Understanding and Exploiting Applied Loads for Guided Wave Structural Health Monitoring

A challenging goal of aircraft structural health monitoring (SHM) systems is to provide continuous assessment of structural integrity during flight. It is now widely recognized that variable environmental and operational conditions can compromise results and thus lead to erroneous maintenance and usage decisions. This issue is particularly evident for SHM systems utilizing ultrasonic guided waves because of their known sensitivity to these variable conditions. Much attention has been given to the effect of temperature variations because they are both unavoidable and readily investigated in the laboratory. More recently, the effects of applied loads on SHM systems have been considered. On the one hand, loads affect both wave speeds and propagation distances similarly to temperature changes, but the anisotropic nature of such changes can compromise SHM techniques that require baseline comparisons. On the other hand, applied loads can open cracks and thus enhance their detectability. Here these loading effects are considered along with their impact on detection and localization of damage via guided wave imaging with a spatially distributed array. A new approach is investigated whereby the load-dependent behavior of the guided wave signals is used for improved damage detection and localization without requiring baseline data from the damage free structure. Experimental results are shown from fatigue tests performed on an aluminum plate specimen that was instrumented with a sparse array of ultrasonic guided wave transducers.