Non-destructive testing techniques based on nonlinear methods for assessment of debonding in single lap joints

Abstract

Nonlinear ultrasonic non-destructive evaluation (NDE) methods can be used for the identification of defects within adhesive bonds as they rely on the detection of nonlinear elastic features for the evaluation of the bond strength. In this paper the nonlinear content of the structural response of a single lap joint subjected to ultrasonic harmonic excitation is both numerically and experimentally evaluated to identify and characterize the defects within the bonded region. Different metallic samples with the same geometry were experimentally tested in order to characterize the debonding between two plates by using two surface bonded piezoelectric transducers in pitch-catch mode. The dynamic response of the damaged samples acquired by the single receiver sensor showed the presence of higher harmonics (2 nd and 3 rd ) and subharmonics of the fundamental frequencies. These nonlinear elastic phenomena are clearly due to nonlinear effects induced by the poor adhesion between the two plates. A new constitutive model aimed at representing the nonlinear material response generated by the interaction of the ultrasonic waves with the adhesive joint is also presented. Such a model is implemented in an explicit FE software and uses a nonlinear user defined traction-displacement relationship implemented by means of a cohesive material user model interface. The developed model is verified for the different geometrical and material configurations. Good agreement between the experimental and numerical nonlinear response showed that this model can be used as a simple and useful tool for understanding the quality of the adhesive joint.

Nonlinearities Associated with Impaired Sensors in a Typical SHM Experimental Set-Up

“…Differences between experimental and numerical signal amplitudes were due to the need of limiting the computational time using particular mesh sizing and contact parameters (not affecting indeed the correlation with the experimental results). As done in [30][31][32], the specimen was excited by harmonic voltage imposed at sensor A and the dynamic response was picked up at receivers B and D. The signal frequency was varied from 2.1 kHz to 138 kHz corresponding to the resonance frequency of sensors with C.L. respectively of 7.5 and 1 mm calculated analytically by Equation (1).…”

“…In this way, it was possible to evaluate the state of each PZT and then to distinguish the fully bonded sensors from those impaired. The identified undamaged sensor was then used to excite the others with harmonic signal sweeping up to high frequencies to find subharmonics induced by LDR ( [31,32]). The presence of subharmonic at f D = f e /2 in the acquired signals allowed to detect and to characterise possible debondings of the sensors mounted on the structure.…”

Second-order harmonic generation of Lamb wave in prestressed plates

“…The generation of higher-order harmonics due to contact nonlinearity has been investigated experimentally for bulk waves [32], Rayleigh waves [33]- [35] as well as for Lamb waves. A number of recent studies have also focused on different types of damage, such as delamination [36]- [38], fatigue cracks [39]- [41], debonding [42] [43], and loosening bolted joints [44] [45].…”

Evaluation of Bonding Quality with Advanced Nondestructive Testing (NDT) and Data Fusion

“…Conventional longitudinal pulse-echo ultrasonic inspection as well as advanced measurement techniques, such as acoustic microscopy, air-coupled ultrasound, and guided waves, have been used to evaluate bonding quality [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. Moreover, nonlinear behavior has been related to bonding quality with nonlinear ultrasonic NDT [ 20 , 21 , 22 ]. Ultrasonic NDT has advantages to detect and position defects since it is a directional technique.…”