T. Adam

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.

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.

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.

A power law fatigue damage model for fibre-reinforced plastic laminates

Following an earlier comparative investigation of the tensile fatigue response, in terms of stress versus log life curves, of three epoxy-resin-based laminates of 0/90 lay-up and reinforced with carbon, glass and Kevlar-49 fibres, the authors now present a study of the accumulation of fatigue damage in terms of the residual strengths of the fatigued laminates. A stress-independent parametric relationship between normalized stress and normalized number of cycles is found to describe the behaviour of all three composites, suggesting a single major mode of fatigue damage despite the widely different mechanical characteristics of the different materials. The model is a two-parameter power law model, derived from a consideration of the appropriateness of various curve-fitting methods, and permits the incorporation of all modes of damage accumulation, from gentle ‘wear-out’ to ‘sudden death’ simply by the adjustment of this pair of power law exponents.

Fatigue behaviour of hybrid composites

A study has been made of the fatigue behaviour of carbon/Kevlar-49/epoxy hybrid composites. Stress-life data have been obtained for both unidirectional and [(±45, 0, 0)2]s laminates in repeated tension and compression-tension cycling tests at various values of the stress (or R) ratio. Goodman (constant life) diagrams are presented for the unidirectional composites which indicate that the fatigue resistance of hybrid mixtures varies linearly with composition. The presence of the Aramid fibre, whose natural resistance to compression loads is suspect, does not appear to exert any unexpected damaging influence on the response of the hybrids either in tensile or tension-compression loading. This is also true of the behaviour of hybrid laminates containing plies at an angle to the main loading direction.

Fatigue of carbon-fibre-reinforced plastics under block-loading conditions

Abstract Following an earlier study of block-loading fatigue on a high-performance T800/5245 carbon-fibre composite laminate of [(± 45, 02)2]s construction, further experiments of a similar nature have now been carried out on other CFRP laminates with the same lay-up but with different fibre/resin characteristics. In this more recent work, comparable T800/924 and IM7/977 materials and a lower-performance HTA/913 laminate appear to behave in a similar manner to that previously found for the T800/5245 composite. The most significant common feature of the behaviour is that combinations of tension and compression loading are more damaging than purely tensile or purely compressive stress cycling regimes.

Environmental fatigue of reinforced plastics

Abstract This paper presents the results of a series of experiments on the fatigue behaviour of cross-plied (0/90) epoxy-based laminates reinforced with glass, carbon and Kevlar-49 fibres and, in particular, on the effects of environmental conditioning on this behaviour. Conditioning treatments used include drying, storage in an atmosphere of 65% relative humidity and boiling in water, prior to fatigue tests in repeated tension and in bending. The fatigue resistance of CFRP is unaffected by conditioning treatment and mode of stressing; for GRP there is no significant difference between the behaviour of the dry material and that conditioned at 65% relative humidity. On the other hand, boiling in water always weakens GRP and KFRP, although the effect is small except for the case of GRP in the 0/90 orientation tested in tension. The large reduction in this case results directly from loss of fibre strength during pre-conditioning. Complete drying of KFRP laminates is more damaging even than boiling.

Fatigue of Hybrid Composites

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.