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Within the automotive industry, a growing interest in lightweight vehicles has led to a need for novel methods of multi-material joining. Development of methods for joining various materials, with aluminum in particular, has become an increasing focus for many vehicle manufacturers. Electromagnetic forming (EMF) provides a method for creation of tubular joints, which have great potential for use within space-frame structures. This thesis attempts to more comprehensively understand the behavior of multi-groove electromagnetically formed joints under axial tension loading, and suggests an overall methodology for analyzing mechanical joints based on micro-displacement under load. This thesis focused on forming aluminum tubes to steel mandrels. Three different mandrel geometries, each with a different basic groove geometry; rectangular, curved, and trapezoidal, were designed and tested. These each utilized elastic averaging, and incorporated three grooves of varying depths, in order to distribute the load evenly across all grooves. A set of fixtures was designed in order to facilitate the creation of consistent electromagnetically formed joints using each of these geometries. These joints were loaded under various axial tension tests, and ezz strains in the grooves were monitored, primarily using a digital image correlation (DIC) optical strain tracking system. This was intended to compare strain distribution between the three grooves and observe micro-movement within the joints.The three different geometry joints were tested under quasi-static loading until failure, and all joints exceeded the full tensile strength of the undeformed aluminum tube. Significantly lower strains were found in the trapezoidal and rectangular geometry joints, as well as higher stiffnesses, than in the curved geometry joint. The trapezoidal joints were then progressively loaded to 20%, 40%, 60%, and 80% of the tube's ultimate tensile strength. Movement of the mandrel within the joint was observed; however, complications prevented further conclusions from this testing. The rectangular joint was then tested under low cycle testing at 20%, 40%, and 75% of ultimate tensile strength. Initial movement of the joints was observed during the first few cycles, but nearly all samples showed signs of global joint shakedown. Finally, a thermoplastic heat-shrink layer was applied to the joints as a method of galvanic insulation, and the resulting joint was quasi-statically loaded. The insulated joint maintained full tube strength, but showed greater strains than those seen within non-insulated joints. |
