The Effect of Fibre Waviness on the Relationship between Compressive and Flexural Strengths of Unidirectional Composites

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Abstract
A non-linear finite element analysis has been carried out which shows that the same mechanism of shear instability due to fibre waviness can occur under pure bending as under pure compressive loading. Results for carbon fibre-epoxy indicate a significantly higher maximum compressive stress in bending than in compression due to the presence of the stress gradient, and resulting through thickness stresses. For thick specimens. where the stress gradient is low, the maximum compressive stress in bending tends towards the value in pure compression. As the thickness decreases. the bending stress increases due to the constraint of the adjacent less highly loaded fibres. The predicted maximum stress in bending was up to 73% higher than in compression. The results show that where failure is controlled by shear instability due to fibre waviness. compressive strength can be expected to be a function of stress gradient and therefore cannot be regarded as a true material property.