Adaptive Health Monitoring Concepts for Spot-welded and Weld-bonded Structural Joints

The health monitoring of structural joints is a major concern of the engineering community and needs to be addressed with the proper consideration. Among joining techniques, the spot welding and weld-bonding (spot weld + adhesive bonding) methods are of great interest in a number of industries. Spot welding is the traditional assembly method for steel-based automotive structures, while weld-bonding is a novel technique that combines the stiffness and productivity benefits of adhesive bonding with the proventechnology attributes of spot welding. Future trends in the design and construction of vehicular structures indicate a strong diversification of material usage, with aluminum and polymeric composites projected to play a major role. While aluminum is amenable to both spot welding and adhesive bonding, composites will, most likely, be entirely adhesive bonded. Hence, the trends towards weld-bonding and adhesive bonding are clearly visible. The paper first presents a short review of the present trends in joining techniques with special emphasis on those in present and future use in the automotive industry. This will be followed by a short review of the existing non-destructive evaluation (NDE) techniques that may be applicable to the health monitoring of spot welded and weld-bonded structures. Next, novel concepts of health monitoring utilizing an adaptive structures approach are presented. Laser ultrasound techniques; tagged adhesive methods; dielectric response techniques; electric potential methods, etc. are briefly mentioned. Particular emphasis is placed on the electro-mechanical impedance method. This novel method is described in some detail. The electro-mechanical impedance method utilizes the coupled electro-mechanical response of piezo-electric interrogators (sensoractuator crystals) intimately bonded to the monitored structure. An array of such structural interrogators can be envisaged to monitor the incipient local damage and crack growth in spot-welded and weld-bonded structures. Preliminary proof-of-concept demonstration tests performed with this method have shown remarkable features and ease of utilization. 1. PRESENT AND FUTURE TRENDS IN JOINING TECHNIQUES FOR AUTOMOTIVE STRUCTURES The workshop "Basic research needs for vehicles of the future, an integrated perspective of academic, industrial, and government researchers" held at the Princeton Materials Institute, in January 46, 1996, under the sponsorship of the National Science Foundation and the Department of Energy in partnership with Chrysler Corporation, Ford Motor, and General Motors Corporation (Anon.,1996) identified light-weight materials as one of the six key areas for future development of vehicle systems. In the light-weight materials area, joining techniques was one of the five focus issues. Besides the continuous use of steel, two additional material classes are being considered for future vehicles: (a) light metals aluminum and magnesium; and (b) polymer matrix fiber composites. Future joining techniques will have to target these material classes too. Hence, the following joining objectives were outlined: 1. Establish molecular basis for next-generation joining technologies to reduce cost and improve reliability. 2. Improve laserand resistance-welding of aluminum, magnesium, and their alloys through in situ thermalprofile monitoring and feedback control. 3. Establish surface chemistry effects on weld quality. 4. Extend NDE techniques for welded joints. 5. Develop adhesives to bond metal-to-metal, metal-tocomposites, and composite-to-composite surfaces; 6. Study the effect of surface cleanliness on adhesive reliability; 7. Develop NDE techniques for adhesive bonds during manufacturing and in service. The finding and recommendations of the workshop indicated that welding and adhesive bonding will be the primary low-cost joining methods. In the automotive industry, the welding technique of choice is the resistance welding, with most of the processing being done through the resistance spot-welding variety. This technique is applicable to both steel and aluminum structures.