Acousto-elastic measurements and baseline-free assessment of bolted joints using guided waves in space structures

Abstract

Integrity of bolted joints is critical for successful deployment and operation of space structures. Conventional structural qualification tests span weeks if not months and inhibit rapid launch of space systems. Recent developments in the embedded ultrasonic acousto-elastic method offer fast diagnosis of bolted joints and opportunities for locating the fault. However, in current acousto-elastic measurement procedures, a baseline representing the healthy condition of the joint is necessary. To mitigate a requirement of the baseline, a new methodology based on relative amplitude and phase measurements is developed. The approach has been validated on laboratory specimens, and modifications were suggested for applications in realistic structures. The paper discusses principles of the baseline-free acoustoelastic method, its practical realization, and respective advantages and disadvantages. Comparison of baseline and baseline-free approaches is presented showing the utility of the recently proposed methodology. Fundamentals of the acousto-elastic response were studied in experiments involving guided wave propagation in a thin plate under tension. The results indicate a difference between acousto-elastic responses collected using sensors oriented parallel and perpendicular to the applied stress. It is suggested that this effect may be used to infer stress orientation in the sample. Practical issues related to acousto-elastic measurements in realistic complex structures are discussed, damage diagnosis algorithms are presented, and potential extensions of the acousto-elastic technique are proposed.