Fatigue life prediction for CFRTP specimens based on plastic strain energy of the resin component

Abstract

The use of Carbon Fiber Reinforced Thermoplastic (CFRTP) is being explored in the field of transport applications, such as in aircraft, where there is a need for both short molding cycle times and recyclability. An essential part of implementing CFRTP as a structural component is the design for fatigue strength, which ensures long-term reliability. However, predicting the fatigue life of CFRTP is challenging due to the significant nonlinear deformation exhibited by thermoplastic resin used as the matrix. This study investigates the effectiveness of a fatigue life prediction method based on Interfacial Plastic Strain Energy (IPSE) for unidirectional CFRTP specimens with polyamide 6 (PA6) as the matrix. Given the high hydrostatic pressure experienced by the PA6 within the CFRTP, we applied the Drucker-Prager yield criterion. A two-scale analysis method was proposed, which accurately predicts macroscopic stress in CFRTP specimens and calculates IPSE from the local mechanical field at the microscale. By applying the internal friction angle obtained from uniaxial tensile and compression tests on PA6, we computed the plastic strain energy within the PA6 of the CFRTP. We then transformed stress-life diagrams from fatigue tests into plastic strain energy-life diagrams, and conclude that the fatigue life of CFRTP can be accurately predicted by applying the IPSE evaluation method, resulting fatigue diagrams both for pure PA6 and that in the CFRTP specimens matched well.