Carbon composites based on multi-axial multi-ply stitched preforms. Part 7: Mechanical properties and damage observations in composites with sheared reinforcement

Biaxial multi-ply carbon fabrics (+45°/−45° and 0°/90°) were sheared up to 30°, 45° and 50° using a shearing frame. The carbon/epoxy composite plates were made using resin transfer moulding. Tensile properties were measured in two principal directions and compared with results computed with the classical laminate theory and meso-scale finite element analysis. Damage initiation in laminates due to tensile loading was investigated using acoustic emission and X-ray radiography. Shearing changes the fibre orientation and hence the behaviour of the composites, leading to the shear angle dependence of damage initiation. Thermal cracks in composites with unsheared reinforcements, induced by post-curing, are not present in the composites with sheared reinforcement. The analysis of the experimental observations of the damage initiation and development is supported by finite element analysis.