Carbon composites based on multi-axial multi-ply stitched preforms. Part 4. Mechanical properties of composites and damage observation

Multi-axial multi-ply fabric (MMF) or non-crimp stitched carbon fabric (NCF) reinforced epoxy composites, produced by the resin transfer moulding process are described. The mechanical properties of the composites-tensile, in-plane shear and out-of-plane shear-are measured in a number of orientations relative to the stitching direction for different NCF based laminates: quadriaxial, biaxial ±45 and 0/90°, as well as unidirectional. As expected, the quadriaxial laminates show a quasi-isotropic behaviour while biaxial laminates show an an-isotropic behaviour. The mechanical properties of the biaxial ±45° laminates are similar to the 0/90° laminates in the corresponding directions. The in-plane shear properties of the materials are strongly dependent on the amount of reinforcement in the +45 and −45° direction relative to the loading direction. Absence of a significant difference in stiffness between experimental results and classical laminate theory predictions reveals that stitching has minor effects on the stiffness of this material. Tensile tests on 0/90° laminates were accompanied by acoustic emission registration. The results were correlated with X-ray and C-Scan damage observation. Early (at 0.3% applied strain for load in fibre direction) initiation of damage has been detected. Damage sites are correlated with the resin-rich zones created by the stitching.