Sensitivity limitations for guided wave structural health monitoring

Abstract

Deployable guided wave systems are commercially used for the inspection of long lengths of pipelines in non-destructive testing (NDT) applications. On this basis it might seem that guided waves could be used in a structural health monitoring system (SHM) that is able to detect damage anywhere in a structure with a relatively sparse array of permanently attached sensors. Furthermore, while guided wave NDT is limited to simple structures because of the problem of signal interpretation, reference signal subtraction can be applied to guided wave SHM hence apparently solving the problem of structural complexity. Despite this, and considerable international research effort, there have been no serious commercial applications of guided wave SHM. In this paper, the concept of guided wave propagation and reference signal subtraction are examined at a fundamental level to analytically estimate the sensitivity of the reference signal subtraction approach. It is argued that the limitation on sensitivity is the size of the residual signal left after baseline signal subtraction. The subtraction is never perfect due to environmental changes and results in imperfect cancellation of the signals from benign structural features, such as welds, edges, flanges etc. It is shown that the sensitivity decreases with propagation distance and therefore sensor spacing. Examples of the required sensor pitch to detect a 6mm hole in a 3mm thick aluminium plate subjected to a 1°C temperature change are given, and show the significant detrimental effect that even small temperature changes can have. It is shown that a significant improvement (typically 20 dB) is possible if signal envelopes rather than RF signals are subtracted but that this leads to the problem of sensitivity functions that vary non-monotonically and which may even include blind spots.