset of delamination in laminated composites Kim, 1LY. and Soni, S.R, Journal of Composite Materials Vol 18 (1984) p 70 A failure criterion based on the average stress (~r3) and the interlaminar tensile strength, which was assumed to equal the transverse strength of the composite, was developed and used to predict the onset of delamination. Comparison with experimentally obtained results, using acoustic emission to detect delamination, shows good agreement. However, the authors point out that there is an indication that transverse cracking affects the prediction, and that there is a critical transverse crack length above which the predictions are significantly different from experimental results. Progress in reinforced plastics Wehrenberg I1, R,H. Materials Engineering(April 1984) pp 46-53 Innovations in compounds, fibres and processes discussed at the 39th Annual SP1 RP/C Conference are reviewed. A thermoplastic composite based on PPS with long carbon or glass fibres allows optimum automated production cycles, and gives good impact strength. Smooth surfaced polyester-based SMCs show increased toughness with good paint and adhesion bonding for auto applications. Developments in resins are reported; polyester/
C O M P O S I T E S . OCTOBER 1 9 8 4
urethane hybrids have increased strength, stiffness and impact resistance over unsaturated polyesters. Pultrusion of composites is expected to have good growth potential in competition with AI extrusions. Advances in automatic processing of large components and computercontrolled filament winding are reviewed. Photocuring prepregs with reinforced polyester containing photoinitiators which are rapidly cured by visible light aid fast production. New reinforcements include high strength nylon and PET fibres. Calcium sulphate whiskers can replace glass in thermoplastic and thermosets. PVDC microspheres have been developed for use with syntactic foams. Pullout tests with metallic glass ribbons in different matrices Tio, T.K,, Friedrich, K,, Hornbogen, E., K~Sster, U. and Fels, A. Journal of Materials Science Letters Vol 3 No 4 (May 1984) p 415 The potential for using metglass ribbons as reinforcement in composites is studied using pull-out tests. A range of matrices and surface treatments of the ribbons are used and it was found that a thermosetting resin matrix provided the best tensile properties. Pull-out testing showed that a critical length, as used in conventional fibre composites, is applicable in these materials. The geometry and materials effects are investigated and their influence on this critical length is studied. The same optimizations for high fracture energies apply to these materials as is applicable to discontinuous fibre composites. A review of recent progress in the studies of molecular and microstructure of coupling agents and their functions in composites, coatings and adhesive joints Ishida, H. Polymer Composites Vol 5 No 2 (April 1984) pp 101-123 An in-depth study under the above title is presented and an improved understanding of the structure of silane coupling agents is claimed. In particular, the structure of aminosilane, the organization and orientation of silane molecules, structural gradients in the interfacial regions, chemical reactions at the interfaces and within the silane layers and the structural changes in the silane layers are discussed. Theoretical model on the fracture behaviour and tensile strength of fibres coated with two brittle surface layers Ochiai, S., Osamura, K, and Murakami, Y. Zeitschrift ff~r Metallkunde Vol 75 (1984) p 231 Composite degradation due to the formation of a reaction layer at the fibre surfaces is particularly important in some boron fibre and silicon carbide fibre-reinforced composites. In this paper the authors develop a general model to predict the fracture behaviour of fibres covered by two brittle layers. The model they develop is based on Weibull statistics and contains seven unifying parameters. This gives rise to 5040 failure modes. From the present model, it is possible to calculate the strength of the fibres in terms of the thicknesses of the two layers.
METAL MATRICES Analytical stress solutions for fibre-reinforced metallic rocket motor cases Stanley, P., Margetson, J. and Groves, A. International Journal of Mechanical Sciences Vol 25 No 2 (1984) pp 119-130 An analytical solution is presented for a
metallic rocket motor case reinforced circumferentially with a prestrained fibre overwind. Considerations include modification of the stress state by different storage temperatures, case buckling during manufacture and yield behaviour of the case for various winding strains and fibre configurations. The system comprises a lightweight alloy case with high strength fibre overwind which induces a compressive stress state in the case, increasing its bursting pressure and specific strength. Tresca and Von Mises yield criteria are used to evaluate the internal pressure which initiates yield. Fatigue failure of an orthotropie plate with a circular hole Tsangarakis, N. Journal of Composite Materials Vo118 (1984) p 47 The effect of pre-drilled circular holes on the fatigue failure of FP/AI-55 composite has been investigated. It has already been shown that the mechanical properties of this composite are dependent on the degree of matrix/fibre bonding, and in this study only a single batch of material was used. Specimens with holes of 1.6 m m and 3.2 m m had endurance stresses of 225 and 167.5 MPa, respectively, compared with 330 MPa for the unnotched material. Thermographic examination of the specimens during fatigue enabled the detection of areas of poorly bonded fibres near a pre-drilled hole; it also confirmed that there is heat generation during fatigue. Comparison between the experimentally measured endurance stresses and those predicted using the average stress model (ASM) showed very good agreement. Fracture mode and tensile strength of fibres in borsic-titanium alloy composites Ochiai, S., Osamura, 1C and Murakami, Y. Zeitschrift ffir Metallkunde Vol 75 (1984) p 238 A study of the degradation mechanism in borsic/titanium alloy matrix composites has been carried out. At elevated temperatures, reaction products form at the fibre/matrix interlace, and premature failure of this layer is responsible for a degradation in strength of the composite. Various fibre surface treatments have been used to try to eliminate the reaction; however, a layer of different composition is often produccd. For example, the boron fibre may have a silicon carbide sheath. In this paper the authors apply a general theoretical model they developed for a fibre coated with two brittle layers to borsic/Ti40A and borsic/Ti-3AI-2.5V composites. They found that the failure of the Ti40A composite was the result of fracture of a titanium-silicide reaction layer which caused simultaneous fracture of the silicon carbide layer and the boron fibre. In the Ti-3AI-2.5V composite, notching of the fibre, by failure of the TiB 2 reaction layer, was responsible for failure. Mierostruetural instability of Ni-Mo-AI unidirectionally solidified eutectics Schwam, D., Dirnfeld, S.F. and Nadiv, S. Journal of MateriaLs Science Letters' Vol 3 No 4 (April 1984) p 363 Characterization of the microstructural changes observed in in situ composite superalloys is described, The morphology of the phases is investigated using optical and electron microscopy. The effect of temperature gradients is to promote morphological transformations. The fibres reject aluminium as they coarsen, which precipitates as a "/' phase in the vicinity of the fibres. The square-section fibres become circular in shape as they become unstable. The orientation relationships for the fibres and their array is obtained.
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Powder metallurgically produced metal-glass composites Kainer, K.U., Bergmann, H.W. and Mordike, B.L. Powder Metallurgy Vol 27 No 1 (1984) pp 30-37 This paper investigates the influence of starting materials and post-fabrication deformation processes on the properties of a metal/glass system. The composite is fabricated by mixing and compacting A1 and glass powders, then annealing. It is subsequently extruded or forged. Optical and electron microscopy are used to investigate the microstructure. It is found that strength and toughness increase with increasing glass content and particle size. During fracture, the brittle phase fractures first and then the ductile phase is drawn out. Wear properties are improved, and the creep rate of the composite is decreased compared with pure A1. Most of the improvements in physical and elastic properties can be explained by the rule of mixtures, and are attributable to dispersionhardening effects, although hardness, strength and wear exhibit changes in excess of values predicted by the rule of mixtures. These Al/glass composites have good joining and finishing capabilities; surface anodizing is also possible. Simple and precise measurements of fibre volume and void fractions in metal matrix composite materials Schoutens, J.E. Journal of Materials Science Vo119 (1984) p 957 This paper discusses methods for measuring fibre volume fraction (V~) and void fraction (Ve) in metal matrix composites, and the degree of precision expected. The simple non-destructive method to determine Vf involves weighing in air and in a liquid, while measuring Vg requires etching away some of the matrix material. The analysis gives equations relating Vf and V~ with numerical results for B/A1 and glass/Al systems. If voids are present, two sequential experiments on the same specimen are required to determine Vf and V~ This involves finding the composite density, then etching away some matrix and finding the new density.
observing the fracture surface morphology. At temperatures below IO0°C, fracture involves breaking lateral bonds between polymer molecules; creep processes dominate at higher temperatures. Failure is initiated at surface irregularities such as kink bands. Microscopy reveals that fracture occurs by cutting through fibrils at defective regions by a step process, after initiating at surface kink bands. The large difference between the measured and theoretical strength, which is based on an infinitely long, aligned extended chain, can be explained in terms of these bands; thin filaments are desirable since they will have fewer surface bands so that the crystalline structure is continuous, and strength is determined by covalent bond strength of the molecule rather than lateral bonding between molecules which limits practical fibre strength.
