Developments in the Science and Technology of Composite Materials

Aramid fibres are very tough and strong polymer materials. The modulus of elasticity of these fibres can range from 60 to 140 GPa, whereas the strength reaches a value of 4.5 GPa at 2.5 cm gauge length with an elongation at break of more than 4%. The origin for these properties is found in the intermolecular interactions resulting in a semirigid conformation of the poly(p-phenylene terephthalamide) chain. X-ray and electron diffraction studies have demonstrated that the structure is paracrystalline. On the basis of a single-phase structural model the elastic properties of this fibre have been fully explained.Another outstanding property of the aramid fibres is the low rate for creep and stress relaxation. Presumably this is due to the semi-rigid nature of the chains, the high crystallinity of the structure and the hydrogen bonding between the chains. For a better understanding of the visco-elasticity of the fibre it is necessary to know the structural phenomena that happen during creep and stress relaxation. A useful tool is provided by the dynamic compliance measurement, because in well-oriented fibres it is linear related to the second moment of the chain orientation distribution. Experiments have shown that during creep and stress relaxation a progressive contraction of this distribution takes place, which is caused by shear deformation of the crystallites. These results have lead to a further development of the series model, which now incorporates viscoelasticity.

The interest and activity in the carbon and ceramic composites field is the result of their new potentialities and increasing promises. These composites meet the requirements of “thermostability”, reliability and specific performances mainly imposed by the new projects of Aeronautical and Spacial Systems.Without being the only reliability parameters, thoughness, damage stress levels and thermochemical resistance of these new materials are now determinative for futher developments and uses.After an analysis of the microstructural rupture of these composites and of the evolution of models describing this rupture, this paper develops breifly the main aspects of carbon composites protection and ceramic composites manufacturing in order to emphasize: the rules governing the relations between microstructure and rupturethe consequences upon architecture, processing and using methods

Peculiar modes of TEM allowing to restitute the three dimensional arrangement of elementary scattering domains (microstructure) of a given material are described. Applications to carbon and ceramic (SiC) fibres and matrices are given as examples. Mechanical and electrical properties related to microstructure are considered mainly for carbon fibres.

Advanced materials is a new and growing business built upon substitution. Although materials scientists have the molecular understanding to design new materials, entry to the market is not guaranteed. Industry participants must face technological, economic and commercial challenges which can take between 20 and 40 years to overcome. The industry is also competitive and profitability is not assured. Nevertheless, significant progress has been made by polymer composites in aircraft structures, aero propulsion and marine applications and by ceramic materials in electronic and structural applications.

Using a series of di- and tetrafunctional prepolymers we have shown the role of non epoxide chain extremities and impurities present in the prepolymers on the reaction mechanisms and kinetics and the final thermal properties of the materials.From a mechanistic standpoint in the 100–180°C temperature range, epoxide/primary amine reactions dominate at the beginning; at the end of the reaction, epoxide/hydroxyl and epoxide/secondary amine reactions dominate. These reactions may be intermolecular (to form cross-linked networks) or intramolecular to form rings, especially at the end of the heating cycle.On prepregs we were able to show the non-negligible role of the fibre in reaction kinetics but change reactions mechanisms only slightly.

Polybismaleimides are a class of thermosetting resins which, due to high cross-link density and aromatic backbone structure, show excellent temperature stability and outstanding hot wet performance. On account of the brittleness of the Bismaleimide resins, they have to be modified to achieve tough networks. This work describes chemical concepts to improve the fracture toughness of Bismaleimide resins, such as — addition of reactive elastomers - Michael addition chain extension — copolymerisation with allyl terminated comonomers — copolymerisation with propenyl terminated comonomers — modification with thermoplastics. Depending on the modification method, fracture toughness improvements of up to twentyfold compared with unmodified Bismaleimides can be attained.

Improved bonding of fiberglass, Kevlar and carbon fiber to a variety of thermoset resins has been achieved via the addition of minor amounts of organotitanate and/or organozirconate coupling agents. Peel strength, fiber pullout and chemical resistance data is provided to demonstrate significant improvements in bonding between these fibers and epoxy, polyurethane, polyester and vinyl ester resins.

The influence of such thickening agents as CaO; MgO; Ca(OH)₂ and Mg(OH)₂ on the viscosimetric behaviour of preimpregnated polyester samples (SMC), has been studied. The results of these studies, allow us to conclude that, even though with all the agents the viscosity increases, the MgO is the most effective, as it originates the faster kinetics of viscosimetric increase, compatible with most of the references of transforming through usual moulding. In addition we try to present and discuss some results we have obtained to prove the influence of amount of water and other of the preimpregnated SMC preparation.

Sol-gel η-alumina fibres containing a proportion of axially-oriented pores have been compared with others containing only random porosity. The former fibres possess better mechanical properties. A simple theory is presented to account qualitatively for these effects which are related to the optical birefringence of fibres containing some axially oriented pores.

Using polymeric fibre strength data from the literature and from our own laboratory tests, we show in the first part of the present paper that simple modifications of Poisson/Weibull concepts, resulting in a new failure probability function, can be used as a modeling scheme for the study of diameter effects on strength in a way at least as satisfactory as previously used LEFM-based schemes. In the second part of the paper a new experimental approach for the study of composite failure is presented. Specially prepared composite monolayer models were tested in simple tension under an optical microscope equipped with crossed polarizers and with a video camera. The potential usefulness of this approach for the characterisation of basic failure modes in fibre-reinforced composites, and as a probe of existing strength theories in such materials, is demonstrated.

Silicon carbide fibres of 100 µm diameter were tested in tension at various gauge lengths in air, embedded in two polymer matrices and with thin coatings of a polymer. It has been shown that the tensile strength distribution is strongly influenced by these variables. It appears that coating or embedding inhibits failure from surface flaws at strains which would otherwise have led to failure.

The sol-gel route to unidirectional fibre reinforced ceramic composites has been demonstrated. The technique has been modified to increase the solid yield of the sol using a particulate filler. After sintering the composites were found to be microcracked as a result of the constrained sintering and thermal shrinkage. The interfacial shear strength has been estimated from the average spacing of the microcracks, and found to be comparatively low for the carbon fibre reinforced composites.

Polymer pyrolysis as a route to fibre reinforced Si-N-C materials was identified and demonstrated. The synthesis and pyrolysis of polysilazane precursors were optimized with respect to ceramic yield and minimum bloating during pyrolysis. Solution infiltration of stacked Nicalon SiC fibre weaves was performed. Repeated impregnations/pyrolyses yielded fairly strong composites exhibiting non-brittle fracture behaviour, especially when a polymer free from low molecular weight oligomers was used and the fibres given a carbon coating.

