Non‐destructive fatigue damage detection of carbon fiber‐reinforced thermoplastics using positron annihilation method

Tensile axial fatigue tests were carried out using carbon fiber‐reinforced thermoplastics (CFRTP) plate with a hole notch up to very high cycle fatigue (VHCF) regime, where number of loading cycles was 2 × 107. The fatigue test was periodically interrupted, and fatigue damage accumulation around the hole notch was non‐destructively detected by two positron annihilation methods. Positron annihilation lifetime spectroscopy was applied to obtain positron lifetime and Doppler broadening method to obtain S‐parameter. Both parameters increased with increasing fatigue loading cycles. Fatigue damage detected by two positron annihilation methods was attributed to the expansion of free volume in the polymer matrix. The estimated size of free volume was sub‐nanoscale according to the experimental value of the positron annihilation lifetime. SEM and μCT scan revealed that the macroscopic fatigue damages, such as surface cracking, transverse cracking and delamination, increased with increasing fatigue loading cycles, which indicated that sub‐nanoscale and macroscopic fatigue damages progressed simultaneously. Consequently, it can be concluded that the positron annihilation methods can be used for the non‐destructive fatigue damage detection of CFRTP. Highlights Fatigue damage of CFRTP was detected by two kinds of positron annihilation methods. Positron lifetime and S‐parameter increased with increasing loading cycles up to 2 × 107. Microscale or nanoscale fatigue damages in CFRTP were related to macroscopic damages.