In this study, the feasibility of monitoring the structural integrity of welded thick aluminum plates was experimentally tested using two widely used SHM methods: impedance and Lamb wave analyses. The test structure was fabricated from two 1/4 inch thick aluminum plates welded together, and various structural defects, such as holes and cuts, were applied. At each of these damage steps, data were collected for both the impedance and Lamb wave techniques. Results consistently revealed the impedance method to be sensitive to damage in and through the weld. The envelopes of the Lamb wave signals were calculated using the S-transformation of the time histories. There was significant change to the curves when different defects were added to the plate. Both of the SHM methods studied detected each of the cuts and holes acting to reduce the overall strength of the structure. Each technique also detected the hole damage on the opposite side of the weld as the sensor(s) used for damage detection. The study further verified that surface waves move across welds allowing SHM methods to detect the defects even if the sensors are located on neighboring plates or geometries.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.