Small Scale Crack Growth Sensor for Determination of AA5XXX Susceptibility to SCC

摘要

Stress corrosion cracking (SCC) of AA5XXX exposed to the atmosphere Is a critical safety issue for DoD assets, often resulting In service failures. Previous research found in literature has demonstrated that SCC is controlled by complex interactions of load, environment, electrochemistry, and alloy properties. Traditional environmental fracture testing conducted under immersion conditions may produce results that are much different than measurements collected under thin electrolyte layers or droplets formed during atmospheric exposure. To more accurately assess AA5XXX susceptibility to SCC, a standalone in situ crack growth sensor has been developed to monitor crack propagation within a surrogate tensile sample under realistic atmospheric conditions. Using tailored test specimens and exacting methods for loading, corrosive conditions and load interaction effects on SCC can be evaluated in accelerated tests and outdoor environments. These measurements can then be used to identify the significant relationships between environmental parameters and SCC failure processes. Systematically varying environmental and mechanical parameters (e.g., relative humidity, salt molarity, degree of sensitization, and stress intensity) has shown a strong dependence of crack velocity on cyclic relative humidity (RH). Specifically, an increase in crack velocity has been observed with decreasing RH. This is contradictory to previous perceptions that cracking increases proportionally with increasing RH, and reflects ongoing uncertainty in literature of the exact mechanism of corrosion and SCC during drying of a thin electrolyte. To extend this analysis to more realistic conditions and further investigate the effects of drying on SCC, susceptibility of UNS A95083 material to cracking has been monitored during standard accelerated corrosion tests, including ASTM B117 and ASTM G85 A5. The effect of specific environmental test cycles and cyclic drying on SCC velocity was quantified and will be presented. The application of these results to establish more aggressive accelerated corrosion test conditions that mimic service environment cracking processes will also be discussed.