Experimental investigation of E/M impedance health monitoring for spot-welded structural joints

Health monitoring results obtained during fatigue testing of a spot-welded lap-sheer structural-joint specimen using the electro-mechanical (E/M) impedance technique are presented. The test specimens were instrumented with piezoelectric wafer transducers, and the base E/M impedance signature was recorded in the 200-1,100 kHz frequency range. Fatigue testing was applied to initiate and propagate crack damage of controlled magnitude. Calibration tests using plain specimens were first performed to correlate stiffness decrease with damage progression and remaining life. During the subsequent health-monitoring tests, the decrease in structural stiffness was used to assess damage progression in the specimen. As damage progressed, the E/M impedance signatures were recorded at predetermined intervals. Signature data were processed, and the RMS impedance change was calculated. Damage index values were observed to increase as crack damage increases. The initiation and propagation of damage was successfully correlated with the E/M impedance measurements. Sensing and the localization principles of E/M impedance method were confirmed, and the rejection of spurious information was verified. These experiments demonstrated that the E/M impedance technique is a potentially powerful tool for damage detection, health monitoring, and NDE of spot-welded structural joints.