B. Harris

Compression strength of carbon, glass and Kevlar-49 fibre reinforced polyester resins

The compression behaviour of a series of polyester resins of various compositions and in different states of cure has been investigated. Their mechanical characteristics having been established, the same range of resins was then used as a matrix material for a series of composites reinforced with carbon, glass and aromatic polyamide fibres. The composites were unidirectionally reinforced, having been manufactured by pultrusion, and were compression tested in the fibre direction after a series of experiments to assess the validity of a simple testing procedure. “Rule of Mixtures” behaviour occurred in glass-polyester composites up to limiting volume fractions (Vf) of 0.31 for strength and 0.46 for elastic modulus, the compression modulus being equal to the tensile modulus, and the apparent fibre strength being in the range 1.3 to 1.6 GPa at this limiting Vf. At a Vf of 0.31 the strengths of reinforced polyesters were proportional to the matrix yield strength, σmy, and their moduli were an inverse exponential function of σmy. For the same matrix yield strength a composite with an epoxy resin matrix was stronger than polyester based composites. At Vf=0.30, Kevlar fibre composites behaved as though their compression modulus and strength were much smaller than their tensile modulus and strength, while carbon fibre composites were only slightly less stiff and weaker in compression than in tension. The compression strengths of the polyester resins were found to be proportional to their elastic moduli.

Life prediction for fatigue of T800/5245 carbon-fibre composites: I. Constant-amplitude loading

Abstract This is the second of two papers describing the fatigue response of a [(±45,0 2 ) 2 ] s laminate of the T800/5245 composite system, a modern aerospace material consisting of high-failure-strain, intermediate-modulus carbon fibres in a toughened bismaleimide resin system. In the first paper, the fatigue response in repeated tension, repeated compression, and mixed tension-compression was determined at constant stress levels over a wide range of R values. The results of those constant-amplitude experiments were then used to define a programme of four- and two-block variable-loading experiments in an attempt to derive predictive methods for such loading conditions. Formulae have been developed to predict life under non-linear cumulative damage conditions and empirical data derived to validate the procedures.

The use of neural networks for the prediction of fatigue lives of composite materials

Constant-stress fatigue data for five carbon-fibre-reinforced plastics and one glass-reinforced plastic laminate have been used to evaluate possible artificial neural network architectures for the prediction of fatigue lives and to develop network training methods. It has been found that artificial neural networks can be trained to model constant-stress fatigue behaviour at least as well as other current life-prediction methods and can provide accurate (and conservative) representations of the stress/R-ratio/median-life surfaces for carbon-fibre composites from quite small experimental data-bases. Although their predictive ability for minimum life is less satisfactory than that for median life, and is non-conservative, the procedures developed in this work could nevertheless be used in design with little further modification. Some success has been achieved in modelling fatigue under block-loading conditions, but this problem is more difficult and requires much more effort before ANNs could be used with confidence for variable-stress conditions.

Fatigue behaviour of carbon fibre reinforced plastics

This paper presents a comparison of the fatigue behaviour of several varieties of carbon fibre reinforced plastics (cfrp), emphasizing the differences between some of the older, conventional materials consisting of xas fibres in Ciba-Geigy Fibredux 913 and 914 epoxy resin matrices, and newer materials based on intermediate modulus (im) fibres in toughened resins. It is shown that the fatigue response of these materials follows a relatively simple pattern which implies that the shape of the stress (or strain) vs. life curve is determined largely by the tensile failure strain of the composite in question. This pattern appears not to be restricted to carbon fibre composites, as demonstrated by reference to results for plain aramid and glass fibre reinforced plastics, for carbon/aramid hybrids, and for materials with unidirectional, 090, and [(±45,0,0)2]s lay-ups. The majority of the results presented relate to repeated tension cycling (R = 0.1), but some results for R ratios which include a compression component are also discussed.

The environmental fatigue behaviour of reinforced plastics

This paper presents the results of an investigation of the effects of hygrothermal conditioning on the mechanical properties and fatigue behaviour of epoxy-based composites reinforced with carbon, glass and aromatic polyamide fibres. Cross-plied (0°/90°) laminates of these materials, of nominal fibre volume fraction ca. 0.6, were conditioned by drying, by exposing to a 65% r. h. (relative humidity) atmosphere and by boiling in water. The effects of these treatments on the tensile and shear strengths and on the tensile fatigue response of the laminates when tested orthogonally and at ±45° to the fibre lay-up are discussed. The 0°/90° tensile properties of the three laminates are relatively little affected by the conditioning treatments except for g. r. p. (glass reinforced plastic) exposed to boiling water, and of K. f. r. p. (Kevlar fibre reinforced plastic) in which the strength is reduced somewhat by complete drying. The shear strengths and ±45° tensile strengths are more sensitive to the effects of moisture, however, and it appears that optimum strengths are developed after conditioning at the intermediate 65% r. h. Tensile fatigue tests have been carried out on laminates in the 0°/90° and ±45° orientations. In c. f. r. p. (carbon fibre reinforced plastic) there is no effect of conditioning on the fatigue behaviour of 0°/90° samples, and in g. r. p. only the boiling water treatment affects the results. Preloading g. r. p. samples before conditioning has no effect on their fatigue behaviour. The tensile fatigue resistance of 0°/90° Kevlar-epoxy laminates is reduced by drying more than by boiling, and in all conditions the stress against logarithm of life curves are characterized by a downward curvature that renders the long term stability of this material suspect. This behaviour is also reflected in differences in the ±45° tensile fatigue response of the three laminates.