SEP has developed composite materials built with refractory reinforcements (carbon or ceramic) associated with a ceramic matrix. These thermostructural composite SiC/SiC and C/SiC materials combine a good resistance to oxidation and to chemical agressions with a high mechanical behaviour, at high temperature and an exceptional toughness-compared to the classical ceramics. News applications have been found for these new composites in the fields of engines on one hand and for very high temperature structures on the other hand. Very large industrial installations have been implemented since 1986 in order to meet the production requirements yielded by these new applications under development.

Slow stable crack growth is a prominent feature of the fracture behaviour of fibre reinforced cement. The main mechanism for resistance crack growth during crack extension in this type of composite is microcracking ahead of the crack tip. This phenomenon is characteristic of a cement matrix, containing many pre-existing defects which will propagate during loading. Its effects can be calculated by applying linear elastic fracture mechanics to the microcracks. Fibres stabilise the crack propagation as long as they bridge the crack. A statistical study, based on fibre bundle behaviour, allows an explaination of the effect of fibre properties on crack growth resistance in this type of composite.

Pseudo-unidirectional composites made of SiC Nicalon fibers and LAS glass-ceramic matrix were obtained via a low temperature sol-gel/hot pressing route. The fibers have been coated with a thin layer of pyrocarbon in order to promote fiber pull out. The elastic constant Cij matrix and engineering elastic moduli at room temperature were derived from US wave propagation experiments. The failure strength was measured for different processing conditions through three point bending testing. The failure energy and dynamic toughness were calculated from instrumented CHARPY test data. Thermal expansion experiments were performed on both unreinforced LAS matrix and SiC/LAS composites.

The toughness of SiC fiber (Nicalon)/SiC matrix composites is increased when an interphase, made of a soft material (e.g. pyrocarbon or boron nitride) and playing the role of a mechanical fuse, is present at the fiber-matrix interfaces. The BN-interphase, deposited from a gaseous BF₃-NH₃ precursor, has been analyzed both chemically and microstructurally by TEM and AES, as well as the associated fiber-interphase and matrix-interphase interfaces. The sequence of materials observed at the fiber-matrix boundary is: SiC matrix/BN interphase/SiO₂/carbon/Nicalon fiber.

Examination of fibre-reinforced metal microstructures produced via a pressure infiltration route shows that for hypoeutectic alloys, primary metal dendrites nucleate and grow from within the interfibre regions, resulting in segregation of second phases to the fibre-matrix interface. If these intermetallics cannot be removed during a subsequent solution treatment then they may act as GriffitrTtype flaws and substantially reduce the tensile strength of the composite.

Thermal cycling tests were performed on the Si₃N₄ whisker/99.9% aluminum and /AC8A alloy composites obtained by squeeze casting, followed by tensile tests, observation of fracture surfaces and microstructures to examine the fatigue of matrices. Little change in tensile strength was observed for the specimen subjected to 1,000 cycles, except T6 treated AC8A alloy matrix composite. After 500 cycles, the effect of T6 treatment was lost. However, fracture surfaces showed somewhat brittle surface, which should be caused by locall plastic flow of matrice.

Reinforcement of silicon carbide whiskers improved significantly the elastic and strength behaviours of aluminium alloys. Unfortunately, the increase of the volume fraction of whiskers is responsible for a very low ductility of these M.M.C. and the sensitivity to stress concentration is very high. In order to use these materials in some structural components, we need to obtain a fracture criterion for the 2124 Al + 20% SiC_w at room and high temperatures (i.e.20°C to 350°C). After a tension and fatigue crack growth characterization, a damage micromechanics modelisation based on the microstructural characteristics such as the aspect ratio and the volume fraction distributions is established.

K₂ZrF₆ enhances the wetting ability of SiC (or C) fibers by liquid aluminum alloys at low temperatures due to (i) a dissolution of the alumina film by fluoride species, (ii) a local evolution of heat related to exothermic reactions and (iii) the formation of a new liquid phase. The K₂ZrF₆ process results in a lowering of the UTS reinforcements due to an activation of the kinetics of formation of Al₄C₃ crystals at the fiber surface. Based on a theoretical modelization, it is established that the impregnation ability of SiC-SiC preforms by aluminum is significantly improved by a K₂ZrF₆, treatment applied to the preforms prior to casting.

The properties of multifilament SiC/Al composites can be enhanced when they are ‘hybridised’ by small additions of particulate or whisker material. Previous studies have shown that a mechanism for this improvement is the reduction in the number of fibre contacts. The present work investigated other possible mechanisms. It showed evidence for the reduction of meniscus penetration defects and for the importance of the increased availability of the reinforcement/matrix interfacial area in reducing interface related degradation. In the Al-Cu matrix system this interfacial effect was manifested in the distribution of intermetallic in the composite.

The advantages of whisker reinforced magnesium are stiffnes, improved hardness, good wear resistance, reduction in thermal expansion and high strength: weight ratio. The disadvantages are expense of whiskers, high production costs of composites as well as health problems due to the carcogenic effects of fine whiskers. In this paper the preparation of whisker strengthened magnesium is demonstrated on the system magnesium-SiC or potassium-titanate whiskers. The properties of such composites eg modulus, strength, wear resistance, hardness and thermal expansion are discussed and compared with commercial Mg-SiC-F9-whisker composites (ARCO).

A vibratory treatment is proposed with a view to obtaining the controlled unidirectional orientation of short reinforcing ceramic fibers in Metal Matrix Composites manufactured with Powder Metallurgy methods. The treatment is applied to the metal-powder and fibers mixture before compaction and sintering, which results in the unidirectional orientation of the fibers that may either be uniform in the whole machine part or differ considerably in different zones. The effectiveness of the proposed mechanism has been proved over a wide range of experimental parameters, various vibratory systems and different powder fiber mixtures being used. The dynamic response of the mixtures as well as their dependence on frequency, acceleration, wave form, amplitude, and energy are discussed.

A review of titanium matrix processing techniques is presented. Among the C.V.D. reinforcements commercially available, those coated with a thick protective layer (e.g. SCS-6 from AVCO or B₄C/B from SNPE) are the only ones which strengthen effectively the titanium matrices. A synthesis of the chemical and mechanical (static and dynamic) behaviours of 1D-SCS-6/Ti-6AI-4V and 1D-B₄C/B/Ti-6AI-4V elaborated by vacuum hot pressing (V.H.P.) is given.

