Cyclic behaviours of superelastic shape-memory alloy plates joined by tungsten inert gas welding

The limitations of connection technology have long been an obstacle to constructing large-dimensional elements made of shape memory alloy (SMA). This is because techniques used to join small SMA elements (e.g., wires) may not be capable of joining the large-dimensional elements required for civil engineering structures. This study explored the feasibility of using tungsten inert gas welding to connect SMA elements. A welding technique was developed and tested on 2.5-mm SMA plates. Experimental studies used three measurement systems (including a smartphone-based digital image correlation system) to capture the cyclic behaviours of the specimens. The initial welded specimen exhibited stable flag-shaped behaviour with a desirable strength and ductility before the weld zone was activated (i.e., yielded). Subsequently, three additional specimens that had 70%, 50% and 30% reductions in the cross-section areas of the weakened segments were fabricated and tested. This weakening was performed to prevent activation of the weld zone and to concentrate any inelastic deformation in the base materials. The test results showed that all of the weakened specimens had desirable performances, showing the limited influence of the welding on cyclic properties. The 70% weakened element exhibited nearly the same behaviour as the unwelded specimen, but the large degree of weakening may result in uneconomical designs. The 50% weakened specimen achieved a good balance between the degree of weakening and delaying the failure. The 30% weakened specimen had a desirable performance, but the fracture occurred in the weld zone. These results successfully demonstrate the feasibility of using tungsten inert gas welding to create connections between large-dimensional SMA specimens, which will allow the production of larger-dimension and complex SMA elements in the future.