Ultrasonic guided waves, due to theirs capability of interrogating a large structure from a single sensor position, has been proven as a promising tool for structural health monitoring (SHM). In this paper, we present two imaging approaches of utilizing guided wave leave-in-place sensors for early detection of defects in plate-like structures as well as for monitoring the defect growth. The first approach is based on a guided wave tomographic technique, in which the region surrounded by a sparse sensor array is monitored. The second one is a phased array approach, in which sensors are attached to a structure in a compact format to form an array. The region subjected to inspection and monitoring is the region outside the array. Both techniques have shown excellent capability of determining damage size, location, and severity.
Multi-Wave and Hybrid Imaging Techniques: A New Direction for Nondestructive Testing and Structural Health Monitoring