Fatigue life prediction for hybrid composites

Abstract The fatigue behaviour of a series of carbon/Kevlar-49/epoxy unidirectional hybrid composites has been established as a function of composition and of the ratio of the maximum to minimum stress in repeated tension and tension/compression cycling. The results are analysed in terms of a fatigue function which permits the representation of all data in a single two-parameter fatigue curve.

Study of carbon fibre surface treatments by dynamic mechanical analysis

Dynamic mechanical and thermal analysis (DMTA) has been used to study the effects of surface treatment of carbon fibres on the viscoelastic properties of composites containing them. The fibres were treated by dip-coating, electro-polymerization, and plasma polymerization, and the behaviour of these fibres is compared with that of fibres treated by ordinary commercial oxidation and sizing procedures. Analysis of the experimental results is made both in terms of conventional approaches to viscoelastic behaviour and of the power-law analysis of the glass transition in an attempt to obtain suitable parameters for the evaluation of the effects of surface treatment.

An empirical fatigue-life model for high-performance fibre composites with and without impact damage

Abstract This article follows earlier work on the development of a life-prediction method for carbon-fibre/epoxy laminates. For comparison with the behaviour of a number of different CFRP laminates already studied, further constant-life fatigue data have now been obtained for a further CFRP composite and a GRP laminate of similar construction – a 16-ply [(±45,0 2 ) 2 ] S lay-up. Fatigue tests have been carried out on these materials in both the virgin condition and after damage by low-velocity impacts. Following analysis of these new data and a re-examination of the older data base, the constant-life model has been appropriately modified. It now offers a prediction procedure for the fatigue response of composite materials in the virgin and impact-damaged conditions which requires, in the first instance, only the tensile and compressive strengths of the composite in question. The model is equally applicable to both GRP and CFRP, despite the fact that the fatigue response of a GRP laminate is different from that of an equivalent CFRP material.

Matrix Cracking and the Mechanical Behaviour of SiC-CAS Composites

An experimental investigation has been carried out on the mechanical properties of unidirectional (0)12, (0, 90)3S, (±45, 02)S, and (±45)3S composites consisting of CAS glass ceramic reinforced with Nicalon SiC fibres. Measurements have been made of the elastic properties and of the tensile, compression and shear strengths of the composites, and these have been supported by a detailed study of the damage which occurs during monotonic and repeated loading. These damage studies have been carried out by means of edge replication microscopy and acoustic emission monitoring. The elastic properties of the composites are, by and large, close to the values that would be predicted from the constituent properties and lay-up sequences, but their strengths are lower than expected, and it appears that the Nicalon reinforcing fibre has been seriously degraded during manufacture. The fracture energy is much higher than predicted from observations of fibre pull-out, and it is suggested that the energy required to form a close three-dimensional network of matrix cracks could account for the high apparent toughness. The matrix cracking stress can be predicted reasonably closely by the Aveston, Cooper and Kelly model of cracking in brittle matrix composites, but it is shown that subcritical microcracks can form and/or grow at stresses well below the predicted critical values without affecting composite properties.

The environmental fatigue behaviour of carbon fibre reinforced polyether ether ketone

The fatigue behaviour of carbon fibre/PEEK composite is compared with that of carbon/ epoxy material of similar construction, particularly in respect of the effect of hygrothermal conditioning treatments. Laminates of both materials were of 0/90 lay-up, and they were tested in repeated tension at 0° and at 45° to the major fibre axis. The superior toughness of the polyether ether ketone and its better adhesion to the carbon fibres results in composites of substantially greater toughness than that of the carbon/epoxy material, and this is reflected in the fatigue behaviour of the carbon fibre/PEEK. The tougher PEEK matrix inhibits the development of local fibre damage and fatigue crack growth, permitting a 0/90 composite with compliant XAS fibres to perform as well in fatigue as an epoxy laminate with stiffer HTS fibres. Hygrothermal treatments have no effect on the fatigue response of either material in the 0/90 orientation. The fatigue response of a cross-plied carbon/PEEK laminate in the ±45° orientation is much better than that of equivalent carbon/epoxy composites, again because the superior properties of the thermoplastic matrix.