Smart Lockwire: A Shape Memory Alloy Lockwire for Improved Reliability in Bolted Fixing in Automotive and Aeronautical Applications

Shape memory alloys (SMA) have a unique behavior due to a reversible phase transformation between two solid crystalline structures: martensite (low temperature and low stiffness phase) and austenite (high temperature and a higher stiffness phase). This transformation can occur either as a result of temperature change or mechanical stress load, both above characteristic critical values of these materials. Due to the reversible nature of this phenomenon, direct transformation occurs when austenite transforms into martensite and reverse transformation when martensite transforms into austenite. The latter is induced by raising the temperature and it is this process that occurs during the generation of significant associated forces through the undergone deformation recovering of the material, being of fundamental importance for the use as actuators. The use of lockwires is common in aviation and automotive industries as an extra precaution to prevent possible screw loosening (due to vibration, temperature increase or other external forces) in bolted joints where it is not possible to use locking nuts. The use of a lockwire is also a requirement for approval in safety technical inspections present in maintenance standards in aviation and motor racing. The operating principle of a conventional lockwire is to lock, through a metal wire, one or more screws that are not using locking nuts. Usually made of stainless steel, these fasteners have the characteristic of acting passively on the screws. In this context, the objective of this paper is to present a lockwire manufactured with a Ni-Ti SMA able to work actively on the bolts through their tendency to contraction with increasing temperature. The force generated by the phase transformation of the Ni-Ti SMA wire promotes an extra torque on the screw union, further reducing the risk of loosening and thus increasing system reliability.