An appropriate stiffness degradation parameter to monitor fatigue damage evolution in composites

This thesis suggests that torsional stiffness be an effective parameter to monitor internal damage evolution in the forms of transverse cracks and interlaminar delaminations in cross-ply laminates caused by· fatigue loading. [ 003 / 9050]5 l cross-ply laminates were fabricated from AS4/3501-6 carbon epoxy prepreg with various spacings of interlaminar delaminations, which were simulated by Dupont® Teflon® films. Transverse cracks were introduced by tensile tests. Torsional and tensile tests were conducted to record torsional and tensile stiffness, and correlate their magnitude with damage levels. Finite Element Analysis (FEA) was employed to computationally investigate the above issue by means of a 3-D model that was generated with ANSYS 5.7. Calculation of torsional and longitudinal stiffnesses for laminates of various crack densities and delamination spacings predicted trends similar to those recorded experimentally. Based on both experimental and computational results, it is concluded that torsional stiffness is indeed more effective in indicating fatigue damage evolution in cross-ply laminates than longitudinal stiffness.