Little attention was paid to zinc matrix composites in the past, due to the high density of zinc, but its very low melting temperature allows a very easy manufacturing process and thus its industrial use as a structural material may be expected in the future.In the case of zinc matrix reinforced by stainless steel filaments, a comparative study is performed in order to establish a relationship between manufacturing process and the mechanical features of this material.In this paper, samples are manufactured by hot press under vacuum of SS/Zn monolayers. The matrix included in such monolayers was apported via electrolytic deposition or by liquid metal infiltration. Just before incorporation in the zinc matrix composite, the stainless steel filament could optionally be surfacially activated by pickling in a bath having controlled acidity.The mechanical performance of SS/Zn composites is strongly influenced by the duration of the sintering period and activation of the steel filament. The fracture energy depends on the failure mode (transfibrilar or flexure breakdown) and surface pre-treatment of filaments. For electrodeposited matrix, short sintering periods yield better toughness, but for infiltrated matrix three hours consolidation are recommended.

The temperature dependence of creep in Al/20% SiC composite has been studied at 150°C to 350°C. The material shows a high stress sensitivity and large temperature dependence of creep strength. The stress levels required for creep rates of 10−6s−1 – 10−7s−1 lie above the first deviation from linearity of the tensile stress strain curve. High temperature creep rupture occurs by failure of the Al matrix. Voiding about the SiC reinforcement is seen at lower temperatures.

A magnesium alloy containing about 11wt.% lithium is shown to be attractive as a matrix for composite materials. The degradation of carbon, alumina and silicon carbide in this matrix is examined, and the effect of lithium on silicon carbide monofilament and multifilament is examined using a vapour phase impregnation technique. Only silicon carbide whiskers are found to be stable in this matrix, although a barrier layer of yttria is shown to protect silicon carbide monofilament from attack.

Preform production, squeeze infiltration and extrusion procedures are described for the preparation of single phase (β) Mg-Li alloys reinforced with SiC whiskers. Observations are reported on the generation of whisker alignment and second phase distribution as a result of preform binder dissolution and re-precipitation.Tensile test data are given, showing that the presence of whiskers can lead to considerable property enhancement. Dynamic stress relaxation processes appear to affect the tensile behaviour at room temperature over the range of strain rates employed during conventional testing. It is thought that this is associated with the very fast diffusion kinetics exhibited by the lithium.

Particle reinfored MMC’s are interesting in application in arduous enviroments. Advantages of these composites are increased modulus, strength, high temperature properties and wear restistance. The thermal expansion is reduced. In this paper the microstructures and properties of composites with different particle additions, eg Sic, TiB₂, Ti(C,N), AlN and Al₂O₃-platelets produced by powder metallurgy techniques are dicussed.

This paper presents the results from an experimental investigation into the age-hardening response of short δ alumina fibre reinforced 6061. It is shown that chemical effects degrade the age-hardening response of these composites. These effects result from subtle interactions between the active matrix alloying elements and the surfaces of the alumina fibres. Such interactions degrade the age-hardening potential of the matrix and reduce the peak hardness attainable from the composites. It is also shown that chemical interactions between the preform binder and matrix alloy can result in anomalous age-hardening

Composites based on the Al-7Si-0.6Mg alloy and reinforced by short SiC (NICALON) fibers were processed by rheocasting giving the matrix a globular microstructure. Tensile tests were performed at high temperatures with various strain rate. The presence of fibers impeds the plastic deformation giving rise to strong strain hardening effect and low strain rate sensitivity compared to the unreinforced matrix deformed in the highest range of temperature. The observation of a stronger activation energy for the composites is correlated with the modification of residual stresses within the matrix.

For the oxidized carbon fibre-EVA system, the existence of an interphase layer between the fibre and the matrix, having an elastic modulus close to the one of the elastomer in its glassy state, can explain the results obtained by fragmentation on single fibre composites. In the present study, viscoelastic measurements on unidirectional composites confirm the existence of such a layer. Hence, the mechanical (average modulus of elasticity) and physical (thickness) properties of this interphase are determined by means of different theoretical approaches. It appears that all the theoretical analyses are in disagreement with experimental observations.

Electrochemical grafting of amino groups at the surface of carbon fibres (high strength and high modulus) is highly described. Amino groups are covalent bonded to carbon atoms forming a thin and dense surface layer. They are able to react, for instance, with epoxy groups of an organic matrix. We compare toughnesses of composite materials containing untreated or treated (oxidised, aminated) carbon fibres. It is shown that aminating surface treatments are a new way to improve brittle matrix composite material toughness or to get a better adhesive matching between matrix and fibres.

Reactive chemical vapour deposition (RCVD) has been used for the coating of the single filaments of a PAN-based carbon yarn by SiC. A continuous process has been developped, it consists to react a mixture SiCl₄-H₂ with the fiber, at T 1000°C, under, atmospheric pressure. By the aid of thermodynamic calculations and experimentation, the conditions of RCVD (gaz composition, temperature, reaction time) are optimized to elabore a coating without degrading the fiber. It is shown that a SiC-β coating, 0.05 micron thick, modified slighty the characteristics and is sufficient to reduce the rate of combustion of the fibers by a factor closed to 100.

Procedures are described for the formation of thin layers on monofilaments by sputter deposition, with the aim of controlling high temperature degradation by interfacial chemical reaction. Factors affecting material selection and coating structure are briefly considered. Advantages are identified in having a relatively high fibre temperature during deposition, in encouraging stored stored compressive stresses and in generating a duplex coating structure, comprising an inner layer of metal and an outer layer of oxide.

Pyrolytic SiC deposits were performed on Nicalon 202 SiC fibers using either a static method designed for fiber structure infiltration or a dynamical method designed for fiber coating. A Carbon deposit was sometimes introduced between the fiber and the SiC matrix. The coated fibers were analysed by Secondary Ion Mass Spectrometry (SIMS). The depth profiles obtained show that (i) the fibers retain their composition after the process (static or dynamical), (ii) the deposits prepared using the static method exhibit an heterogeneity of microstructure and composition; these changes are due to a transitory stage at the beginning of the CVD reaction and (iii) the deposits prepared using the dynamical method exhibit a constant carbon content higher than the stoechiometric one (C/Si = 1.4).

The effect of aqueous conditioning on the bond strength of AR glass fibre-vinyl ester resin composites has been investigated using the embedded single filament tensile test. Two fibre coatings were compared, one containing A1100 (γ amino propyl triethoxysilane) silane coupling agent and the other without. The results provide evidence for the existence of physisorbed and chemisorbed layers on the fibre surface. The former was removed by immersion in warm water whereas the latter is tenaciously bound to the surface, providing protection against boiling water and sulphuric acid.

