Comparative study on the behavior of woven-ply reinforced thermoplastic or thermosetting laminates under severe environmental conditions

Abstract

This work aims at determining whether thermoplastic-based composites can be used in secondary aircraft structures to replace thermosetting-based composites or not. In order to answer this question, the mechanical behaviors of carbon fiber fabric reinforced thermoplastic (PPS or PEEK) and thermosetting (epoxy) laminates subjected to different stress states under severe environmental conditions (120°C after hygrothermal aging) have been compared. In addition to usual mechanical tests (tensile, open hole tensile), single-bolt double lap joint and single-bolt single lap joint tests were also performed. Severe conditions help enhance the ductile behavior of the epoxy matrix, but degrade the fiber/matrix interface, resulting in lower stiffness and strength of laminates with a quasi-isotropic lay-up. In thermoplastic-based laminates, the degree of retention of mechanical properties is quite high even for PPS-based laminates when T>Tg. In laminates with a [45]7 lay-up, severe conditions adversely affect the mechanical properties of the three composite systems. However, the combination of matrix ductile behavior, and the strain gradient near the hole, lead to an extensive plastic deformation along the ±45° oriented fibers bundles in notched A-P laminates. It results in decreasing significantly the hole-sensitivity of C/PPS and C/Epoxy under severe conditions. In bolted joints, a severe environment has a limited impact on the bearing strength of epoxy-based laminates. In the case of thermoplastic-based laminates, it increases the strength of double lap joints, but is detrimental to the strength of single lap joints.