Recovery of Preload in Bolted Joint Using Shape Memory Alloy Elements

One of the main problems with a bolted joint is losing its preload. In this situation, it cannot provide the required clamping force needed to keep the joint members together or prevent fluid leakages. Although every effort is usually made at the design stage to prevent such failure, because of the numerous factors present in the problem, these efforts are not always successful. After loosening occurs, the joint should be retightened to regain its preload. However, there are circumstances where the joint is very important but it is not easily accessible and retightening cannot be done manually. The shape memory effect (SME) property can be used in such circumstances to produce the necessary preload. The shape memory alloy (SMA) element should be activated if monitoring the bolt preload through the application of strain gauges shows that the preload has fallen below a pre-determined threshold level. This paper presents a mathematical model for the SMA element and the whole joint behavior. The relation between SMA activation (corresponding to the amount of phase change in the SMA) and the resulting preload is estimated. To this end, it is assumed that the SMA element behaves in such a way that either its cross-sectional area or its volume remain constant. The analysis of this model shows the feasibility of the application of SMA for producing the required preload. Hence, if used properly, the required preload is achieved and a self-healing joint is obtained.