Peter W. R. Beaumont

Matrix cracking and stiffness reduction during the fatigue of a (0/90)s GFRP laminate

Abstract Stiffness reduction due to matrix cracking in a (0/90) s glass fibre reinforced plastic (GFRP) laminate has been studied under both quasi-static and fatigue loading. The stiffness reduction is shown to be directly proportional to the density of cracks which accumulate in the transverse ply. A model for the transverse ply crack growth during fatigue gives good agreement with the experimentally determined stiffness reduction curves.

Fatigue damage mechanics of composite materials. I: Experimental measurement of damage and post-fatigue properties

A new approach for modelling the post-fatigue strength and stiffness of notched fibre composite laminates has been developed. It is based on the observation of notch tip damage which can be quantified by the extent of the individual failure processes, splitting in the 0° plies and delamination between the 0° ply and off-axis plies. The notch tip damage zone grows stably under tensile cyclic loading in a self-similar manner and the size and shape of this damage zone is dependent on laminate geometry and constituent properties of the fibre, matrix and interface. The post-fatigue strength and stiffness of the laminate can be related uniquely to the split length, which defines the extent of damage growth. In this first paper in a series of four, observation is made and measurements taken of the damage growth mechanisms that make up the damage zone in carbon fibre/epoxy laminates. Radiographs are used to characterise the notch tip damage zone and to establish a qualitative relationship between post-fatigue strength (or stiffness), cyclic stress, damage size and numbers of cycles.

Failure of brittle polymers by slow crack growth: Part 3 Effect of composition upon the fracture of silica particle-filled epoxy resin composites

The mechanical properties of a silica particle-filled epoxy resin composite system have been investigated in air as a function of volume fraction of particles for volume fractions ranging from 0 to 0.52. The Youngs modulus and the compressive yield stress both increase as the volume fraction of silica particles is increased and various models of particle strengthening have been used to explain this behaviour. Slow crack growth in the various particulate composites has been studied using a fracture mechanics approach. The variation of crack velocity (V) with stress intensity factor (KI) has been measured for each of the compositions investigated. In each case, a unique relationship between V and KI has been found with KI increasing with volume fraction of particles at a given value of V. The failure mechanisms and the variation of other fracture mechanics parameters, for example, crack opening displacement and plastic zone size with increasing particle volume fraction have been discussed.

The measurement and prediction of residual stresses in carbon-fibre/polymer composites

Experiments have been carried out that measure the residual stress in a thermoplastic matrix and a toughened thermosetting matrix, both reinforced with carbon fibre. Some of these experiments are based on the determination of the extent of curvature of unbalanced laminates as they cool from their processing temperature to ambient conditions. Other experiments involve measurements of the first-ply cracking stress. Both methods are compared and modelled by using classical lamination theory. It was demonstrated that, under certain conditions, the residual tensile stress can approach closely to the transverse ply tensile strength. After 200 days at ambient conditions, these stresses can decay by approximately one-quarter.

Failure of brittle polymers by slow crack growth: Part 1 Crack propagation in polymethylmethacrylate and time-to-failure predictions

Slow crack growth in two forms of polymethylmethacrylate (PMMA) has been studied from a fracture mechanics viewpoint. It has been found that in Perspex acrylic sheet and surgical Simplex acrylic bone cement the crack velocity, V, for each material depends upon the intensification of stress at the tip of the crack. Experimental measurements have been made of V as a function of the stress intensity factor, KI, at the crack tip, and a derived V(K) relationship has been used to predict the times-to-failure of components made from Perspex and Simplex bone cement. Direct measurements of times-to-failure for Perspex are entirely consistent with the predicted values.

The interlaminar and intralaminar fracture toughness of carbon-fibre/polymer composites: The effect of temperature

The mode I interlaminar toughness and intralaminar fracture toughness, and mode II interlaminar shear fracture toughness of a thermoplastic and a toughened thermosetting polymer reinforced with aligned and cross-ply carbon fibres are investigated over a wide range of temperature. The nature of the fracture process and fracture resistance are explained qualitatively on the basis of measurements of toughness, crack-tip and crack-wake events, and post-mortem fractographic analysis.

A STRESS INTENSITY FACTOR APPROACH TO THE FATIGUE GROWTH OF TRANSVERSE PLY CRACKS

Abstract A model has been developed for the stiffness reduction due to transverse ply crack growth during the fatigue of a (0/90)s glass-fibre reinforced plastic laminate. A stress intensity factor is derived for a transverse ply crack and related to the stiffness reduction rate by the Paris law. The model gives good agreement with experimental data and can be used to maintain a constant stiffness reduction rate by incremental load-adding. The effect of frequency is considered.

The fatigue damage mechanics of notched carbon fibre/PEEK laminates

A model is presented for the strength, post-fatigue residual strength and damage propagation in notched, cross-ply carbon fibre/polyetheretherketone (PEEK) laminates. Fracture mechanics principles are used to predict quasi-static damage growth, and the application of a Paris law permits extension to fatigue damage. Strength is predicted by applying a failure criterion based on the tensile stress distribution in the 0° plies, as modified by damage (either quasi-static or fatigue). The volume dependence of strength is included by using a simple Weibull distribution. The parameters of the model are determined from independent experiments. Good agreement with experimental results is obtained. Comparisons are made with previous results from carbon fibre/epoxy laminates. The behaviour of the carbon fibre/PEEK is similar, although the extent of delamination and matrix cracking is reduced owing to the higher inherent toughness of the matrix.

A failure analysis of the micromechanisms of fracture of carbon fibre and glass fibre composites in monotonic loading

The micromechanisms of crack extension of carbon fibres, glass fibres and hybrid composites containing glass fibres and carbon fibres in epoxy and polyester resins have been studied. A new collection of failure data based on observations of fibres debonding, snapping and pulling out has been summarized in cumulative probability diagrams and analysed using Weibull distribution parameters. This data, together with models of failure processes and information of work of fracture, is used to construct fracture-mechanism diagrams. These diagrams, together with the Weibull parameters may help in distinguishing between mechanisms of fracture, give guidance in selecting a material system and in isolating aging and environmental effects.

Structure and properties of injection-moulded nylon-6

The distribution of residual stress through the thickness of injection-moulded nylon-6 has been investigated. It was found that the stress distribution is parabolic with a compressive stress at the surface of the moulding and a tensile stress in the centre of the moulding. The magnitude of these residual stresses is inversely proportional to mould temperature. Exposure of the moulding to boiling water resulted in a reversal of the residual stress distribution, with a tensile stress at the surface and a compressive stress at the centre of the moulding. This effect was attributed to the inhomogeneous volume changes that occurred in the mouldings as a result of water-induced crystallization in nylon-6.