Stress corrosion cracking of aluminum structural alloys and statistics of failure

Stress corrosion cracking (SCC), a premature failure mode due to the synergistic action of sustained tensile stresses and aggressive environment, is commonly observed in aircraft structural materials. The breaking load method has shown considerable promise as an accelerated test for quantifying SCC in terms amenable to interpretation using statistics and fracture mechanics. This method was employed to investigate development of SCC damage in alloy 2024 in naturally aged (T3) and artificially aged conditions (T8). The statistical procedure for data analysis, based on analysis of variance (ANOVA) methods, was applied to evaluate the effects of test parameters on experimental results. Corrosion damage in these alloy tempers was measured by comparing the post-exposure fracture stress of short transverse tensile specimens with the original tensile strength. Specimens were exposed to 3.5% salt water alternate immersion at various stress levels and time intervals. Experimentally determined fracture stress versus flaw size variations were compared to analytical results. A model based on a single semi-elliptical flaw geometry allowed prediction of maximum flaw depth from post-exposure fracture stress data. Breaking load test results are further interpreted in terms of their potential applicability for durability assessment of structural components. Resistance to cracking in AA 2024-T351 was improved significantly when specimens were coated with the Fluid Film\sp\circler corrosion preventive compound. The effect of alloy microstructure on SCC resistance is also discussed