Experimental and numerical fatigue damage characterization in multidirectional thermoplastic glass/polypropylene laminates based on in-situ damage observations

Quantifying microscopic damage mechanisms in opaque composite laminates with multiple off-axis plies throughout fatigue experiments is challenging. However, if successful, important relations can be revealed between actual evolving damages and their contribution to the mechanical laminate response. This study characterizes the evolution of matrix cracking in ±45 and 90 embedded off-axis plies together with effects of delamination and relates them to the global mechanical property reductions in Glass/Polypropylene laminates. [0/45/0/-45]s and [0/45/90/-45]s laminates are tested under tension-tension fatigue over 500 000 cycles at three different stress levels. In-situ edge damage detection by optical measurements are performed as well as the validation of damages by detailed post-mortem microscopy. Moreover, through-the-width sectioning is applied to reveal the damages inside the laminate after testing. Additionally, the effects of different fabrication histories on damage progression are assessed. Influences of 45-ply thickness and different ply neighbouring orientations on the damage behaviour are discussed with their contribution to the normalized stiffness degradation and Poisson's ratio evolution. Moreover, experimental results are compared with a modelling approach by which the effects of damages in multiple plies can be considered, showing very good accordance.