Bolt Failure Investigation and Remedial Actions for Non-explosive Holddown Mechanism Used in Dual Axis Drive Mechanism for Space Application

Holddown mechanisms are used for rigidly holding an appendage or a steering mechanism during the launch phase of a satellite and deploying the same in orbit unambiguously. There are various technologies used for holddown mechanisms categorized as explosive-based and non-explosive based. The explosive-based or pyro-based holddown devices are most extensively used and they support large preloads but places where large release shocks are prohibited, the non-explosive devices are incorporated. A new holddown mechanism for dual axis drive mechanism was realized comprising of tensioned bolt and nut pair with SMA actuator as release device and load sensing washer. During the qualification level vibration testing of the system, load drop was monitored in the low level tests and during the full level test, the bolt got fractured mid-way during the test and the test got aborted. A detailed investigation was carried out including microscopic imaging for understanding the failure mode, dimensional re-inspection of all the components, scrutiny of the holddown load at the time of failure, shear load test of the holddown bolt and re-visiting vibration loads. Based on all the above-mentioned actions, root cause of the failure could be established. This paper provides the details of a non-explosive holddown mechanism which is actuated using an SMA actuator for releasing a tensioned bolt. The paper summarizes the anomaly during the qualification test of the non-explosive SMA-based tensioned bolt mechanism. It also chronologically brings out the investigation activities and the root cause for the anomaly is clearly established. Subsequent to the mitigation of the same, re-test of the mechanism was carried out successfully and details of testing are illustrated. The paper helps in suggesting a methodology for investigating the failure of a holddown mechanism and highlights criticality of dimensional tolerances for load components subjected to dynamic loading.