Variation of PEEK matrix crystallinity in APC-2 composite subjected to large shearing deformations

This paper presents an experimental investigation of matrix cracking and microstructure variations in order to evaluate their relative contributions to the shear stiffness loss of AS4/PEEK composite material subjected to large shear deformations. Specimen observations by X-radiography allow us to characterize the matrix cracking, and measurements of the matrix crystallinity at different levels of loading give information on the microstructure alteration of the PEEK matrix. These measurements have been performed by using three different methods that have been compared: wide-angle X-ray scattering (WAXS), conventional differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (MDSC). The relationship between the matrix cracking development, the change in the PEEK crystallinity and the secant shear stiffness variation is discussed. This study shows that, neither the presence of inter- or intra-ply cracks nor the decrease of the PEEK matrix crystallinity degree, are sufficient to induce the significant loss of the ply shear stiffness measured in [±45]2s APC-2 laminates tested in tension.