Loading criteria for the fatigue failure of polyamide 66 fibres Oudet, C.H. and Bunsell, A.R. Journal of Materials Science Letters Vol 3 No 4 (April 1984) p 295 Nylon 66 fibres were observed to exhibit two regimes of failure type. At greater than 52% load cycling, there were predominantly creep-type failures. Below this level, a fatigue morphology was observed. A comparison is made between a range of polyamide 66 fibres. Examination of the fractured fibre ends shows that the end is always curved inwards, indicating that the surface is in compression as the fibres are unloaded. Residual stresses in the fibres would be expected to modify the m i n i m u m load criterion for crack initiation. It is suggested that this may also be influential' in the apparent acceleration of creep failures under cyclic loading.
RESIN MATRICES
ORGANIC FIBRES
The design of ends of cylindrical pressure vessels in glass reinforced plastic Barton, D.C., Amos, A.R., Soden, P.D. and Gill, S.S. International Journal of Mechanical Sciences Vo126 No 3 (1984) pp 177-199 This paper presents an analysis for the design of torispherical and semi-ellipsoidal ends of cylindrical GRP chopped strand mat pressure vessels, in accordance with BS4994 to calculate unit loading of laminates in such vessels. Here, the solution is of an a x i s y m m e t i c thin shell problem for a linear elastic material taking account of the effect of change of shape of the ends due to pressurization. All reinforcement layers must be of the same type. The design for G R P ends is based on limiting the m a x i m u m tensile strain to a nominated value, although a m a x i m u m stress criterion is also examined. Stresses in a laminate having layers with different properties vary greatly across the thickness depending on arrangement; therefore, the m a x i m u m bending stress is not necessarily at the laminate surface. A design based on limiting the m a x i m u m strain may be more valid since the m a x i m u m strain is always on one of the surfaces.
The fracture process of ultra-high strength polyethylene fibres Smook, J.. Hamersma, W. and Pennings, A.J. Journal of Materials Science Vol 19 (1984) pp 1359-1373 This paper examines the fracture behaviour of PE fibres by measuring the lifetime of fibres in dead-load tests at various temperatures, then
The detailed structure of delamination fracture surfaces in graphite/epoxy laminates Johannesson, T., Sjrblom, P. and Selden, R. Journal of Materials Science Vol 19 (1984) pp 1171-1177 This paper investigates delamination fracture surfaces in angle-ply graphite/epoxy tension specimens using SEM to look at microscopic structural features to understand delamination
Yield and deformation of metallic glasses strengthened by post-extrusion addition of second phase particles Zielinski, P.G. and Ast, D.G. Acta Metallurgica Vo132 No 3 (1984) pp 397-405 This paper studies the stress/strain behaviour of metallic glasses with second phase particles by SEM and bend tests on hairpin configuration specimens. The materials are melt spun Ni75SisBI7 in ribbon form with homogeneously dispersed TiC and WC particles. The effect of unshearable second-phase particles blocking shear bands on the yield stress is analysed using the one-parameter theory of work hardening (Ashby). The presence of the particles increases yield stress and modulus in agreement with the rule of mixtures. The value derived for the friction stress agrees with analysis of bending.
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mechanisms. Fractures showed some fibre pullout with resin residue indicating a high interfacial bond strength. The general delamination surface shows uncovered fibres, fibre imprints and matrix serrations, which are plate features, lying perpendicular to the surface. It is a twoplanar surface where the crack path alternates from one side of the interlaminar region to the other. The delamination process involves fibre debonding due to stress concentration at the interface, followed by formation of slanted resin cracks normal to the major principal stress; these features are linked by microscopic crack propagation into the resin, forming the two-planar delamination surface.
Effect of particulate reinforcement on creep behaviour of polyurethane foams Alperstein, D., Narkis, M., Kenig, S. and Siegmann, A. Polymer Composites Vol 5 No 2 (April 1984) pp 155-158 The compressive creep behaviour of rigid unreinforced and particulate-reinforced polyurethane foams at different densities, filler contents, temperatures and loads was investigated. It was found that the addition of high modulus particulate fillers significantly enhanced the creep resistance of rigid polyurethane foams, especially at high reinforcement contents and high foam densities. Effect of temperature, moisture and radiation exposures on composite mechanical properties Mazzio, V.F. and Huber, G. SAMPE Journal Vo120 No 2 (March/April 1984) pp 14-23 The results of a programme carried out to generate data to support the design and application of advanced composites for space systems structures are presented. In the programme, four different materials (three types of carbon fibre and Kevlar 49 in epoxy resin matrices) were exposed to three levels of electron beam radiation, moisture saturation at 90% RH and 71.5°C and a combined 'moisture and space environment exposure'. After exposure, specimens were assessed for tensile, compression, in-plane shear and short-beam shear properties. Only results of tests carried out in the fibre axial direction are detailed. Fatigue and reliability evaluation of unhatched carbon epoxy laminates Radhakrishnan, K. Journal of Composite Materials Vo118 (1984) p 21 Most of the models that predict the fatigue life from a proof loading require a considerable amount of testing before the model can be used. In this paper the author examines the model developed by Yang, which requires only a m i n i m u m of setting up. The model predicted the m i n i m u m number of fatigue cycles that a carbon/epoxy laminate would survive for various loadings. However, at probabilities of above 0.3, it overestimated the fatigue life and underestimated the residual strength. The fracture of particulate-filled epoxide resins Moloney, A.C., Kausch, H.H. and Stieger, H.R. Journal of Materials Science Vol 19 No 4 (1984) p 125 In this paper, the effect of glass beads on the fracture behaviour of epoxides as a model system for rough mineral particles in plastic is investigated. Resins are filled with varying fractions of glass beads (diameters 50-100 btm and 0-50 pm), both treated with coupling agent and untreated. Hollow silica spheres are also included. Double-torsion tests measure KIc; flexural strength is measured in 3-point bending and yield stress in plane-strain compression. Fracture surfaces are examined in the SEM. The authors consider toughening
COMPOSITES. OCTOBER 1984
mechanisms due to second-phase particles by the bowing of a crack front between particles and crack blunting by debonding of the particles from the matrix. For poorly bonded glass beads, crack growth is unstable due to blunting. For a well-bonded composite, crack propagation is stable. It appears that a m i n i m u m filler/resin adhesion is required for the crack bowing mechanism. Caution is required when extrapolating results for spherical fillers to more irregular mineral particles. Fracture toughness of Kevlar-epoxy composites with controlled interfaciul bonding Mai, Y.W. and Castino, F. Journal of Materials Science Vol 19 (1984) pp 1638-1655 This investigation into toughness uses intermittent bonding; the fibres are partly coated with a viscous fluid, deliberately introducing regions of weak interfacial bonding which absorb energy by blunting cracks, increasing pull-out length and hence improving toughness. The coating materials were a silicone vacuum fluid and a polyurethane varnish. Fracture toughness was measured in the range -60 -40°C. Intermittent bonding increases toughness without a significant drop in tensile strength, although the varnish gave the best results. Hygrothermal ageing degraded the varnish-coated material, with little effect on the other type of material. Analysis of the fracture mechanisms indicates that fibre fracture work and fibre pull-out toughness contribute to total fracture toughness in these composites. There is reasonable agreement between predicted fracture toughness values and experiment. Free dieyandiamide in erosslinked epoxy resins Kasturiarachchi, K.A. and Pritchard, G.Journal of Materials Science Letters Vol 3 No 4 (April 1984) p 283 A study into the effect of residual curing agents in cross-linked epoxy resin/E-glass fibre composites is made. The curing agent dicyandiamide dissolves in water at temperatures between 40 and 80°C. This is shown in humidity/temperature tests followed by various analysis techniques. Three observations have been made: (1) regions of porosity develop in resin-rich regions; (2) a m m o n i a is developed as a decomposition product between water and the curing agent; and (3) pockets of dissolved solutes may be pressurized, these cavities can develop and initiate cracks in the resin. It is recommended that the gelation process in the production route be made at higher temperatures where practicable. Jute reinforced polyester structures Semsarzadeh, M.A., Lotfali, A.R. and Mirzadeh, H. Polymer Composites Vol 5 No 2 (April 1984) p 141-142 Large diameter pipes and structures were manufactured in polyester resin reinforced with jute cloth, using a modified filament winding machine. The results of the effect of catalysts, accelerators and fillers on the mechanical properties of the structures and the effects of loads on the final structures are reported. Kevlar epoxy substrate for interconnecting leadless chip carrier Packard, D.C. SAMPE Journal Vol 20 No 1 (January/February 1984) pp 6-14 The development of the leadless ceramic chip carrier (LCCC) presented the US military electronics industry with the opportunity of achieving higher packaging densities than had been possible with fiat-pack or dual-in-line packages, since the LCCC could be soldered directly to the substrate material. This neces-