The fracture properties of short glass fibre reinforced polypropylene were studied in relation to micro-structural changes caused by annealing at higher temperature. Two types of injection molded composites were compared. The microstructure was characterized with help of X-ray diffraction and DSC methods. Fracture energy was evaluated on base of notched impact tests, morphology of fracture surfaces by means of scanning electron microscope.

PAN-based (T300) and Pitch-based (P55 and P100) carbon fibers were isothermally heat-treated for 1000–120h at 450–700°C in saturated vapour pressure of Mg. Characterization of the resulting samples by mechanical testing, XRD, SEM, EMA and chemical analysis led to the conclusion that the chemical compatibility of carbon fibers towards Mg depends on their nature: pure and highly graphitised Pitch-based fibers exhibit an excellent inertness towards Mg whereas impure and disorded Pan-based fibers may slightly react with this metal.

Fracture experiment were carried out on unidirectional glass epoxy composite material with the presence of a crack parallel to the fiber direction. Fatigue tests were performed using tensile split specimens, the crack being parallel to the fiber direction. Split specimens loaded in fatigue tension parallel to the fibers were used. A damaged zone consisting in fiber bridging the major crack was observed and microfractographic observations revealed that the crack extension lad to mode one fracture mechanism. A three dimensional finite element method was employed to calculate the values of the strain energy release rates.Crack growth rates versus ∆K₁ and ∆K₂ were studied and compared to those of pure mode I and mode II obtained with DCB specimens. It was shown, that this “analysis” corrolate microfractographic observations.

A Mixed Mode crack analysis is applied to simulate the failure mechanism of the Iosipescu shear test for a fibre-reinforced composite. The stability of the curvilinear crack trajectory appears to be sensitive to the ratio between the principal strengths. For a strongly orthotropic material, the zig-zag trajectory results to be alternately parallel to the fibres or directed towards the loading point.

Static indentation laws for laminated composite thin shells and a rigid ball are proposed in this research. The contact behaviors of the rigid ball and target shells are divided into three stages i.e., the loading, unloading and reloading stages. While a 2.5 power is suggested to use in unloading stage, it is found that Hertzian type 1.5 power law is adequate to represent the loading and reloading paths. The effect of the curvature is considered by imposing an unknown function in force-indentation equation for describing the contact behavior at loading stage. This unknown function which characterizes the curvature effect is obtained by modifying the Hertzian law. Once this unknown function is determined, the unloading and reloading equations could be derived from it.

A finite-difference version of the Dynamic Relaxation (DR) method is used to generate elastic solutions of the large deflection laminated Mindlin plate equations. These solutions are combined with the Tsai-Hill failure criterion to produce initial failure data for simply supported and clamped, specially orthotropic and cross-ply laminated square plates subjected to uniform pressure loading. Pressure and deflection versus (slenderness)2 and span: thickness ratio plots are presented in order to quantify the effects of: plate thickness, in-plane edge restraint and membrane action on the initial failure response.

Results of experimental and analytical study of the post-buckling of selected flat stiffened Graphite/Epoxy panels, loaded in axial compression are presented. The initial buckling, post-buckling and failure characteristic are defined and described. The axial load pattern included static test to failure and cyclic loading up to a total of 250,000 cycles. The analytical part consists of results from NASTRAN — a finite elements package, PBCOMP an in-house developed program which performs linear and non-linear buckling analysis and approximate methods.A reduction in the initial buckling load after cycling with an increase in the failure load was observed The study proves that the performance of such structures is only slightly affected by cyclic compression.

A finite element approach to panel flutter problem of finite laminated plate has been developed. Linear small deflection structural theory and quasi-steady aerodynamic theory are employed for the analysis. In order to study the effect of panel configurations on the flutter behavior of a composite laminated panel, a fully compatible general quadrilateral plate bending element is used. The effects of fiber orientations, stacking sequences, anisotropic properties, boundary conditions, aspect ratios, and panel configurations are studied. Calculations show that the flutter stability can be improved if the fiber is properly orientated.

Rail vehicle structural bodyshell designs are usually welded fabrications of aluminium or steel, the plasticity of which dominates the energy absorption when structural collapse occurs during severe frontal impact. Composite materials have been shown to offer significant increases in energy absorption, when compared with metals during controlled collapse regimes. This experimental work confirms the advantages of such materials, at force and energy levels compatible with rail vehicle design constraints and identifies the influence of geometry on cylindrical energy absorbing modules manufactured from glass fibres and polyester resin.

When an end fitting is bonded to a composite tube it is normal practice to use thick joint theory to estimate joint shear stresses. Because a composite is much stiffer in the longitudinal than the through thickness direction, the whole tube acts as the joint. Thus the peak stresses depend on the geometry of the joint and the material properties. Mathematical analysis and finite element analysis illustrate stress curves. Consideration is given to an end fitting comprising a mandrel and cuff-nut with mechanical locking.

A computer code, designated as “PDHOLEC”, was developed for analyzing compression failure of laminated composites containing an open hole. The code was developed based on the progressive damage analysis which was proposed by the authors. The analysis consists of a stress analysis for calculating stresses and strains inside the laminates, and a failure analysis for predicting damage and determining the residual stiffnesses and strengths of the laminates. Not only can the ultimate load of the laminates be determined by the code, but also the types and the extent of damage as a function of the applied load can be evaluated. The code was verified extensively by experimental data. The code can be used as a design tool for designing and sizing laminated composites containing holes and subjected to compression loading.

New technical opportunities with regard to manufacturing and mechanical properties are opened up by the development of new integrally woven sandwich structures. In these fibre constructions, two fabric layers are separated by a woven-in system of z-directional fibres. After impregnation with a resin, the double-wall fabrics form the sandwich structure. The mechanical properties can be adjusted in a wide range through the number and the arrangement of the thread-linking system. A comparison with foam and honeycomb sandwich structures shows the potential of this new material class.

The extension of the Multi Live-feed Moulding (MLFM) process to four independently controlled live feeds is described. Three mould cavities, a rectangular plaque, a tapered fin and a circular ring are used to illustrate the degree of control over the orientation of fibres that is possible using the MLFM process. Contact microradiography and mechanical test results are used to illustrate the enhancement of fibre orientation control and mechanical properties that is possible with injection moulded glass fibre reinforced thermoplastics.

Fiber reinforced silicon carbide, silicon nitride, and mullite composites, derived from continuous fiber, have been developed for applications requiring extreme toughness. They consist of unidirectional carbon fiber reinforcing matrix of the composites. The composites are fabricated by a process consisting of slurry impregnation followed by hot pressing. Measurement of sintering characteristics, identification of crystal phase and observation of metallographic structure in the composites are carried out. Flexural strength and fracture toughness of the composites are measured.

