In this study, 48-ply AS4/3501-6 (carbon fiber reinforced epoxy) laminates were delaminated in essentially pure Mode III using a modified split cantilever beam (SCB) Mode III test. For this test, two loads are applied directly to each arm of a SCB specimen using a pair of identical grips. The complex loading configuration is necessary to reduce the Mode II loading which is dominant at the free edges when a single load is applied to each arm. Finite element analysis of a three-dimensional mesh confirmed that the strain energy release rate, determined using the virtual crack closure technique, was dominated by the Mode III component for the proposed loading configuration. Scanning electron microscopy of the delaminated surfaces revealed matrix cracking (analogous to Mode II shear hackles) at 45° to the plane of the delamination and parallel to the fiber direction. Crack stability, data reduction techniques, and the limitations of the new method are also discussed. Further work is required before the method could be considered for widespread use.
Fracture tests were performed with carbon/polymer laminates and analyzed for the purpose of developing translaminar fracture toughness test and analysis procedures. Notched specimens were tested of two types of symmetrical layups — quasi-isotropic [0/45/90] and [0/90]; two carbon fiber/epoxy materials — a relatively brittle T300 fiber/976 epoxy and a tougher AS4 fiber/977-2 epoxy; two laminate thicknesses — 2 mm and 4 mm; and three specimen configurations — the standard three-point bend and compact configurations, and an extended compact specimen with arm-height to specimen-width ratio of 1.9. Stress and displacement expressions were obtained for the extended compact specimen, including those for stress intensity factor, K, and crack mouth opening displacement, V, in terms of relative notch length, a/W, and for a/W in terms of V. Relationships for the bending stresses that control self-similar and off-axis cracking for the extended compact specimen were derived. Damage was characterized in the tests, including that associated with arm breakage in the standard compact specimen and load-point damage in the bend specimen. Two types of notch-tip damage were characterized using radiography, that which extends perpendicular to the notch in predominantly 0‡ fiber layups, and that which occurs ahead of the notch in quasi-isotropic and 90‡ fiber layups. The applied K at maximum load, Kmax, determined in a way that took account of the effective crack growth up to the maximum load point, was used as a measure of fracture toughness. For deviations from the linear P-V plot corresponding to Δa/W ≤ 0.04, Kmax gave consistent measurements of fracture toughness. This criterion also excluded tests with damage of the type that violates the basic concept of fracture toughness measurement. Plots of Kmax vs δa/W showed increasing resistance to crack growth for quasi-isotropic layups and constant resistance to crack growth for predominantly 90‡ fiber layups.
Heterogeneous surface wear (HSW) models have been derived to predict the specific wear rates of woven carbon fibre reinforced epoxy composite materials. The specific wear rates of unidirectional carbon fibre/epoxy composites, with fibres orientated both parallel and antiparallel to the direction of sliding, are used as input variables. The first model (EW mode) is based on an assumption of uniform thickness reduction during wear, but uneven surface pressure. The second model (EP mode) is based on an assumption of even surface pressure throughout the test. The specific wear rates of plain and 5HS woven composite panels were measured to validate the accuracy of the models. It was found that the EW model was able to accurately predict the specific wear rates of the two types of woven composites under mild abrasive conditions (120 grit sandpaper). However, under more severe abrasive conditions (36 grit sandpaper), the woven panels exhibited a new wear mechanism caused by tearing of the out-of-plane fibres at the crossover points of warp and weft fibres. This mechanism caused both models to under-predict the specific wear rates of the woven composites in severe abrasive conditions. However, the EW model can be used with confidence under less abrasive conditions, where the asperities do not have significant interactions with the out of plane fibres.
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