Structural damage detection using nonlinear and noncontact laser ultrasonic techniques

Abstract

Damage often causes a structure to exhibit severe nonlinear behaviors, and the resulting nonlinear features are much more sensitive to damage than their linear counterparts. Also in structural damage detection field, noncontact laser ultrasonic techniques have gained great popularity with their noncontact nature and high spatial resolution. This dissertation mainly aims at structural damage detection by taking advantages of both nonlinear ultrasonic and noncontact laser ultrasonic techniques. The detailed objectives are: (1) Nonlinear ultrasonic modulation with a wideband laser pulse input

(2) Nonlinear damage feature extraction from wideband ultrasonic responses

and (3) Numerical and experimental validation of the developed techniques. When two distinct frequencies are applied on a target structure, nonlinear modulation (sideband) components are created due to nonlinear ultrasonic modulation mechanism at the existence of structural damage. However, the choice of two input frequencies needs to satisfy some binding conditions for nonlinear modulation generation and can be affected by environmental and operational variations of the target structure. Here, nonlinear ultrasonic modulation is extended by using a pulse laser as the driving signal. Nonlinear ultrasonic modulation can occur among multiple frequency peaks, which highly increases the chance for the binding conditions to be satisfied. For extracting nonlinear features from the wideband ultrasonic responses, first, a sideband peak count (SPC) based damage detection technique is developed to analyze the nonlinearity caused by structural damage in the frequency domain. A nonlinear damage feature is defined by keeping track of the relatively weak spectral peaks generated due to damage induced nonlinearity. This SPC based technique is further optimized by operating SPC in the spectral correlation domain, which increases its sensitivity to damage and its robustness against noise interference. Second, a state space based damage detection technique is proposed by projecting the wideband response into a high-order state space and reconstructing its state space attractor. Another nonlinear damage feature is obtained by checking the geometrical variations of the reconstructed attractors. Moreover, to eliminate the influence caused by varying operational and environmental conditions, with a laser ultrasonic scanning system, a baseline-free damage detection technique is proposed by using the ultrasonic responses acquired from adjacent scanning points as references. Thus, damage can be detected and even visualized without relying on the baseline data obtained from the intact condition. A multi-physics simulation scheme is developed for simulating laser-induced ultrasonic waves on aluminum plates and validating the proposed damage detection techniques with a simulated micro crack. The proposed damage detection techniques are also experimentally validated by detecting fatigue crack in an aluminum plate, delamination in a carbon fiber reinforced polymer (CFRP) plate, and delamination/ debonding in a glass fiber reinforced polymer (GFRP) wind turbine blade.