The weight reduction due to the utilization of the S.M.C. makes this material appropiate for several applications. The final property of a S.M.C. product depends on the process employed for the transformation. Therefore it shall be interesting to know how the processing parameters affect the mechanical and thermal properties of the product. This work intends to study the effect of mold pressure mold temperature and cure time on the flexural properties and on the Tg of the material.

Heat transfer characteristics of gas turbine blades can be measured in an intermittent short steady-pulse wind tunnel. Such a tunnel has been constructed at RAE Farnborough in which a stable hot gas pulse is generated by an ultra-lightweight, large diameter piston. Manufacture of such a piston is made possible by composites fabrication techniques recently developed for aerospace applications. Two pistons have been fabricated and found to exceed their design and operational requirements.

This paper will show a state of the art idea in the sector dealing with the design of domes that are accomplished in Filament Winding (F.W.). Later they can be used for either a pressure reservoir (only membranal loads) or as a covering for the rocket’s motor (membranal and concentrated loads). While undertaking the study of winding in the above cases we used two different hypothesis: without friction between the fibers and the underlying surfaces as well as conditions with friction. A computer aided winding system of composite structure has been developed. Such approaches opportunely adapted, can be utilized to determine the superior winding method of an assigned axisymmetric design.

It is shown how experimental data on composites and the component fibres may be used to make inferences about the stress concentration factors and length of stress overload region. The method employed is numerical maximum likelihood, involving detailed combinatorial calculations, and is therefore highly computationally intensive. The method is illustrated using experimental data on hybrid composites consisting of carbon fibre tows embedded in glass-epoxy composite, particular emphasis being placed on the consequence of varying the distance between tows.

This paper describes the activities of a task group of the European Group on Fracture examining interlaminar fracture tests, with a view to standardization. Despite the increasing use of such tests to characterise composite material toughness no widely accepted standard test procedures exist. In order to highlight contentious areas which might hinder the adoption of a standard, a group of university and industrial laboratories from 9 countries have been involved in two series of round robin tests on four different materials, over the last two years. This has enabled test protocols to be drafted for mode I and mode II tests, which are currently being examined in a third series of tests.

This paper deals with the evaluation of modal characteristics of a carbon fiber reinforced laminated panel in the range of frequencies 0–200 Hz. Although composite materials are largely adopted because of their specific properties in structural parts of many kind of systems, their dynamic behaviour is not fully investigated. In this work a single point excitation technique is adopted with impulse and random excitation devices, in order to carry out a modal parameter extraction of the panel under test. Experimental results were processed by a FFT spectral analyzer and finally rearranged to get information on eigenfrequencies and structural damping coefficients. A comparison between the results obtained with impulse and random excitation is also made.

Two types of advanced carbon fibre reinforced thermoplastic have been tested and the effects of fabrication conditions on their microstructures revealed. The APC-2 composite is revealed to bond extremely well to the AS-4 carbon fibres and this, as well as transverse properties, are improved by heat treatments. This behaviour is shown to be due to the carbon fibres acting as nucleation sites and promoting transcristalline growth. The AC40-60 composite showed no such behaviour and subsequently had poor interfacial properties. Heat treatment of the PPS polymer increased crystallinity due to an increase in the proportion of low molecular weight polymer.

The existence of a strength functional, which is a function of the stress tensor, is assumed. The invariants for the strength function for each class of composites in the third-order approximation are established. Consequently, the strength functions proposed by Tsai and Wu for triclinic and rhombic materials, and by Gol’denblat and Kopnov for rhombic materials can be obtained readily.

Linear flexure-response-studies (EFlex), through-thickness fracture toughness tests (Kc) and interlaminar mode I and mode II fracture energy measurements (GIc, GIIc) were carried out with different laminates of a carbon, aramide and glass fiber/thermoplastic polyamide 12 composite system. Specimens were prepared from fiber bundles interspersed with polymer powder and surrounded by a polymer sheath. The results, which must be considered as prelimary data because of very limited availability of specimen material, reflect an overall good fracture toughness profile of the different laminates tested. This is also highlighted by the fracture surface micrographs achieved with SEM-analysis.

The volume fraction of fibres in glass/epoxy unidirectional laminates was varied by inserting layers of the epoxy film (Ciba Geigy 913 resin) into the lay-up. Tensile-tensile fatigue tests were carried out on composites with fibre volume fractions of 69% and 47%. Under a given stress, the composite having the lower fibre volume fraction exhibits significantly longer fatigue lifetime. The micromechanics of damage accumulation and failure has been investigated in order to explain this beneficial effect of resin-rich regions on the fatigue performance of UD laminates.

It is evaluated the static and fatigue properties of fibre dominated glass reinforced plastics with polyester, vinylester and epoxy resins, in plain specimens and in presence of holes and bolts. In spite of the quite different aspect of fracture surfaces, matrix quality do not influence the static nor dynamic strength. In bolted joints, failure mode shift from bearing failure in static tests to tensile failure under dynamic conditions, meaning that bearing strength does not degrade at all, or at least do it at a much minor rate that tensile strength. Fatigue degradation rate is more severe for plain that for notched specimens.

Composite materials need a good stress transfer between the fibres and the resin. Therefore, carbon fibres are surface treated. The influence of this surface treatment on the mechanical properties and the damage development in a cross ply (O₂°, 9O₂°)S carbon epoxy laminate during monotonic tensile testing was investigated. It is shown that the mechanical properties of a cross ply laminate improve for a low surface treatment level, while they decrease strongly for higher treatment levels.

Three-dimensional effective moduli are used in the analysis of thick laminates consisting of a large number of identical sublaminates. A global-local method is developed in which the effective modulus theory is used to model the thick laminate except for the local region where accurate stresses are desired. Evaluative examples involving free edge stresses and interlaminar cracks are presented.

For a well-founded failure analysis of parts made of composites we are still lacking sufficient information, especially in the case of materials with a brittle matrix. This study therefore was aimed at investigating the influence of loading conditions on the fracture surface structure of a carbon fibre-reinforced bismaleimide. The results have revealed that also in the case of brittle matrix materials an allocation of fracture surface structure characteristics to conditions existing during damage development is possible.Fracture surface structures in carbon fibre-reinforced bismaleimides partly differ from those in carbon fibre-reinforced epoxy resins.

