Protoqualification (PQ) testing, typically performed on the first assembled electronic units and spacecraft, establish random vibration and thermal cycling fatigue margins for subsequent builds that are only subjected to the acceptance test program (ATP). Considering the fatigue damage induced by both unit- and spacecraft-level testing, fatigue margins are calculated for equipment exposed to ATP-level testing. Probabilistic analyses are also performed to determine the relationship between the fatigue margin and the demonstrated reliability for the unit to survive the mission. Parameters such as the number of retests and the material dependent fatigue acceleration exponent are considered.
Using MIL-STD-1540E,1 PQ random vibration test guidelines of three decibels (+3 dB) above the acceptance input level for 2-minute-per-axis duration, the assessment shows that the test establishes ample fatigue margin for units subjected to the less stringent ATP. The probabilistic analysis shows that the established reliability to survive the launch environment without a fatigue induced failure is adequate. It is further shown that the desired high-cycle fatigue life factor of greater than four is maintained even after one unit retest is performed. However, since this conclusion is general in nature, it is important that for future spacecraft, assessments be performed for both PQ and ATP hardware, taking into account each specific design and application to verify that adequate fatigue margin exists to survive launch.
Because mission thermal cyclic environments vary greatly with regard to the number of cycles and the extent of the thermal range for each cycle, MIL-STD-1540E PQ guidelines for unit-level testing were designed to verify the design and workmanship and not intended to demonstrate fatigue margin for specific missions. Rather, MIL-STD-1540E specifies that thermal cyclic fatigue margin for the combination of pre-launch and orbital environments should be demonstrated with life testing. Accordingly, the analysis shows that PQ testing only demonstrates the desired low-cycle fatigue life factor of 2 for subsequent units which are subjected to a single ATP. The fatigue life factor falls below 2 when retests and/or mission thermal cyclic environments are considered. Because acceptable fatigue margin is not established with the standard PQ thermal cycle test procedure, it is essential that other means be taken to ensure that adequate fatigue margin exists. This paper provides guidelines for using analyses and offline life testing to assess the fatigue life of critical units and their internal components for pre-launch and orbital thermal cyclic environments.