Shape-memory alloy mechanical contact devices

The effect of current cycling on the performance of bolted aluminum-to-aluminum connections employing different mechanical contract devices has been studied. Current-cycling conditions and different initial contact loads (0.7 and 2 kN) have been used. The devices considered were lock-spring, conventional disc-spring (Belleville) and shape-memory alloy disc-spring washers. The results show that the mechanical and electrical integrity of the connections is strongly affected by the joint configuration, which differs according to the mechanical device used. The CuAlMn shape-memory alloy disc-spring washer was found to significantly lower and stabilize the contact resistance in a bolted joint, even when incorrectly installed (low initial contact force). The observed effect is explained in terms of a strong temperature dependence of the shape-memory alloy mechanical properties (spring rate) over a narrow temperature range. The mechanism responsible for this dependence is reversible martensite-austenite phase transformation which enables the shape-memory alloy to act as a sensor and a force actuator. This is of significant practical importance since field experience has shown that very often joint failures are linked to the incorrect installation of the joints.