Particle reinfored MMC’s are interesting in application in arduous enviroments. Advantages of these composites are increased modulus, strength, high temperature properties and wear restistance. The thermal expansion is reduced. In this paper the microstructures and properties of composites with different particle additions, eg SiC, TiB₂, Ti(C, N), A1N and Al₂O₃-platelets produced by powder metallurgy techniques are dicussea.

The evolution of matrix cracks during incremental monotonic loading was followed using in situ radiography and was documented as a function of strain. Three commercial available matrices with a matrix failure strain ranging from low to very high were selected. From these tests can be concluded that the matrix properties greatly influence initiation and propagation of matrix cracks.

Composite materials reinforced with continuous fibres offer superior tensile properties under static as well as fatigue loading. However, under compressive loading the properties seem to be not as pronounced. Buckling of the test coupons falsifies the results in a way that not the real material properties, but specimen geometry, laminate lay-up, fibre diameter or even the unsupported length of the test coupons determine the results achieved /1–3/. While under tensile loading the composite properties are mainly dependent on the fibres, under compression the matrix gaines more and more influence.

This paper presents an experimental evaluation of static strength and damage growth resistance under spectrum loading of impacted carbon fibre reinforced epoxy sandwich panels. The program includes determination of the influence of impact energy on damage visibility, damage size and distribution, and strength losses with respect to different skin thicknesses and core densities. In the case of barely visible impact damage the susceptibility of sandwich panels to impacts is reflected in large strength reduction and furthermore in damage growth during fatigue loading. The visibility of damage during the fatigue loading has been subsequently reduced. Damage growth and failure also occurred for non-visual impact damages.

Some results of static and fatigue tests in complex stress states of tension and torsion are shown. The static capacities of three glass/epoxy materials are compared. Finally, the sensitivity of the average stress in the form of isonumber cycles to failure curves are shown.

CFRP specimens, containing an idealized discontinuity, were subjected to static tension tests and an impact damaged CFRP stringer stiffened panel was compression fatigue loaded to show some of the capabilities of the ultrasonic-insitu method. H-scans obtained with this method show a feasible way to correlate stiffness degradation and ultrasonic attenuation measurements.

Glass/polyester materials of high quality have been studied under long time fatigue loading, with special reference to the associated stiffness and strength changes. The continuous recording of stiffness parameters during fatigue testing constitutes a non-destructive method by which microstructural changes can be monitored. The combined information on the material modulus E and the secant modulus Eg allows a schematic stress-strain curve to be invoked.

The skin-to-core adhesion strength in sandwich panels is very often the weak link in composite structures. For this problem, a novel solution has been developed, based on a new type of fabric. A three-dimensional fabric is woven in one step; pile threads connect the top and bottom twodimensional fabric. When the 3D-fabric is impregnated, cured and filled with a PU-foam, a sandwich structure with interconnected skins is created. Preliminary mechanical test results are encourageing.

For structural applications an essential problem is the analysis of the long term behaviour under a general, mechanical and environmental, loading history. This durability analysis, or life-time prediction, must be performed on the basis of a certain number of tests carried out on limited, and if possible short, time scale by the use of accelerating factors. Acceleration of the thermomechanical behaviour of polymer based systems can be realised by temperature and stress level. In order to analyse the results of such tests nonlinear viscoelasticity theory must be used.

A comparative study of durability in distilled water of two various polyester laminates reinforced by fibre glass type E is presented. A three point bending apparatus has been developped allowing immersion of samples in distilled water at various temperature and at high stress level. (30, 50 and 70% of the nominal breaking point). Results show that tetrapolyester laminate have a least resistance (failure after quite short time and elevated rate of deflection) than isopolyester laminate. At same level stress, time rupture of samples and deterioration mechanisms depend of the orientation of the fibre. The prediction of the long time comportment seems more exactly by use an Arrhenius’s model than by extrapolation of tests realized at high stress level.

The design of a new thermal fatigue unit based on semiconductor thermoelectric devices to provide rapid and programmable thermal cycling is described. The unit has been calibrated to measure the temperature-time response of a typical laminate to establish the optimum conditions for short cycle times. The effects of cycling between -50oC and +50oC on a range of cross-ply carbon fibre/BMI laminates has been studied. A qualitative analysis of the effects of thermal cycling on three [0₃/90₆] laminates is presented.

Cross-ply cracking in specimens out of the laminate 0/90₄/0 was investigated in dry condition and after saturation with water at RT (moisture uptake 1.44%) in the temperature range of −100°C to +100°C.These investigations resulted in Weibull transverse fracture strain distributions, with which the strain at first ply failure (FPF) and at interface failure was determined. At increasing temperature in the dry material two conflicting phenomena influence transverse cracking: these are the ductility of the matrix and the strength of the fibre-matrix interface. The improvement of ductility increases the transverse fracture strain, whereas the reduction of the interface strength reduces the transverse fracture strain. For the strain at FPF the improvement of ductility dominates, except in the range between RT and 60°C where the interface strength drops considerable. Saturation with moisture at RT and testing at RT leads to a further improvement of ductility and an additional decrease of interface strength. The overall effect on the strain at first ply failure is positive. The wet material tested at −100°C showed both an increase in strain at FPF and at interface failure.

When Westland Helicopters realised the potential of epoxy based composites for structural applications, a programme of work was instigated to investigate the properties of aged composites. After some initial work on ageing specimens artificially, a three stage technique was adopted to accelerate the ageing process. This method was chosen because it would result in a specimen with a consistent moisture content throughout the laminate. The conditions required to achieve this were predicted using a computer programme but it was found that the empirical results differed considerably from those predicted because the matrix system employed for these laminates does not follow Ficks laws of diffusion at higher temperatures.

The kinetics of thermal degradation and its consequent effects on the mechanism of microcracking in carbon fibre composites fabricated from PMR-15, an end-capped bismaleimide resin system, have been studied under oxidative conditions. The overall degradation involves three stages. For the unidirectional laminates, oxidative decomposition of resin appears to control the micromechanics of their durability, whereas changes in thermomechanical properties of the matrix seems to be responsible for the formation of transverse cracks in the cross-ply laminates.

The presence of moisture in a composite causes the composite to expand and to lose some of its strength. In composites made with glass or carbon fibers, the moisture is absorbed by the matrix alone. In aramid-epoxy composites, the moisture is absorbed by both the epoxy matrix and the fibers. It has been proposed that a surface treatment of the aramid fiber may improve the hygromechanical properties of aramid-epoxy composites. An analysis of these properties is made and a correspondence between the location of fracture and the local moisture content has been established.

