Far side defect detection in composite materials through threshold dynamics effect of local defect resonance frequency

• Threshold dynamics in LDR is utilized for inspection of deeper defects in sweep vibro-thermography (SVT). • Parametric resonance drains the energy from the fundamental LDR frequency to higher and fractional harmonics. • Threshold dynamics is found to be beneficial for single sided inspection of deeper defects. Detection of deeper defects (depth more than mid layer thickness) are found to be challenging in local defect resonance (LDR) based non-destructive testing methods. In this paper, threshold dynamics in LDR is utilized for inspection of deeper defects in sweep vibro-thermography (SVT). First, CFRP composite samples are fabricated using vacuum assisted resin transfer moulding (VARTM) method. In first sample, a flat bottom hole (FBH) is created, and a barely visible impacts damage (BVID) is created in the second sample with an in-house developed portable impactor. The impacted CFRP sample is inspected using X-ray computed tomography (CT). Then, LDR based laser Doppler vibrometry (LDV) has been performed to understand the threshold dynamics in case of FBH. Thereafter, SVT has been used for inspection of BVID from the near and far (or, deeper) side. In both cases, a circular piezoelectric disc is used to vibrate the sample with desired resonance frequency range and voltages. The specimens are tested with six different voltage level. It has been found that temperature in the near side increases with an increase in input voltage to a certain voltage level. Thereafter, the temperature decreases with an increase in input voltage. In case of far or deeper side, the defect is found to be visible above that threshold voltage. Parametric resonance is found to be responsible for this observation which drains the energy from the fundamental LDR frequency to higher and fractional harmonics. Threshold dynamics effect is utilised to visualise far side defect in composite sample.