Nonlinear shear-horizontal waves for structural health monitoring through incipient defect detection

Abstract

Early damage diagnosis using nonlinear ultrasonic guided waves is of great significance to ensure the safety and integrity of structures. Very recent work shows high-order nonlinear shear horizontal (SH) waves are susceptible to incipient defects, pointing at the possibility of achieving Structural Health Monitoring (SHM) based on SH waves. Despite some past efforts, research on the nonlinear SH waves to characterize the material degradation and micro-scale damage are still in infancy. Moreover, a general SHM strategy alongside an effective system design using the nonlinear SH waves is still much needed. This thesis provides a systematic analysis of the underlying mechanisms of damage-related nonlinear SH wave generation, propagation and interaction. It also explores the feasibility of the nonlinear SH waves for SHM applications. More specifically, incipient microstructural change detection inside thin-walled structures using the third harmonic SH (3rd SH) waves and micro-scale crack detection by the second harmonic SH (2nd SH) waves are investigated, respectively. This thesis first addresses the efficient generation and reception of the fundamental SH waves using Magnetostrictive transducers (MsTs). A theoretical shear-lag model is established along with the normal mode expansion (NME) method. The coupling of the MsT with a host plate under inspection is achieved by a bonding layer. Model validations are carried out through the finite element (FE) simulations and Scanning Laser Doppler Vibrometer (SLDV) tests. Influences of the coil configuration and bonding conditions are investigated through theoretical analyses. The model is applied to guide the design of an effective SHM system for the detection of material degradation using the 3rd SH waves. A quantitative comparison of detection sensitivity between the 2nd Lamb waves and the 3rd SH waves is provided. A theoretical framework is established to link the dislocation dipole density with the second and third harmonic wave responses. The primary S0-secondary S0 mode pair and primary SH0-tertiary SH0 mode pair are selected for the experimental studies. After successful mitigation of the undesired and adverse effects of the adhesive nonlinearity (AN) from the actuating part, an experiment is conducted to demonstrate the higher sensitivity of the 3rd SH waves over the 2nd Lamb waves to microstructural changes. The capability of a 3rd-SH-wave-based SHM detection approach for microstructural characterization and monitoring is further investigated. A material characterization technique—X-ray Diffraction test (XRD)—is conducted to witness the second phase precipitation in the material. The optimized 3rd-SH-wave-based SHM system is applied to monitor the material degradation in a plate treated by a thermal aging scheme, cross-checked by Vickers Hardness tests. In a different application, the detection of micro-scale cracks using the nonlinear SH waves is also investigated. The 2nd SH wave generation mechanism due to the contact acoustic nonlinear (CAN) is demonstrated. The influence of the AN on the 2nd SH wave responses is also discussed. An initially-closed breathing crack is simulated by the contact analysis using FE models. In the experiments, the damage is created through the point force pressing in a glass panel. The variation of AN is achieved through a low-temperature heating process. It is shown that the 2nd-SH-wave-based SHM systems offer high sensitivity to breathing cracks while strong robustness and immunity to the undesirable yet unavoidable adhesive nonlinearities. As a whole, the work presented in this Ph.D. thesis offers a comprehensive analysis of the properties and utilization of the high-order nonlinear SH waves in view of their applications for the detection and monitoring of incipient micro-structural damage in structures and material degradations.