COMPOSITES. OCTOBER 1984
sitated a substrate with a similar coefficient of thermal expansion to that of the LCCC. The details of the development of a suitable substrate material are given, leading to the successful development of a wide variety of multilayer boards based on Kevlar-reinforced epoxy material. Long-term moisture absorption on graphite/ epoxy angle-ply laminates Blikstad, M., Sjrblom, P.O.W. and Johannesson. T.R. Journal of Composite Materials Vo118 (1984) p32 The water absorption of fiberite T300/1034, Ciba-Geigy T300/920 and Ciba-Geigy T300/914 prepreg materials have been studied. Samples. in the form of rectangular specimens cut from angle ply laminates, were exposed to environments of different relative humidites, and the degree of moisture absorption determined from the change in specimen weight. The T300/1034 material showed two absorption pleateaus (at 100 and 400 days), whereas the T300/914 and T300/920 materials only showed the former plateau. The discrepancy between the measured and predicted absorptions, assuming Fickian diffusion, indicated that a two-stage process might be operating. However, a threedimensional analysis was found to adequately describe the data. Mechanism of fracture of short glass fibrereinforced polyamide thermoplastic Sato, N., Kurauchi, T., Sato, S. and Kamigaito, O. Journal of Materials Science Vol 19 (1984) pp 1145-1152 Acoustic emission is used during tensile tests on a thermoplastic polyamide 6.6 with short glass fibres in an attempt to detect different failure origins. Ring-down counting and amplitude distribution analysis are used. Activity begins at approximately 0.25 Lmax, and microscopy is used to correlate damage events with acoustic amplitude peaks. Interfacial cracks are seen to initiate at fibre ends at a low load, corresponding to the onset of acoustic emission. These propagate by shear deformation of the matrix along the interfaces, which continues up to 0.95 Lmax as indicated by the amplitude peaks. At higher load, the high amplitude peaks reflect matrix cracking, which continues up to failure of the composite. SEM observations show that most fibres are pulled out. A theoretical analysis based on stress levels in the matrix, fibre and interface for the composite just before the failure stress is used for various values of Vf; this correlates with observations of interfacial cracking of low load and matrix cracking near Lrnax, as well as absence of fibre breakage. Metallographie investigation of the damage caused to GRP by the combined action of electrical, mechanical and chemical environments Harris, S.J., Nobel, B. and Owen, M.J.Journal of Materials Science Vol 19 (1984) pp 1596-1604 The effect of high electrical voltages on GRP rods subjected to different levels of mechanical loading and environment is investigated. The pultruded E-glass/epoxy rod is encapsulated in a cladding, and tried on a high voltage line either as suspension or tension insulators. After a period of 1-3 years, sections have been examined and damage found to be present in the cladding material, allowing electrical activity at the surface and interior of the pultruded rod. Examples of material under high and low loads are investigated; where loads are low, damage is caused by electrical breakdown of dielectric properties and removal
of resin and glass. Where stress is high, brittle fracture of the rod occurs before electrical damage becomes severe. Electrical damage is aggravated by presence of interfaces, voids and moisture absorption; it allows contaminants to reach the glass fibre surface and promotes fracture in a m a n n e r similar to stress corrosion in GRP. Microscopic in-situ observation of craze formation in polystyrene-glass bead composites Dekkers, M.J. and Heikens, D. Journal of Materials Science Letters Vol 3 No 4 (April 1984) p 3O7 Confirmation is given for the difference in craze formation in poor and well interfacially bonded composites. The study shows that lor the poorly bonded case the craze initiation is preceded by the interface dewetting. Crazes are then initiated from a cap-shaped cavity at a direction which is typically 60° to the applied cord. The craze tip bends towards a direction perpendicular to the loading direction when it has left an influential region surrounding the beads. Processing and flammability parameters of bismaleimide and some other thermally stable resin matrices for composites Kourtides, D.A~ Polymer Composites Vol 5 No 2 (April 1984) pp 143-150 Prepregs of a selection of thermoset and thermoplastic resins reinforced with 133 graphite cloth were prepared. From each of these, laminates containing 10 plies were formed. The samples were then assessed lot thermal stability, ease of ignition and propensity to burn. smoke emission, moisture absorption and mechanical properties (tensile, compressive and short-beam shear strengths) at ambient and elevated temperatures, Results of these tests are presented and discussed. Shear fatigue degradation and fracture of random short-fibre SMC composite Wang, S.S., Goetz, D.P. and Corton, H.T. Journal of Composite Materials Vol 18 (1984) p2 The efl~cts of fatigue on the shear degradation and fracture of an SMC composite were studied. Double V-notched (losipescu) test specimens were fabricated from SMC R50 short fibre-reinforced composite, and tested under conditions of reverse loading cyclic fatigue at a frequency of 1 Hz. It was found that the relationship between cyclic shear stress amplitude, fatigue loading history and composite damage could be described in terms of a power law. Shear fatigue fracture was found to be the result of an accumulation of macroscopically homogeneous damage until nearing the ultimate cycle, when non-homogeneous deformation and damage, eg microcracking, occurred. Short silk fibre reinforced nitrile rubber composites Setua, D.K. and De, S.K- Journal of Materials Science Vol 19 (1984) pp 983-999 This study looks at the role of 'resorcinolhexamethylenetetramine-silica" bonding agents in increasing the adhesion in a silk fibre/nitrile rubber composite. Tensile and tear testing is carried out at 30°C and hardness, compression set, heat build-up, resilience, abrasion loss and wear properties measured. Increasing fibre concentration improves tensile strength, tear strength, hardness and modulus while decreasing the resilience. The effect of ageing for 48 h at 100°C on fibre/matrix adhesion is investigated: properties are retained better lbr properly bonded fibre-filled mixes than lbr an unfilled mix, since the matrix deteriorates rather than the fibres.
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Stress relaxation properties of some reaction injection molded (RIM) and reinforced RIM systems Turner, R.B. Polymer Composites Vol 5 No 2 (April 1984) pp 151-154 Three different urethane RIM systems were prepared. 1.6 m m glass fibre reinforcement was added to one of the systems. Stress relaxation data for all samples over a temperature range of 23°C to 100°C are presented, together with differential scanning calorimetry and dynamic mechanical spectroscopy data. It was found that the state of hard segment (amorphous or crystalline) influenced the reaction of the polymer to stress. Also, stress relaxation and creep performance were improved by the addition of 25% glass fibre.
composite have been compared. Measurements of the coefficient of expansion were made using strain gauges, thermal mechanical analysis and an optical dilatometer. Good correlation between the three independent methods was found for the thin composites. For the thick composites, the expansion coefficient as measured by strain gauges was found to be dependent on the position of the gauge. This was thought to be due to the formation of localized stresses. Furthermore, measurements of the thick composite using the optical dilatometer revealed a large amount of hysteresis, which was considerably reduced on post-curing. The authors concluded that this was the result of residual curing stresses being annealed-out during post-curing`
Thermoeinstic behaviour of thick glass/epoxy composite laminates Raghava, R.S., Valentich, J. and Nathenson, IL D. Journal of Composite Materials Vo118 (1984) p 81 The thermal expansion properties of a thick (54 mm) and a thin (5.4 mm) glass/epoxy
Three-phase syntactic foams Narkis, M., Kenig, S. and Puterman, M. Polymer Composites Iiol 5 No 2 (April 1984) pp 159-164 Three-phase syntactic foams comprising a close-packed microballoon bed, binding resin and air voids were prepared and their mechanical properties investigated. Modified modulus-composition equations are shown to
al a'det
be applicable for the prediction of moduli values of these three-phase systems.