New considerations about fatigue behaviour of GFRP through the roles of the interfaces are presented. For bending fatigue tests, interfaces are described as privileged ways of cracking. Developed from the main idea of the “resistance of the materials” to the cracks nucleated at the very first loadings, this work analyses the subsequent growth of cracks and damage: weaker shear strengthes in interfaces can induce higher lifetimes.

Fretting wear and fretting fatigue often limit the lifetime of structural parts under cyclic loading. Although fibre reinforced plastics are widespread in such applications, there is only little known about their fretting wear and fretting fatigue performance. The present paper investigates the influence of an additional fretting component on the fatigue behavior of a CF/EP — laminate. A quantitative model for the degree of fretting fatigue damage is proposed. Separate studies on the fretting wear of these materials are employed to explain the fretting fatigue results.

In dimensioning components made with composites one has to consider, besides the mechanical stress, the influence of the surrounding atmosphere. Moisture picked up by the polymer matrix system of composites diminishes the stiffness and therefore the thermal stability of the system. The moisture absorption as well as the influence of the moisture in the laminate on the physical and mechanical behaviour are discussed.

Moisture absorption in resins and composite materials has been an area of concern for a number of years. In most previous investigations a simple concentration independent Fickian diffusion process has been invoked to describe the moisture absorption into what is generally assumed to be a single-phase material. In this paper a model is developed which enables the diffusion characteristics of a two-phase polymeric material to be described, and the volume fraction of the phases to be calculated.

The hygrothermal aging induces physical and chemical damages into composites leading to lost of mechanical properties and change in the damage mechanism and fracture behaviour. This paper describes the mechanical behaviour under internal pressure with closed end pressure testing procedure of glass filament wound pipes. This paper reports the interaction between physico-chemical and mechanical damage mechanism and the effect of both ageing time and matrix flexibility on the mechanical properties of the tubes.

The different properties of the high performance composite materials may suffer certain modifications as a result of environmental effects. This report deals with the influence of the hygrothermal conditions over the static mechanical properties of a laminated made of an epoxy resin reinforced with carbon fibre. Previous to the mechanical characterization of the material, the kinetic of the moisture absorption under different conditions has been studied. This lead to a relationship between the modifications suffered by the laminate and the variation of the mechanical properties.

This study deals with the fracture mechanical behavior of composite materials consisting of different kinds of single fiber bundles embedded in a polymeric matrix. The fracture toughness and the failure mechanisms of these model composites are discussed in terms of fiber volume fraction and fiber/matrix interfacial bond quality. The reinforcement effect of the embedded fibers is described by a reinforcement effectiveness parameter “R_MV” The normalized fracture toughness of the EP-composite systems increases strongly with increasing R_MV, whereas an opposite trend was observed for the PC-composites. An improvement in toughness of the PC-composites could be achieved by impregnating the fiber bundles with a PC-solution before embedding.

The triggering mechanisms in energy absorbing glass cloth-epoxy tubes were investigated using microscopic examination supported by finite element analysis.It is found that the initial stages of the crushing process are dominated by the continuous formation of wedges that generate lateral cracks that cut off small rings of material. This process is at a later stage modified to a front-wedge-front geometry.

The micromechanics of model 0°/Sand/0° glass fibre hybrid laminates has been studied. The tensile moduli are closer to the upper rather than lower bound laws of mixtures predictions. The enhanced moduli associated with particle agglomeration in resin castings is maintained in the composite. The microdamage is shown to be analogous to transverse cracking in 0°/90°/0° composite by application of the shear-lag theory.

Experimental data on the compressive strength of glass carbon hybrids are presented. Carbon/glass ratios of 0, 12.5, 25, 37.5, 50, 75 and 100% were tested in 1 and 2mm thick unidirectional laminates. It was found that lower strength was obtained with carbon plies on the surface of the specimen. 8-ply were found to be stronger than 16-ply specimens, a result confirmed by a finite element investigation of the specimen stress distribution. The rule of mixtures was found to give a good estimate for hybrid modulus, provided the stiffness of the constituents was taken at the appropriate strain level.

A study has been carried out of the fatigue behaviour of hybrid composites of carbon/Kevlar-49 and carbon/glass in epoxy resin. For these two families of materials a comparison is presented of the properties of unidirectional composites and laminates with a [(±45,0₂)₂]_s structure. The unidirectional composites have been tested in repeated tension and tension/compression at R ratios between +0.1 and −1.2. All other materials have been tested in repeated tension fatigue only (R = +0.1). The results have been analyzed by normalizing relative to the elastic modulus and to the ordinary tensile strength, revealing a homogeneous pattern of behaviour for both families of hybrids.

A numerical study of the thermally-induced response of two widely used composites has been conducted. This study was performed using a newly developed numerical model which, unlike previous models, does not include the assumption of local thermal equilibrium between the solid matrix and gases generated as a result of the decomposition processes. The results of this study include temperature, mass loss, pressure and expansion profiles. These results were used to evaluate the effects of material composition and processing on the overall response of the materials.

Materials selection aimed at structural applications calls for notions of “efficiency factor” and “fibre/resin matching” that are based mainly upon a comparison of the constituents characteristics with those of the corresponding moulded composites. It is shown in the presentation that a full a better knowledge of the fibre/matrix relation at the interface. Example is taken of two widely used carbon fibres (T300 and AS4) associated with the Ciba 914 resin.

The damage behavior of composite material under compression load is, specially when there exists stress concentration, a very important factor in its design. The present paper is aimed at the comprehension of mechanism and the characterization of failure in a carbon/epoxy composite plate. In order to the usual difficulties in compression investigation, the mechanical tests were performed with compact specimen. The damage evolution was recorded to the variation of mechanical behavior, which yielded the criterion and law of mechanical behavior. Since the compression damage has a 3 dimensions form, the damage criterion were established in term of the energy by unit volume. The applications of these criterion shown that the linear fracture mechanic is a efficient tool for the characterization of the compression failure of the carbon/epoxy laminate materials.

In order to develop data and tools useful for the design of structures made of GRP, a programme of mechanical testing and a finite element code are currently being developed. An outline of the work conducted so far is presented.The influence of size, specimen geometry and applied strain rate on the mechanical properties of GRP has been studied. The fracture toughness of GRP has also been determined. K_c has been determined using SENT (single edge notched tensile) specimens and plates with a central notch. Three different types of K calibrations were used for the notched plates.The behaviour of this material in the tensile test, although non-linear, may be approximated by two straight lines with different inclinations. A simple finite element programme was developed to model this type of behaviour, and the study of a notched plate is given as an example of application.

