Compressive Failure Due to Shear Instability: Experimental Investigation of Waviness and Correlation with Analysis

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Abstract
Tv1 panels of unidirectional T800/924 carbon fibre-epoxy were fabricated with a kink running across the width to produce fibre waviness. One panel had maximum fibre misalignment varying through the thickness from about 1.70 to 3.9° with a wavelength of about 6.0 mm, and the other varied from 2.20 to 5.60 with a wavelength of about 6.6 mm. Specimens loaded in compression showed increasing bending followed by catastrophic failure at average stresses of 1085 and 840 MPa. This is well below the compressive strength of 1816 MPa reported elsewhere for nominally straight specimens, confirming the critical effect of fibre waviness. The stress-strain response of the specimens was predicted well from finite element analysis accounting for large fibre rotations and non-linear shear properties. Failure was predicted at stresses 5.6% and 8.0% below the measured values. The model showed that the failure was precipitated by shear instability rather than classical compressive instability. The good correlation obtained validates the modelling approach and shows that it can be applied to predicting the effects of fibre waviness in composite components.