Transverse tensile strength anisotropy in thick filament wound RINA composites Miyase, A. Journal of Materials Science Vol 19 (1984) pp 923-928 Transverse tensile strength in glass/epoxy thick filament wound ring composites is found to depend on specimen size and orientation due to the presence of elongated 'pancake" shaped voids between strands. Winding atmosphere also affects the properties. The Mode I KIC value is measured by width-tapered double cantilever beam tests on specimens of three different orientations. Therefore, the elongated voids will give different effective flaw sizes depending on specimen orientation. Transverse strength anisotropy and specimen size effects are caused by differences in void content, shape, size and distribution. Linear elastic fracture mechanics is applicable to the correlation of flaw size and transverse strength in a qualitative way only, due to complexities arising from void distribution and geometry.
l A2n
Abstracts of British Patents are prepared by Dr F.R Jones, Department of Materials Science and Engineering, University of Surrey, UK
U K PATENTS COMPONENTS Plastic transmission brake band (D.A.B. Industries Inc, USA) GB 2 122 132 A (11 January 1984) A continuous filament (eg glass fibre) reinforced epoxy resin strap, with a bonded friction lining, is described. Fibre reinforced plastics leaf spring (The Budd Company, USA) GB 2 125 514 A (7 March 1984) An FRP leaf spring of constant cross-sectional area tapering to the ends but thicker in the middle, with an appropriate reccsss, is described. The recess can be used to locate the fixing pins without damaging the composite. Various assemblies are described, one of which includes a metal leaf spring to facilitate transverse mounting. A rotary filament moulding technique is also described. Glass and/or vitreous ceramic laminate system for microwave shielding applications (Carl Zeiss Stiftung` FRG) GB 2 125 732 A (14 March 1984) A metal microwave shielding grid of various types is sandwiched in a plastic matrix between two glass panels which form a microwave oven door. Dry friction composition (Borg-Warner Corp, USA) GB 2 126 594 A (28 March 1984) An aramid pulp/glass fibre-reinforced rubbermodified phenolic resin laminate for use in clutch facings is described.
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Ferrite containing fibres and fabrics (TDK Corp, Japan) GB 2 127 739 A (18 April 1984) A cloth is made by weaving slices from a polymer/ferrite/fibre composite. These cloths can be further laminated with a ferrite/resin mixture, thus making the manufacture of complex shapes possible.
MATERIALS Structures of reinforced impregnated graphite and their production (Le-Carbone-Lorraine, France) GB 2 120 975 A (14 December 1983) Resin-impregnated graphite for heat exchangers etc is reinforced at the surface by a phenolicresin based, woven CFRP. Fibre-reinforced composite materials (Dart Industries Inc) GB 2 123 838 A (8 February 1984) The permeability of plastics housings, for electronic equipment, to electromagnetic interference is reduced by reinforcing with stainless steel fibre strands. Chopped random fibres are ineffective. Injection-moulded reinforced polycarbonates are exemplified. The strands are 3-30 m m long and 1.6-6.4 m m in diameter. Epoxy resins containing metal (Hitco, USA) GB 2 125 056 A (29 February 1984) The properties of epoxy resins can be improved by incorporating metal elements into the structure. Metallized (eg B,W and Zr) prepolymers are described. Thermosetting resin composition (Mitsui Toatsu Chemicals Inc, Japan) GB 2 126
240 A (21 March 1984) Bismaleimide resins with high temperature resistance (>400°C) and low moisture absorption are described. Heat resistant laminate (T & N Materials Research Ltd, UK) GB 2 126 526 A (28 March 1984) A glass/jute fibre phenolic laminate is described, which can replace an asbestos equivalent. The laminate configuration is found to be significant. Fibre reinforced resin molded articles (Ina Seito Co Ltd, Japan) GB2 130141A (31 May 1984) A laminated GRP article which uniformly reflects and shields electromagnetic waves comprises laminae of an electro-conductive non-woven fabric (prepreg of aluminiumcoated glass fibre), a woven fabric (eg glass fibre) and an SMC. The intermediate woven layer prevents defects from forming in the electro-conductive layer.
PROCESSES Reinforced concrete (Square Grip Ltd (UK)) GB 2 122 126 A (11 January 1984) A device for supporting the reinforcing rods during the casting of reinforced concrete is describe& Moulding composite articles; panels (KMMCO Structural Foam Inc, USA) GB2 122 130 A (11 January 1984) By intermittently introducing inert gas into the
COMPOSITES. OCTOBER 1984
resutvev
Prepared by M . E . Adams, R.S. Bailey, L. Boniface and d . W . Rock
CARBON FIBRES
absorbed by the composite at static loading rates can be one or two orders of magnitude higher than that of the unreinforced matrix.
CEMENT MA TRICES
Repair and protection of reinforced concrete in high-rise buildings (Part 1) Ovens, A. Concrete Vol 18 No 5 (May 1984) p 21 The article discusses the current approach in repairing reinforced concrete in situ, using state of the art information. This first part of the article is concerned with the structural repairs of those buildings in which an inadequate quality or thickness of concrete cover over the steel reinforcement has been used. This results in loss of passivating alkalinity, active corrosion of the steel and eventual spalling of the concrete. The relationship between balancing the economics and the individual aspects of survey, diagnosis, repair and protection and the use and life expectancy of the structure is realized. The second part of this report is concerned with the coating recommended.
Durability of cement sheets reinforced with fibrillated polypropylene networks Hannant, D.J.Magazine of Concrete Research Vol 35 No 125 (December 1983) pp 197-204 The durability of cement sheets reinforced with fibrillated polyproylene networks is examined in terms of the effects upon the tensile stress/ strain curve of three years of natural weathering or of a similar period of indoor storage. It is shown that three years of exposure has little effect upon the strengths of the networks, but that increases in the strength and elastic modulus of the matrix in the naturally weathered specimens alter some features of the stress/strain curve in comparison with similar material stored indoors. (Author's abstract)
Steel fibre reinforced concrete beams in torsion Narayanan, R. and Kareem-Palanjian International Journal of Cement Composites and Lightweight Concrete Vol 5 No 4 (November 1983) pp 235-246 The results of 28 torsion tests on steel fibrereinforced beams are presented and discussed. The parameters varied were the type, aspect ratio and the volume fraction of the fibres used, as well as the cross-sectional dimensions of the beam. Methods of predicting the cracking torque and the ultimate torque are proposed; the predictions obtained by the suggested methods are shown to be satisfactory by comparing them with the torques observed in the tests carried out by the authors and previous investigators. (Authors' abstract)
XPS studies by use of digital difference spectrum technique of functional groups on the surface of carbon fibres Takahagi, T. and Ishitani, A. Carbon Vo122No 1 (1984) p 43 A technique is described which has proved effective for the analysis of the functional groups introduced by surface oxidation treatment of carbon fibres. The major functional groups introduced to the carbon fibre surface by surface oxidation are hydroxyl and carboxyl groups. Disordering of the crystal lattice is also reported, together with the generation of carboxyl groups in the case of surface oxidation of graphite fibres.
Flax fibres as a reinforcement in cement mortars Coutts, R.S.P. International Journal of Cement Composites and Lightweight Concrete Vol 5 No 4 (November 1983) pp 257-262 The flexural strength and fracture properties of a cement mortar reinforced with New Zealand flax fibres were studied. The effect of refining the fibres before incorporation into the matrix was also investigated. It was found that at a fibre loading of approximately 8-10% by mass flexural strengths in excess of 20 MPa could be obtained, which are comparable to Pinus radiata fibre-reinforced mortars. However, the fracture toughness values were approximately half that of the P radiata composite. (Author's abstract) Impact properties of steel fibre reinforced concrete in bending Naaman. A.E. and Gopalaratnam International Journal of Cement Composites and Lightweight Concrete Vol 5 No 4 (November 1983) pp 225-233 The effects of strain rate of loading including impact, on the bending properties of steel fibrereinforced mortar were studied using an instrumented drop-weight impact machine. The results of the tests indicate that, depending upon the fibre reinforcement, the energy
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CERAMIC MA TRICES Advanced ceramics and composites. Shortfalls surface at CMS Workshop Fisher, G. American Ceramics Society Bulletin Vo163 No 2 (February 1984) p 247 A report is made of the 8th A n n u a l Conference on Advanced Ceramics and Composites. The applicability of ceramic matrix composites, and ceramics is discussed. The proceedings include; a recommendation for a rational programme on ceramics for gas turbines, the reliability and characterization of these materials, modelling, defects and their effect on fracture toughness and fabrication. It was concluded that ceramic matrix composites could fill a gap in gas turbine and high temperature applications, but that there is a place for both these and monolithic ceramic materials for which reliability and progress in development has been proved. Composites, engineering the ultimate material Fisher, G. American Ceramics Society Bulletin Vo163 No 3 (March 1984) p 360 A brief review of currently used composites is made. The limitations of organic, glass and metal matrix composites are discussed and it is
pointed out that higher temperature applications indicate ceramic matrix composites to be of potential. The state of the art of these materials is discussed, and their reliability assessed. It is pointed out that ceramic matrix composites is a growth area in terms of materials development and testing. It remains to be seen whether these toughened materials or the high strength monolithic ceramics will prevail in the advanced gas turbine applications. Optical properties and solar selectivity of coevaporated Co-AI203 composite films Niklasson, G.A, and Granqvist, C.G. Journal of Applied Physics Vo155 No 9 (May 1984) p 3382 These deposits were analysed using a variety of techniques. A uniform separated-grain structure with regular HCP Co particles embedded in A1203 has been developed using Co contents up to 30% by volume. The complex dielectric permeability was evaluated for a range of wavelengths and numerical accuracy and internal consistancy investigated. At large Co contents discrepancies were observed to the derived classical scattering theory and evidence for anisotropy was obtained. Computer optimization was carried out for the system and experimental evidence reinforced the results of varying composition, and improving the solar selectivity.
