Quantification of early-stage fatigue damage on cross-ply CFRP laminates by lock-in thermographic thermal diffusivity measurement

Abstract

In this paper, thermal diffusivity measurements using a lock-in thermography based on the laser-periodic-heating method was used to quantify the micro-scale damage that occurred in the early-stage of fatigue. Measurement system using square-core optical fiber with a mode mixing was developed to ensure heating uniformity and enlarge the measurement area. A developed model was fitted to the frequency dependence of phase delay, which was corresponded to a time delay from the input periodical laser heating, to obtain the thermal diffusivity. Cross-ply CFRP laminates were prepared for fatigue tests and thermal diffusivity measurement. Two type of tensile fatigue load applied sinusoidally at maximum loads of 30% and 50% of static tensile strength, respectively. Fatigue load cycle reached to 105 times. Post-fatigue sample were confirmed by X-ray CT that there was no clack growth inside the sample. The results showed that the thermal diffusivity of the post-fatigue sample decreased by up to 3.1% compared to the unloaded sample. This result indicate that thermal diffusivity measurement has possibility to quantitate the early-stage fatigue damage. However, further experiment was needed to extract only the fatigue loading effect, since this result includes differences in the initial state before fatigue loading.