Details of the Electro-Mechanical (E/M) Impedance Health Monitoring of Spot-Welded Structural Joints

Health monitoring of structural joints is a major concern of the engineering community. Among joining techniques, spotwelding play a major role. Spot welding is the traditional method of assembly for steel-based automotive structures, while spot-welding of aluminum is being considered for future vehicular structures. Though spot welding of steel is well researched and understood, the spot-welding of aluminum still poses a considerable challenge. The durability and health monitoring of aluminum spot-welded joint is of major importance. The present paper addresses the use of electro-mechanical (E/M) method and piezoelectric active sensors for health monitoring spot-welded structural joints. Experiments were performed on aluminum-alloy spot-welded lap shear specimens under fatigue loading to determine a stable pattern of crack propagation in correlation with applied fatigue cycles. The specimen stiffness method was used to correlate crack advancement through the specimen with specimen stiffness reduction. In this way, an independent method of assessing the degree of structural deterioration as a function of applied fatigue cycles and remaining life was established for that class of specimens. Then, representative test specimens were instrumented with piezo-electric wafer transducers, and the base E/M impedance signature was recorded over the relevant frequency range. Subsequently, the specimens were subjected to fatigue loading such that crack-type damage was initiated and propagated under controlled conditions. During tests, the specimens were continuously monitored for stiffness reduction, and E/M impedance signature was recorded at predetermined damage levels. The crack damage initiation and propagation was correlated with E/M impedance measurements. Damage index values were compared and correlated with the crack damage propagation. Sources of experimental errors were identified and discussed.