Shape Memory Alloy Ring-based Actuators for Thermally Activated Emergency Venting System

Storage of rocket motors in containers, hangars, or other enclosed spaces pose significant safety and security risks under abnormal conditions, such as accidental fires. For such pressurized vessels, mitigation plans depend on engineering solutions such as a thermally initiated venting system (TIVS), consisting of a temperature sensor and linear shape charge (LSC) that controllably detonates and cuts the motor casing at a critical temperature, relieving pressure. However, this and similar venting approaches depend on external systems and sensors requiring batteries and/or monitoring, which increases the dry mass of the rocket motors due to permanent integration. This can also alter the aerodynamic conditions in flight.

In this work, the development of a non-energetic shape memory alloy (SMA) ring-based actuator designed to deliver high-force, high-displacement for a novel venting system is presented, which passively actuates at abnormal temperatures, generating sufficient force to breach or clamshell the pressure vessel and allow venting. The device consists of NiTi teardrop-shaped rings formed into custom teardrop shapes using a modified ASTM-E3097 procedure with a custom thermomechanical rig. Rings with a 3mm wire diameter and a hoop diameter of 138mm were found to produce 2225 N of force (~ 580 MPa) and over 8mm of displacement under load. To increase the total force required for cracking across various materials, multiple rings were stacked in series to increase the cumulative force. Finally, a custom SMA was developed to meet the specific critical temperature requirements of the rocket motor, including conditions such as ignition temperature and blocking force requirements.