Nonlinear Ultrasonic Evaluation Method For Early Debonding Of Bonded Structures

Conventional ultrasonic testing is not sensitive to early debonding defects.Although this kind of damage can be effectively identified by nonlinear ultrasonic testing method,the damage signal is weak and vulnerable to the interference of high-order harmonic components produced by testing equipment and testing links,which leads to false damage alarm in actual detection.In this context,a nonlinear ultrasonic detection method based on local defect resonance(LDR)is proposed.Combined with the subsequent signal processing method,the early damage of this kind of bonding structure / material is accurately identified,quantified and evaluated.Firstly,the mechanical analysis of the bonding structure with micro debonding damage is carried out,and then a single degree of freedom nonlinear spring oscillator model is established.The multi-scale method is used to solve the model.The results show that the detection method based on LDR effect can enhance the nonlinear effect of the defect,and the micro damage can be accurately identified by subharmonic.In addition,the conditions of subharmonic generation are analyzed and verified by numerical simulation and experiment.Secondly,the relationship between LDR frequency and damage parameters of specimens under different debonding conditions is theoretically deduced.The specimens with different damage conditions are detected and the micro debonding defect area is quantitatively estimated by the measured LDR frequency.Finally,considering the interference of noise to ultrasonic signal in actual detection,singular spectrum analysis is used to denoise the experimental signal,and then HHT is used to locate the micro debonding damage by using the sub harmonic generation time of Hilbert time spectrum.Based on the LDR effect,the results of simulation and experimental show that the nonlinear coefficient and the second harmonic excitation efficiency are greatly improved,which indicates that the detection method can improve the sensitivity of nonlinear ultrasonic testing.The accuracy of defect identification by using subharmonic is higher than using higher harmonic.In the aspect of damage quantitative assessment,the relationship between LDR frequency and damage parameters is deduced theoretically and tested experimentally.The maximum error in actual damage quantitative assessment is 7.1%.As for of damage location evaluation,SSA and HHT are combined to process the signal.The sub harmonic time-frequency information in Hilbert time spectrum is used to calculate the damage location of the test specimen,and the error is 12%.