GENERAL An advanced theory of the strength of hybrid composites Hiroshi Fukuda Journal of Materials Science Vol 19 (1984) pp 974-982 This paper presents a statistical approach to the strength of a unidirectional hybrid sheet considering a modification of Zweben's theory. The concept of the hybrid effect is first explained with reference to two hybrid systems in terms of enhancement of initial failure strain, ie failure strain of the low elongation fibre. The systems are graphite/E-glass (Bunsell, Harris) and Kevlar/graphite (Zweben). This study uses a 'di-plet' criterion to evaluate failure of the hybrid composite. Zweben's theory is modified by calculating more precise stress concentration factors using a shear lag analysis (appendix). Experimental data for stress/strain behaviour is compared with theoretical predictions of the hybrid effect. Analysis of the effect of embedded fibre length on fibre debonding and pull-out from an elastic matrix (Part 1 review of theories) Gray, R.J. Journal of Materials Science Vol 19 (1984) pp 861-870 Theories to explain the processes involved in pull-out tests on single fibres are reviewed. This gives an estimation of the resistance to fibre/ matrix interfacial debonding in composites with ductile fibres in a brittle or elastic matrix This is a function of the interfacial bond shear strength and interfacial bond area. Good experimental agreement is obtained for ductile/ plastically deforming matrices with uniform interfacial shear strength, where debonding
COMPOSITES
. OCTOBER 1984
load is proportional to embedded fibre length. .For a brittle/elastic matrix, a non-uniform shear stress distribution along the interface gives a non-linear relation between pull-out load and embedded fibre length. Major theories take account of the frictional resistance to slipping over the debonded part of the interface, as well as the effect of embedded fibre length, and give numerical estimates of interfacial shear strength and frictional resistance to slipping. Analysis of the effect of embedded fibre length on fibre debonding and pull-out from an elastic matrix (Part 2 application to a steel fibrecementitious matrix composite system) Gray, R.J. Journal of Materials Science Vol 19 (1984) pp 1680-1691 This paper tests the applicability of the theories discussed in part 1 to explaining interfacial debonding and pull-out in a brass-coated steel wire/cementitious mortar composite. For an embedded fibre length of 100 mm, failure occurred by fibre debonding and pull-out whereas for length of 125 mm, failure was by fibre fracture rather than pull-out. The total resistance to interfacial debonding comprises adhesional bonding and frictional resistance to slipping after debonding (for a sufficiently long embedded fibre length). Those theories which ignore the frictional term cannot explain failure by fibre fracture instead of pull-out but give reasonable values of m a x i m u m shear strength of the fibre/matrix interfacial bond. Design of laminated plates for maximum stiffness Taucherl, T.R. and Adibhatla, S. Journal of Composite Materials Vol 18 (1984) p 58 The design of symmetrical fibre-reinforced laminated plates for m a x i m u m stiffness is discussed. An analysis, based on the criterion of m i n i m u m strain energy, is developed. Deformation of the laminate plate is obtained using the Rayleigh-Ritz method. For optimum laminate design, the strain energy must be minimized and this was achieved using a quasiNewton procedure. Examples of results obtained for a graphite/epoxy (T300/$208) composite system are presented. Experimental and analytical studies on the onset of delamination in laminated composites Kim, 1LY. and Soni, S.R, Journal of Composite Materials Vol 18 (1984) p 70 A failure criterion based on the average stress (~r3) and the interlaminar tensile strength, which was assumed to equal the transverse strength of the composite, was developed and used to predict the onset of delamination. Comparison with experimentally obtained results, using acoustic emission to detect delamination, shows good agreement. However, the authors point out that there is an indication that transverse cracking affects the prediction, and that there is a critical transverse crack length above which the predictions are significantly different from experimental results. Progress in reinforced plastics Wehrenberg I1, R,H. Materials Engineering(April 1984) pp 46-53 Innovations in compounds, fibres and processes discussed at the 39th Annual SP1 RP/C Conference are reviewed. A thermoplastic composite based on PPS with long carbon or glass fibres allows optimum automated production cycles, and gives good impact strength. Smooth surfaced polyester-based SMCs show increased toughness with good paint and adhesion bonding for auto applications. Developments in resins are reported; polyester/
C O M P O S I T E S . OCTOBER 1 9 8 4
urethane hybrids have increased strength, stiffness and impact resistance over unsaturated polyesters. Pultrusion of composites is expected to have good growth potential in competition with AI extrusions. Advances in automatic processing of large components and computercontrolled filament winding are reviewed. Photocuring prepregs with reinforced polyester containing photoinitiators which are rapidly cured by visible light aid fast production. New reinforcements include high strength nylon and PET fibres. Calcium sulphate whiskers can replace glass in thermoplastic and thermosets. PVDC microspheres have been developed for use with syntactic foams. Pullout tests with metallic glass ribbons in different matrices Tio, T.K,, Friedrich, K,, Hornbogen, E., K~Sster, U. and Fels, A. Journal of Materials Science Letters Vol 3 No 4 (May 1984) p 415 The potential for using metglass ribbons as reinforcement in composites is studied using pull-out tests. A range of matrices and surface treatments of the ribbons are used and it was found that a thermosetting resin matrix provided the best tensile properties. Pull-out testing showed that a critical length, as used in conventional fibre composites, is applicable in these materials. The geometry and materials effects are investigated and their influence on this critical length is studied. The same optimizations for high fracture energies apply to these materials as is applicable to discontinuous fibre composites. A review of recent progress in the studies of molecular and microstructure of coupling agents and their functions in composites, coatings and adhesive joints Ishida, H. Polymer Composites Vol 5 No 2 (April 1984) pp 101-123 An in-depth study under the above title is presented and an improved understanding of the structure of silane coupling agents is claimed. In particular, the structure of aminosilane, the organization and orientation of silane molecules, structural gradients in the interfacial regions, chemical reactions at the interfaces and within the silane layers and the structural changes in the silane layers are discussed. Theoretical model on the fracture behaviour and tensile strength of fibres coated with two brittle surface layers Ochiai, S., Osamura, K, and Murakami, Y. Zeitschrift ff~r Metallkunde Vol 75 (1984) p 231 Composite degradation due to the formation of a reaction layer at the fibre surfaces is particularly important in some boron fibre and silicon carbide fibre-reinforced composites. In this paper the authors develop a general model to predict the fracture behaviour of fibres covered by two brittle layers. The model they develop is based on Weibull statistics and contains seven unifying parameters. This gives rise to 5040 failure modes. From the present model, it is possible to calculate the strength of the fibres in terms of the thicknesses of the two layers.
