Real-time Monitoring Of Damage Evolution In Aerospace Materials Using Nonlinear Acoustics

This work deals with the development of a novel non‐destructive technique based on nonlinear acoustics, enabling real‐time monitoring of material degradation in aerospace structures. When a sinusoidal ultrasonic wave of a given frequency and of sufficient amplitude is introduced into a nonlinear or an‐harmonic solid, the fundamental wave distorts as it propagates, so that the second and higher harmonics of the fundamental frequency are generated.The measurement of the amplitude of these harmonics provides information on the coefficient of the second and higher order terms of the stress‐strain relation for a nonlinear solid. It is demonstrated here that the material bulk nonlinear parameter for titanium alloy samples at different fatigue levels exhibits large changes compared to linear ultrasonic parameters such as velocity and attenuation. However, the use of bulk ultrasonic waves has serious disadvantages for the health monitoring of aerospace structures since it requires the placement of ultrasonic transducers on two, perfectly parallel, opposite sides of the samples. Such a setup is hardly feasible in real field conditions. For this reason, surface acoustic waves (SAW) were used in this study enabling the in‐situ characterization of fatigue damage. The experimental setup for measuring the material nonlinear parameter using SAW was realised and the feasibility of the technique for health monitoring of aerospace structures was evaluated.