Practical issues in self-repairing bolted joints

The goal of this research is to reduce the likelihood of failure and the cost of maintenance of critical bolted joints. To reduce the self-loosening mode of failure, the concept of a self-sensing and self-healing bolted joint has been developed. This concept combines piezoelectric-based health-monitoring techniques with shape memory alloy (SMA) actuators to restore tension in a loose bolt. Many practical issues need to be addressed before the self-healing bolted joint can become a reality. One of the primary issues is the actuation of the SMA washer. The relatively large mass of the SMA washer and low resistance because of its short length make resistive heating particularly difficult. In addition, the large mass of the members connected by the joint can often act as a heat sink for what heat is generated. Therefore, a series of models was developed to assess the viability of resistive heating and provide an estimate for the power requirements for effective actuation. Modeling and experimental testing have shown that the use of external heater can be used to actuate SMA actuator with conventional power sources. By making the SMA washer substantially easier to actuate, this method provides a convenient alternative to the resistive heating, which requires very large currents needed for heating, and make the adaptive joints more accessible to real-field applications. This paper summarizes considerations needed to design SMA actuators, experimental setup and procedures, and several implementation issues and can be used as a guideline of future investigation.