METAL MATRICES Analytical stress solutions for fibre-reinforced metallic rocket motor cases Stanley, P., Margetson, J. and Groves, A. International Journal of Mechanical Sciences Vol 25 No 2 (1984) pp 119-130 An analytical solution is presented for a
metallic rocket motor case reinforced circumferentially with a prestrained fibre overwind. Considerations include modification of the stress state by different storage temperatures, case buckling during manufacture and yield behaviour of the case for various winding strains and fibre configurations. The system comprises a lightweight alloy case with high strength fibre overwind which induces a compressive stress state in the case, increasing its bursting pressure and specific strength. Tresca and Von Mises yield criteria are used to evaluate the internal pressure which initiates yield. Fatigue failure of an orthotropie plate with a circular hole Tsangarakis, N. Journal of Composite Materials Vo118 (1984) p 47 The effect of pre-drilled circular holes on the fatigue failure of FP/AI-55 composite has been investigated. It has already been shown that the mechanical properties of this composite are dependent on the degree of matrix/fibre bonding, and in this study only a single batch of material was used. Specimens with holes of 1.6 m m and 3.2 m m had endurance stresses of 225 and 167.5 MPa, respectively, compared with 330 MPa for the unnotched material. Thermographic examination of the specimens during fatigue enabled the detection of areas of poorly bonded fibres near a pre-drilled hole; it also confirmed that there is heat generation during fatigue. Comparison between the experimentally measured endurance stresses and those predicted using the average stress model (ASM) showed very good agreement. Fracture mode and tensile strength of fibres in borsic-titanium alloy composites Ochiai, S., Osamura, 1C and Murakami, Y. Zeitschrift ffir Metallkunde Vol 75 (1984) p 238 A study of the degradation mechanism in borsic/titanium alloy matrix composites has been carried out. At elevated temperatures, reaction products form at the fibre/matrix interlace, and premature failure of this layer is responsible for a degradation in strength of the composite. Various fibre surface treatments have been used to try to eliminate the reaction; however, a layer of different composition is often produccd. For example, the boron fibre may have a silicon carbide sheath. In this paper the authors apply a general theoretical model they developed for a fibre coated with two brittle layers to borsic/Ti40A and borsic/Ti-3AI-2.5V composites. They found that the failure of the Ti40A composite was the result of fracture of a titanium-silicide reaction layer which caused simultaneous fracture of the silicon carbide layer and the boron fibre. In the Ti-3AI-2.5V composite, notching of the fibre, by failure of the TiB 2 reaction layer, was responsible for failure. Mierostruetural instability of Ni-Mo-AI unidirectionally solidified eutectics Schwam, D., Dirnfeld, S.F. and Nadiv, S. Journal of MateriaLs Science Letters' Vol 3 No 4 (April 1984) p 363 Characterization of the microstructural changes observed in in situ composite superalloys is described, The morphology of the phases is investigated using optical and electron microscopy. The effect of temperature gradients is to promote morphological transformations. The fibres reject aluminium as they coarsen, which precipitates as a "/' phase in the vicinity of the fibres. The square-section fibres become circular in shape as they become unstable. The orientation relationships for the fibres and their array is obtained.
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Powder metallurgically produced metal-glass composites Kainer, K.U., Bergmann, H.W. and Mordike, B.L. Powder Metallurgy Vol 27 No 1 (1984) pp 30-37 This paper investigates the influence of starting materials and post-fabrication deformation processes on the properties of a metal/glass system. The composite is fabricated by mixing and compacting A1 and glass powders, then annealing. It is subsequently extruded or forged. Optical and electron microscopy are used to investigate the microstructure. It is found that strength and toughness increase with increasing glass content and particle size. During fracture, the brittle phase fractures first and then the ductile phase is drawn out. Wear properties are improved, and the creep rate of the composite is decreased compared with pure A1. Most of the improvements in physical and elastic properties can be explained by the rule of mixtures, and are attributable to dispersionhardening effects, although hardness, strength and wear exhibit changes in excess of values predicted by the rule of mixtures. These Al/glass composites have good joining and finishing capabilities; surface anodizing is also possible. Simple and precise measurements of fibre volume and void fractions in metal matrix composite materials Schoutens, J.E. Journal of Materials Science Vo119 (1984) p 957 This paper discusses methods for measuring fibre volume fraction (V~) and void fraction (Ve) in metal matrix composites, and the degree of precision expected. The simple non-destructive method to determine Vf involves weighing in air and in a liquid, while measuring Vg requires etching away some of the matrix material. The analysis gives equations relating Vf and V~ with numerical results for B/A1 and glass/Al systems. If voids are present, two sequential experiments on the same specimen are required to determine Vf and V~ This involves finding the composite density, then etching away some matrix and finding the new density.
observing the fracture surface morphology. At temperatures below IO0°C, fracture involves breaking lateral bonds between polymer molecules; creep processes dominate at higher temperatures. Failure is initiated at surface irregularities such as kink bands. Microscopy reveals that fracture occurs by cutting through fibrils at defective regions by a step process, after initiating at surface kink bands. The large difference between the measured and theoretical strength, which is based on an infinitely long, aligned extended chain, can be explained in terms of these bands; thin filaments are desirable since they will have fewer surface bands so that the crystalline structure is continuous, and strength is determined by covalent bond strength of the molecule rather than lateral bonding between molecules which limits practical fibre strength.
Loading criteria for the fatigue failure of polyamide 66 fibres Oudet, C.H. and Bunsell, A.R. Journal of Materials Science Letters Vol 3 No 4 (April 1984) p 295 Nylon 66 fibres were observed to exhibit two regimes of failure type. At greater than 52% load cycling, there were predominantly creep-type failures. Below this level, a fatigue morphology was observed. A comparison is made between a range of polyamide 66 fibres. Examination of the fractured fibre ends shows that the end is always curved inwards, indicating that the surface is in compression as the fibres are unloaded. Residual stresses in the fibres would be expected to modify the m i n i m u m load criterion for crack initiation. It is suggested that this may also be influential' in the apparent acceleration of creep failures under cyclic loading.
RESIN MATRICES
ORGANIC FIBRES
The design of ends of cylindrical pressure vessels in glass reinforced plastic Barton, D.C., Amos, A.R., Soden, P.D. and Gill, S.S. International Journal of Mechanical Sciences Vo126 No 3 (1984) pp 177-199 This paper presents an analysis for the design of torispherical and semi-ellipsoidal ends of cylindrical GRP chopped strand mat pressure vessels, in accordance with BS4994 to calculate unit loading of laminates in such vessels. Here, the solution is of an a x i s y m m e t i c thin shell problem for a linear elastic material taking account of the effect of change of shape of the ends due to pressurization. All reinforcement layers must be of the same type. The design for G R P ends is based on limiting the m a x i m u m tensile strain to a nominated value, although a m a x i m u m stress criterion is also examined. Stresses in a laminate having layers with different properties vary greatly across the thickness depending on arrangement; therefore, the m a x i m u m bending stress is not necessarily at the laminate surface. A design based on limiting the m a x i m u m strain may be more valid since the m a x i m u m strain is always on one of the surfaces.
The fracture process of ultra-high strength polyethylene fibres Smook, J.. Hamersma, W. and Pennings, A.J. Journal of Materials Science Vol 19 (1984) pp 1359-1373 This paper examines the fracture behaviour of PE fibres by measuring the lifetime of fibres in dead-load tests at various temperatures, then
The detailed structure of delamination fracture surfaces in graphite/epoxy laminates Johannesson, T., Sjrblom, P. and Selden, R. Journal of Materials Science Vol 19 (1984) pp 1171-1177 This paper investigates delamination fracture surfaces in angle-ply graphite/epoxy tension specimens using SEM to look at microscopic structural features to understand delamination
Yield and deformation of metallic glasses strengthened by post-extrusion addition of second phase particles Zielinski, P.G. and Ast, D.G. Acta Metallurgica Vo132 No 3 (1984) pp 397-405 This paper studies the stress/strain behaviour of metallic glasses with second phase particles by SEM and bend tests on hairpin configuration specimens. The materials are melt spun Ni75SisBI7 in ribbon form with homogeneously dispersed TiC and WC particles. The effect of unshearable second-phase particles blocking shear bands on the yield stress is analysed using the one-parameter theory of work hardening (Ashby). The presence of the particles increases yield stress and modulus in agreement with the rule of mixtures. The value derived for the friction stress agrees with analysis of bending.
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mechanisms. Fractures showed some fibre pullout with resin residue indicating a high interfacial bond strength. The general delamination surface shows uncovered fibres, fibre imprints and matrix serrations, which are plate features, lying perpendicular to the surface. It is a twoplanar surface where the crack path alternates from one side of the interlaminar region to the other. The delamination process involves fibre debonding due to stress concentration at the interface, followed by formation of slanted resin cracks normal to the major principal stress; these features are linked by microscopic crack propagation into the resin, forming the two-planar delamination surface.
