Vibrothermographic spectroscopy with thermal latency compensation for effective identification of local defect resonance frequencies of a CFRP with BVID

Abstract

Vibrothermography using sinusoidal vibration excitation at the resonance frequencies of a defected area (so-called local defect resonance, or LDR) is a promising technique to boost the defect's deformation and its interfacial interactions and as such enhance resultant vibration-induced heating. Contrary to the classical high-power vibrothermography, low power excitation at an LDR frequency results in a reproducible thermal response and adequate quantification of the corresponding damage features. However, the technique is mainly limited by the fact that it requires a priori knowledge of the LDR frequencies (e.g. obtained from prior vibrational measurements). To overcome this limitation, a stand-alone vibrothermographic spectroscopy procedure is introduced in this paper. The proposed technique applies two consecutive broadband sweep vibrational excitations with ascending and descending frequency modulation rates to the sample. The surface of the excited sample is monitored with an IR camera. Both time derivative analysis and superposition of the recorded thermal responses are performed in order to compensate for the thermal latency of the defect-induced heating. This compensation approach enables proper identification of the actual LDR frequencies based on the apparent LDR frequencies of the thermal response. The method is applied on a carbon fiber reinforced polymer (CFRP) with barely visible impact damage (BVID), and multiple LDR frequencies are readily identified. The identified LDR frequencies are also individually evaluated by both lock-in vibrothermography and 3D scanning laser Doppler vibrometry, confirming the competence of the proposed technique for extracting LDR frequencies in a proper and fast way.