Quantification of Nitride Thickness and Damage Levels in Orbiter/SRB Auxiliary Power Unit Injector Fuel Stems using Distributed Source Positron Annihilation

Injector fuel stems in Orbiter and SRB Auxiliary Power Units (APU) gas generators are subject to operational damage effects that can result in fuel stem failure. In addition, Orbiter APU gas generator fuel stems are subject to a buildup of nitride on the chromized interior surfaces due to the presence of hydrazine and high temperature conditions near the bottom of the fuel stem that can result in an increased susceptibility to corrosion. Over time, the Orbiter nitride layers may degrade resulting in microcracks, which expose the underlying chromized layer to potential corrosion and failure. Nitride thickness variations are a critical parameter in this application and provide an indicator of susceptibility to corrosion and microcracking. Left undetected, damage to both Orbiter and SRB gas generators can lead to an in-flight failure with possible catastrophic consequences. Current inspection technologies, including borescope inspections, may indicate some anomalies, but no nondestructive testing capability exists that can quantify the nitride thickness and/or damage inside the injector fuel stem. The development of an inspection technology that can detect stress effects, degradation of the nitride layer, and the presence of microcracking inside the fuel stem is a key concern for Orbiter/SRB APU gas generator reliability. Positron Systems' new material inspection technology, Distributed Source Positron Annihilation (DSPA), has demonstrated the capability to quantify the effects of material degradation and the onset of microcracking in metals and composite materials at any point in life. The DSPA probe was successfully utilized to nondestructively evaluate changes in nitride thickness and damage for both Orbiter and SRB injector fuel stems (0.095-0.100" ID) in-situ. These measurement results were then compared to as manufactured and in-service APU injector fuel stems to assess accumulated damage. Results from these measurements indicated that DSPA could provide nondestructive, quantitative data to assess nitride layer thickness and damage effects inside the fuel stem such as strain and related damage effects