Effect of particulate reinforcement on creep behaviour of polyurethane foams Alperstein, D., Narkis, M., Kenig, S. and Siegmann, A. Polymer Composites Vol 5 No 2 (April 1984) pp 155-158 The compressive creep behaviour of rigid unreinforced and particulate-reinforced polyurethane foams at different densities, filler contents, temperatures and loads was investigated. It was found that the addition of high modulus particulate fillers significantly enhanced the creep resistance of rigid polyurethane foams, especially at high reinforcement contents and high foam densities. Effect of temperature, moisture and radiation exposures on composite mechanical properties Mazzio, V.F. and Huber, G. SAMPE Journal Vo120 No 2 (March/April 1984) pp 14-23 The results of a programme carried out to generate data to support the design and application of advanced composites for space systems structures are presented. In the programme, four different materials (three types of carbon fibre and Kevlar 49 in epoxy resin matrices) were exposed to three levels of electron beam radiation, moisture saturation at 90% RH and 71.5°C and a combined 'moisture and space environment exposure'. After exposure, specimens were assessed for tensile, compression, in-plane shear and short-beam shear properties. Only results of tests carried out in the fibre axial direction are detailed. Fatigue and reliability evaluation of unhatched carbon epoxy laminates Radhakrishnan, K. Journal of Composite Materials Vo118 (1984) p 21 Most of the models that predict the fatigue life from a proof loading require a considerable amount of testing before the model can be used. In this paper the author examines the model developed by Yang, which requires only a m i n i m u m of setting up. The model predicted the m i n i m u m number of fatigue cycles that a carbon/epoxy laminate would survive for various loadings. However, at probabilities of above 0.3, it overestimated the fatigue life and underestimated the residual strength. The fracture of particulate-filled epoxide resins Moloney, A.C., Kausch, H.H. and Stieger, H.R. Journal of Materials Science Vol 19 No 4 (1984) p 125 In this paper, the effect of glass beads on the fracture behaviour of epoxides as a model system for rough mineral particles in plastic is investigated. Resins are filled with varying fractions of glass beads (diameters 50-100 btm and 0-50 pm), both treated with coupling agent and untreated. Hollow silica spheres are also included. Double-torsion tests measure KIc; flexural strength is measured in 3-point bending and yield stress in plane-strain compression. Fracture surfaces are examined in the SEM. The authors consider toughening
COMPOSITES. OCTOBER 1984
mechanisms due to second-phase particles by the bowing of a crack front between particles and crack blunting by debonding of the particles from the matrix. For poorly bonded glass beads, crack growth is unstable due to blunting. For a well-bonded composite, crack propagation is stable. It appears that a m i n i m u m filler/resin adhesion is required for the crack bowing mechanism. Caution is required when extrapolating results for spherical fillers to more irregular mineral particles. Fracture toughness of Kevlar-epoxy composites with controlled interfaciul bonding Mai, Y.W. and Castino, F. Journal of Materials Science Vol 19 (1984) pp 1638-1655 This investigation into toughness uses intermittent bonding; the fibres are partly coated with a viscous fluid, deliberately introducing regions of weak interfacial bonding which absorb energy by blunting cracks, increasing pull-out length and hence improving toughness. The coating materials were a silicone vacuum fluid and a polyurethane varnish. Fracture toughness was measured in the range -60 -40°C. Intermittent bonding increases toughness without a significant drop in tensile strength, although the varnish gave the best results. Hygrothermal ageing degraded the varnish-coated material, with little effect on the other type of material. Analysis of the fracture mechanisms indicates that fibre fracture work and fibre pull-out toughness contribute to total fracture toughness in these composites. There is reasonable agreement between predicted fracture toughness values and experiment. Free dieyandiamide in erosslinked epoxy resins Kasturiarachchi, K.A. and Pritchard, G.Journal of Materials Science Letters Vol 3 No 4 (April 1984) p 283 A study into the effect of residual curing agents in cross-linked epoxy resin/E-glass fibre composites is made. The curing agent dicyandiamide dissolves in water at temperatures between 40 and 80°C. This is shown in humidity/temperature tests followed by various analysis techniques. Three observations have been made: (1) regions of porosity develop in resin-rich regions; (2) a m m o n i a is developed as a decomposition product between water and the curing agent; and (3) pockets of dissolved solutes may be pressurized, these cavities can develop and initiate cracks in the resin. It is recommended that the gelation process in the production route be made at higher temperatures where practicable. Jute reinforced polyester structures Semsarzadeh, M.A., Lotfali, A.R. and Mirzadeh, H. Polymer Composites Vol 5 No 2 (April 1984) p 141-142 Large diameter pipes and structures were manufactured in polyester resin reinforced with jute cloth, using a modified filament winding machine. The results of the effect of catalysts, accelerators and fillers on the mechanical properties of the structures and the effects of loads on the final structures are reported. Kevlar epoxy substrate for interconnecting leadless chip carrier Packard, D.C. SAMPE Journal Vol 20 No 1 (January/February 1984) pp 6-14 The development of the leadless ceramic chip carrier (LCCC) presented the US military electronics industry with the opportunity of achieving higher packaging densities than had been possible with fiat-pack or dual-in-line packages, since the LCCC could be soldered directly to the substrate material. This neces-
COMPOSITES. OCTOBER 1984
sitated a substrate with a similar coefficient of thermal expansion to that of the LCCC. The details of the development of a suitable substrate material are given, leading to the successful development of a wide variety of multilayer boards based on Kevlar-reinforced epoxy material. Long-term moisture absorption on graphite/ epoxy angle-ply laminates Blikstad, M., Sjrblom, P.O.W. and Johannesson. T.R. Journal of Composite Materials Vo118 (1984) p32 The water absorption of fiberite T300/1034, Ciba-Geigy T300/920 and Ciba-Geigy T300/914 prepreg materials have been studied. Samples. in the form of rectangular specimens cut from angle ply laminates, were exposed to environments of different relative humidites, and the degree of moisture absorption determined from the change in specimen weight. The T300/1034 material showed two absorption pleateaus (at 100 and 400 days), whereas the T300/914 and T300/920 materials only showed the former plateau. The discrepancy between the measured and predicted absorptions, assuming Fickian diffusion, indicated that a two-stage process might be operating. However, a threedimensional analysis was found to adequately describe the data. Mechanism of fracture of short glass fibrereinforced polyamide thermoplastic Sato, N., Kurauchi, T., Sato, S. and Kamigaito, O. Journal of Materials Science Vol 19 (1984) pp 1145-1152 Acoustic emission is used during tensile tests on a thermoplastic polyamide 6.6 with short glass fibres in an attempt to detect different failure origins. Ring-down counting and amplitude distribution analysis are used. Activity begins at approximately 0.25 Lmax, and microscopy is used to correlate damage events with acoustic amplitude peaks. Interfacial cracks are seen to initiate at fibre ends at a low load, corresponding to the onset of acoustic emission. These propagate by shear deformation of the matrix along the interfaces, which continues up to 0.95 Lmax as indicated by the amplitude peaks. At higher load, the high amplitude peaks reflect matrix cracking, which continues up to failure of the composite. SEM observations show that most fibres are pulled out. A theoretical analysis based on stress levels in the matrix, fibre and interface for the composite just before the failure stress is used for various values of Vf; this correlates with observations of interfacial cracking of low load and matrix cracking near Lrnax, as well as absence of fibre breakage. Metallographie investigation of the damage caused to GRP by the combined action of electrical, mechanical and chemical environments Harris, S.J., Nobel, B. and Owen, M.J.Journal of Materials Science Vol 19 (1984) pp 1596-1604 The effect of high electrical voltages on GRP rods subjected to different levels of mechanical loading and environment is investigated. The pultruded E-glass/epoxy rod is encapsulated in a cladding, and tried on a high voltage line either as suspension or tension insulators. After a period of 1-3 years, sections have been examined and damage found to be present in the cladding material, allowing electrical activity at the surface and interior of the pultruded rod. Examples of material under high and low loads are investigated; where loads are low, damage is caused by electrical breakdown of dielectric properties and removal
of resin and glass. Where stress is high, brittle fracture of the rod occurs before electrical damage becomes severe. Electrical damage is aggravated by presence of interfaces, voids and moisture absorption; it allows contaminants to reach the glass fibre surface and promotes fracture in a m a n n e r similar to stress corrosion in GRP. Microscopic in-situ observation of craze formation in polystyrene-glass bead composites Dekkers, M.J. and Heikens, D. Journal of Materials Science Letters Vol 3 No 4 (April 1984) p 3O7 Confirmation is given for the difference in craze formation in poor and well interfacially bonded composites. The study shows that lor the poorly bonded case the craze initiation is preceded by the interface dewetting. Crazes are then initiated from a cap-shaped cavity at a direction which is typically 60° to the applied cord. The craze tip bends towards a direction perpendicular to the loading direction when it has left an influential region surrounding the beads. Processing and flammability parameters of bismaleimide and some other thermally stable resin matrices for composites Kourtides, D.A~ Polymer Composites Vol 5 No 2 (April 1984) pp 143-150 Prepregs of a selection of thermoset and thermoplastic resins reinforced with 133 graphite cloth were prepared. From each of these, laminates containing 10 plies were formed. The samples were then assessed lot thermal stability, ease of ignition and propensity to burn. smoke emission, moisture absorption and mechanical properties (tensile, compressive and short-beam shear strengths) at ambient and elevated temperatures, Results of these tests are presented and discussed. Shear fatigue degradation and fracture of random short-fibre SMC composite Wang, S.S., Goetz, D.P. and Corton, H.T. Journal of Composite Materials Vol 18 (1984) p2 The efl~cts of fatigue on the shear degradation and fracture of an SMC composite were studied. Double V-notched (losipescu) test specimens were fabricated from SMC R50 short fibre-reinforced composite, and tested under conditions of reverse loading cyclic fatigue at a frequency of 1 Hz. It was found that the relationship between cyclic shear stress amplitude, fatigue loading history and composite damage could be described in terms of a power law. Shear fatigue fracture was found to be the result of an accumulation of macroscopically homogeneous damage until nearing the ultimate cycle, when non-homogeneous deformation and damage, eg microcracking, occurred. Short silk fibre reinforced nitrile rubber composites Setua, D.K. and De, S.K- Journal of Materials Science Vol 19 (1984) pp 983-999 This study looks at the role of 'resorcinolhexamethylenetetramine-silica" bonding agents in increasing the adhesion in a silk fibre/nitrile rubber composite. Tensile and tear testing is carried out at 30°C and hardness, compression set, heat build-up, resilience, abrasion loss and wear properties measured. Increasing fibre concentration improves tensile strength, tear strength, hardness and modulus while decreasing the resilience. The effect of ageing for 48 h at 100°C on fibre/matrix adhesion is investigated: properties are retained better lbr properly bonded fibre-filled mixes than lbr an unfilled mix, since the matrix deteriorates rather than the fibres.
