Automatic determination of local defect resonance frequencies and corresponding defect mode shapes in composite structure using air coupled acoustic inspection

The local defect resonance (LDR) based air coupled acoustic testing is found to be a good tool for non-contact inspection of large structure. In this study, automatic determination of local defect resonance frequencies and corresponding defect mode shapes in composite structure is presented using air coupled acoustic testing. In the first step of proposed methodology, the maximum pressure in every frame is recorded in the time domain and then the time frame corresponding to LDR frequencies ( N L ) are extracted from the maximum slope and curvature profile. Next, assuming linear chirp signal excitation, a relation has been derived between frequency frame ( N f ) and time frame ( N L ) corresponding to LDR frequency with respect to the total number of frames (N T ). Then this relation is used to obtain individual defect mode shapes corresponding to each LDR frequency. The proposed methodology is verified with air coupled acoustic testing using broadband backward linear chirp excitation signals on a glass fiber reinforced polymer (GFRP) plate with multiple artificial delaminations and impact damage. Also, the Laser Doppler Vibrometry (LDV) is carried out to validate the LDR frequencies and mode shapes. Automatic defect segregation will help to obtain individual defect depth from the known defect area and LDR frequency which can generate thickness scan also.