During vibration testing there was contact between the aft cap and SM side actuator bracket and the  forward lug and text fixture, with significant wear observed during post-test inspection. This produced a constraint and source of induced reaction force off-axis from the strut. The strut design also permitted a  CG offset due to a combination of sliding contact interfaces, machining tolerances, non-axisymmetric nature of select piece parts, elastically buckled spring, and gravity loads as installed in the random vibration test fixture. The design also did not include an alignment feature integral to the strut, therefore  forward lug and aft cap strut end fitting alignment was not guaranteed and permitted rotational  misalignment between the two rod ends.   In the first test configuration (Z-axis), this contact constraint, combined with inertial forces from an offset  of the strut CG and maximum SM side environments, resulted in an induced loosening torque on the aft  cap. The lack of prevailing torque from an undersized aft cap locking patch increased the joint’s  susceptibility to loosening. The aft cap was observed to have loosened after Z-axis testing whereupon it  was retightened to the required torque. Lockwire was then re-applied in larger quantities and gauge thickness to further increase the torque reaction capability of the joint.   In the last test configuration (Y-axis), a similar contact condition induced a torque at the forward strut  interface. The maximum umbilical side environments were applied during Y-axis testing, which, when  combined with the CG offset, induced a significant torque about the forward lug. Given a contact condition, again observed through post-test inspection, the forward lug relative to the secondary pistoncould have been free to rotate in the direction of tightening while contacting the test fixture off-axis from the strut centerline in the direction of loosening. The contact condition would have induced a torsional load to loosen the forward cap-to-secondary piston interface. Once the inertia-induced torque exceeded  the capability of the joint which includes a combination of breakaway torque, prevailing torque from the locking patch, and lockwire capability, the secondary piston rotated relative to the forward lug breaking the lockwire. At this point the resistive torque was limited to that provided by the undersized locking patch. The rotation continued until the CG offset was small enough relative to the applied force vector that the  prevailing torque of the locking patch exceeded the applied torque (i.e., approximately 90 degrees).  Due to the rotation, strut tooling holes, nominally in the neutral bending axis during the Y-axis vibration  test, were placed in maximum bending. Once the fatigue life capability at the tooling hole was exceeded,  the secondary piston failed by crack growth and overload.   Additional Development Testing After assessing the results of the NESC-led test and identifying proximate causes of the failure, LM  reclaimed responsibility for further strut development testing. Programmatic pressure drove LM to make  immediate, low-impact design changes based on proximate cause. The NESC continued the investigation into root cause in parallel with the spring strut redesign effort.  The NESC team was kept apprised of the redesign activities by the MPCV CM subsystem management  team. LM made design changes intended to prevent loosening of the threaded parts and eliminate fatigue  initiation sites, then retested the strut. These changes included an increase in the size of the secondary  locking patches, quantity and diameter of lockwire, and preload at the end fitting joints (50 percent of Fty).Additionally, the tooling holes were removed and a new retention tool design was devised. Once the initial redesign effort was complete, LM began with the Y-axis test, which was the worst case and the axis in  which the secondary piston failed. Shortly after reaching qualification levels, the strut suffered another  failure. This time, the forward lug fractured at the secondary piston interface. A subsequent investigation by LM determined fatigue failure was the cause and that the corrections made appeared to be working up  to the point at which the testing was stopped. The LM retest had reused a majority of the hardware from the original test unit (including the forward lug) without a residual life assessment. Analysis revealed the fractured rod end had exceeded its fatigue life capability. Gapping in the joint between the rod end and  the secondary piston exacerbated the problem. 401