Fatigue Life Variability in Large Aluminum Forgings with Residual Stress

As part of a larger residual stress modeling and standardization program, a computational assessment of the variability in fatigue crack growth life that can result from variability in residual stresses has been conducted. A detailed finite element analysis of the

forge/quench/coldwork/machine process was performed in order to predict the bulk residual stresses in a fictitious aluminum bulkhead. The residual stress profiles on ten critical planes were used to calculate residual stress intensity factors for a range of crack types that are typical for aircraft structural details. These stress intensity factors were used in a standard, linear-elastic-fracture-mechanics-based fatigue crack growth algorithm in order to predict fatigue life for typical fighter aircraft spectrum loading. Calculations were made for residual stress profiles which reflected variability due to machined part placement within the parent forging. For the conditions considered in this study (forging process, machined part placement, critical location / crack geometry, material and fatigue spectrum) we have found that variability in fatigue crack growth life due to part placement is on the order of +/- 20% about the mean. Furthermore, we have found from scaling the residual stress distributions for these conditions, that fatigue crack growth life sensitivity is on the order of 15% to 50% change in crack gowth life with every 5ksi change in peak residual stress.