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Stress relaxation properties of some reaction injection molded (RIM) and reinforced RIM systems Turner, R.B. Polymer Composites Vol 5 No 2 (April 1984) pp 151-154 Three different urethane RIM systems were prepared. 1.6 m m glass fibre reinforcement was added to one of the systems. Stress relaxation data for all samples over a temperature range of 23°C to 100°C are presented, together with differential scanning calorimetry and dynamic mechanical spectroscopy data. It was found that the state of hard segment (amorphous or crystalline) influenced the reaction of the polymer to stress. Also, stress relaxation and creep performance were improved by the addition of 25% glass fibre.
composite have been compared. Measurements of the coefficient of expansion were made using strain gauges, thermal mechanical analysis and an optical dilatometer. Good correlation between the three independent methods was found for the thin composites. For the thick composites, the expansion coefficient as measured by strain gauges was found to be dependent on the position of the gauge. This was thought to be due to the formation of localized stresses. Furthermore, measurements of the thick composite using the optical dilatometer revealed a large amount of hysteresis, which was considerably reduced on post-curing. The authors concluded that this was the result of residual curing stresses being annealed-out during post-curing`
Thermoeinstic behaviour of thick glass/epoxy composite laminates Raghava, R.S., Valentich, J. and Nathenson, IL D. Journal of Composite Materials Vo118 (1984) p 81 The thermal expansion properties of a thick (54 mm) and a thin (5.4 mm) glass/epoxy
Three-phase syntactic foams Narkis, M., Kenig, S. and Puterman, M. Polymer Composites Iiol 5 No 2 (April 1984) pp 159-164 Three-phase syntactic foams comprising a close-packed microballoon bed, binding resin and air voids were prepared and their mechanical properties investigated. Modified modulus-composition equations are shown to
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be applicable for the prediction of moduli values of these three-phase systems.
Transverse tensile strength anisotropy in thick filament wound RINA composites Miyase, A. Journal of Materials Science Vol 19 (1984) pp 923-928 Transverse tensile strength in glass/epoxy thick filament wound ring composites is found to depend on specimen size and orientation due to the presence of elongated 'pancake" shaped voids between strands. Winding atmosphere also affects the properties. The Mode I KIC value is measured by width-tapered double cantilever beam tests on specimens of three different orientations. Therefore, the elongated voids will give different effective flaw sizes depending on specimen orientation. Transverse strength anisotropy and specimen size effects are caused by differences in void content, shape, size and distribution. Linear elastic fracture mechanics is applicable to the correlation of flaw size and transverse strength in a qualitative way only, due to complexities arising from void distribution and geometry.
l A2n
Abstracts of British Patents are prepared by Dr F.R Jones, Department of Materials Science and Engineering, University of Surrey, UK
U K PATENTS COMPONENTS Plastic transmission brake band (D.A.B. Industries Inc, USA) GB 2 122 132 A (11 January 1984) A continuous filament (eg glass fibre) reinforced epoxy resin strap, with a bonded friction lining, is described. Fibre reinforced plastics leaf spring (The Budd Company, USA) GB 2 125 514 A (7 March 1984) An FRP leaf spring of constant cross-sectional area tapering to the ends but thicker in the middle, with an appropriate reccsss, is described. The recess can be used to locate the fixing pins without damaging the composite. Various assemblies are described, one of which includes a metal leaf spring to facilitate transverse mounting. A rotary filament moulding technique is also described. Glass and/or vitreous ceramic laminate system for microwave shielding applications (Carl Zeiss Stiftung` FRG) GB 2 125 732 A (14 March 1984) A metal microwave shielding grid of various types is sandwiched in a plastic matrix between two glass panels which form a microwave oven door. Dry friction composition (Borg-Warner Corp, USA) GB 2 126 594 A (28 March 1984) An aramid pulp/glass fibre-reinforced rubbermodified phenolic resin laminate for use in clutch facings is described.
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Ferrite containing fibres and fabrics (TDK Corp, Japan) GB 2 127 739 A (18 April 1984) A cloth is made by weaving slices from a polymer/ferrite/fibre composite. These cloths can be further laminated with a ferrite/resin mixture, thus making the manufacture of complex shapes possible.
MATERIALS Structures of reinforced impregnated graphite and their production (Le-Carbone-Lorraine, France) GB 2 120 975 A (14 December 1983) Resin-impregnated graphite for heat exchangers etc is reinforced at the surface by a phenolicresin based, woven CFRP. Fibre-reinforced composite materials (Dart Industries Inc) GB 2 123 838 A (8 February 1984) The permeability of plastics housings, for electronic equipment, to electromagnetic interference is reduced by reinforcing with stainless steel fibre strands. Chopped random fibres are ineffective. Injection-moulded reinforced polycarbonates are exemplified. The strands are 3-30 m m long and 1.6-6.4 m m in diameter. Epoxy resins containing metal (Hitco, USA) GB 2 125 056 A (29 February 1984) The properties of epoxy resins can be improved by incorporating metal elements into the structure. Metallized (eg B,W and Zr) prepolymers are described. Thermosetting resin composition (Mitsui Toatsu Chemicals Inc, Japan) GB 2 126
240 A (21 March 1984) Bismaleimide resins with high temperature resistance (>400°C) and low moisture absorption are described. Heat resistant laminate (T & N Materials Research Ltd, UK) GB 2 126 526 A (28 March 1984) A glass/jute fibre phenolic laminate is described, which can replace an asbestos equivalent. The laminate configuration is found to be significant. Fibre reinforced resin molded articles (Ina Seito Co Ltd, Japan) GB2 130141A (31 May 1984) A laminated GRP article which uniformly reflects and shields electromagnetic waves comprises laminae of an electro-conductive non-woven fabric (prepreg of aluminiumcoated glass fibre), a woven fabric (eg glass fibre) and an SMC. The intermediate woven layer prevents defects from forming in the electro-conductive layer.
PROCESSES Reinforced concrete (Square Grip Ltd (UK)) GB 2 122 126 A (11 January 1984) A device for supporting the reinforcing rods during the casting of reinforced concrete is describe& Moulding composite articles; panels (KMMCO Structural Foam Inc, USA) GB2 122 130 A (11 January 1984) By intermittently introducing inert gas into the
COMPOSITES. OCTOBER 1984