For cost effective design of GRP composite structures in aggressive environments it is important that realistic safety factors are used which are based on testing experience and an understanding of the mechanisms affecting long term failure. Accepting the complexities of defining an accurate model suitable for all GRP systems, this paper utilises an expression for failure time of stress rupture specimens based on slow crack growth, which has been experimentally verified for two extremes of composite behaviour. The approximation is shown to give an acceptable fit to unidirectional and CSM glass composites in water at ambient temperature.

In this paper, we describe a two dimensional model of plates which takes into account a parabolic variation of the transverse shear strains through thikness, so that there is no need to use shear correction coefficients in computing the shear stresses. These properties are very important for the modeling of moderately thick laminated structures. The contribution of the present work is to take into account exactly the contact between the layers of the structure. The present method uses a classic displacement approach for higher-order shear deformation. The form of the displacement, field is dictated by the satisfaction of the following conditions: i) continuity of displacements and stresses at the interfaces; ii) disappearance of the transverse shear stresses on the plate surfaces while non-zero elsewhere. This requires the use of a displacement field in which the inplane displacements are expanded as cubic functions of the thickness coordinate and the transverse deflection is constant throughout the plate thickness. Consequently, the normal transverse deformation is discarded, because we want to have a displacement field which contains the same dependent unknowns as in the first-order shear deformationtheory. So, it is possible to obtain numerical results by a C1 finite element approximation for the displacements. In the same way, a higher-order model with the five classic independent generalized displacements, and which takes the normal transverse deformation into account, involves a C2 finite element approximation for the displacement /1/.

For a multilayered plate, the stress field can become singular in the neighbourhood of the interlayer/free edge intersections. The calculation of such stress singularities involves the determination of both the order and the magnitude of the singularities. The aim of this work is to describe methods that allow to compute these two quantities. At first, the orders are computed using a semi- analytical method and then, the intensity factors are expressed as the ratio of two energy terms and computed using finite element solutions of the free edge problem. Numerical results are presented.

A finite element formulation is presented for static and vibration analysis of multilayer composite plates. We consider a mixed plate theory including transverse shear deformation and stresses. The proposed element is based on the introduction of transverse shear equations in a discrete way. The element has three nodes and nine degrees of freedom. Numerical applications dealing with three and nine layers and sandwich plates show that our model is effective for a wide range of structures.

Intralaminar shear properties of four 40-ply unidirectional carbon-fibre reinforced polymer matrix composites were investigated using the Iosipescu shear test. The apparent shear strength and shear moduli were measured using specimens with two different fibre orientations. Finite element analysis was applied to determine the stress distribution within the Iosipescu specimen. The experimental shear moduli when corrected to account for the non-uniform shear stress distribution in the gauge section, were within 8% of micro-mechanics predictions. The failure modes in the two fibre orientations were different, resulting in a change in apparent shear strength.

This paper examines the application of interlaminar fracture testing to carbon fibre reinforced PEEK (poly-ether-ether-ketone) composites An experimental study of over 100 specimens is reported, in which the influence of specimen thickness, stiffness, width and defect type have been systematically varied. This has revealed a significant effect of specimen stiffness in certain cases, which must be taken into account if useful results are to be obtained. Following the optimisation of the specimen the influences of fibre type (AS4 or IM6) and rate of cooling after moulding (50 °C/min. down to 0.3 °C/min.) were studied.

Polydiacetylene fibres were placed within a carbon fibre composite laminate in the ply direction during the lay-up procedure. Laser light was guided to the polydiacetylene (polyDCH) by means of a fibre optic. After curing the laminates were tested in four-point bending and the internal ply strains measured. The results showed that strains could be measured at any specific point within the laminate, they were consistent and the technique proved to be repeatable.

The technique of Laser Raman Spectroscopy (LRS) applied in the study of fibres, composite interfaces is reviewed in this paper. An optical mechanical strain gauge has been developed for the determination of the internal strain in composite materials. The strain sensitive property of the high performance fibres is a vibrational frequency which may be determined by LRS. Examples are given for applications such as fibre characterisation, strain mapping in composites and examination of the strength of fibre/ matrix interfaces.

Acoustic emission (EA) is a technique by means of which the inspection specifications (CARP Code) for composite materials have been produced over the last few years. This article describes the methodology used for implementing the “CARP” procedure concerning the qualification and in-service tests of tubular structures of glass fiber reinforced resin. The results obtained are also shown by comparing products from different suppliers.

The development of microdielectrometry has made possible production monitoring and cure control in a variety of thermoset resin based systems. As the demands of the technology grow, existing sensors must be demonstrated to have applicability or new sensors must be designed.

Laser moiré interferometry is a recently developed optical technique which allows high resolution measurements of deformation fields in stressed materials. This paper describes the technique and illustrates its use in strain concentration measurements around circular holes in CFRP laminates and in a stressed adhesive bond. In the latter example the initiation and growth of damage have been successfully followed.

Ultrasonic non destructive evaluation of thick glass fiber reinforced plastic composites was carried out using a complete waveform digital acquisition and analysis technique. It is shown that this method yields detailed information on the entire material volume, providing for the accurate localization of defects. The availability of complete waveform data files makes it possible to generate images acting as interfaces between user and ultrasonic database and to statistically process the data to obtain information on the material global properties. Material quality, determined through this technique, was found in good agreement with the results of macroscopic examinations.

Results are given for impact tests for a mass of 4.3 kg dropped from a height of up to 3m onto a GRP sandwich beam in a three point bend configuration. Force versus deflection traces are derived from accelerometer data and the effect of span and impact velocity discussed. A two degree of freedom inertial model is developed for the 180 mm span test to give the local deformation of specimen in the vicinity of the impactor and hence the effect of local stresses on failure. High speed photographic data is used to supplement experimental results.

A range of thermoplastic matrix carbon fibre composites with varying degrees of crystallinity has been subjected to falling weight impact tests. Tests performed at excess energies, sufficient to cause total penetration, indicate that the thermoplastic composites are superior to toughened epoxies in terms of energy absorbed. After low energy impacts, the thermoplastic composites absorb less energy then epoxies with this difference being attributed to a reduction in the amount of microcracking. PEEK based composites are shown to be superior to PPS based composites and amorphous systems superior to crystalline equivalents. The generation of a plastic dent on the surface of a thermoplastic composite at an early stage of the impact event is thought to modify the deformation behaviour of thermoplastics relative to thermosets.

The aim of this paper is to study the shock behaviour of composite tubes under compressive loading conditions. We prove that the type and nature of the fibers, the mechanical properties of the matrix, the geometry of the structure and the fiber arrangement affect significantly the energy absorption capabilities of the composite tube.