Nondestructive evaluation of residual stress at cold expanded holes

The United States Air Force (USAF) has significant experience applying hole cold expansion (Cx) to extend fatigue life and commensurately increase inspection intervals for safety-of-flight structure. At the present time, however, the maximum allowable credit for Cx (per USAF guidelines) is limited to the improvement expected for a reduced initial flaw size in a crack growth assessment (e.g., a reduced 0.005-inch radius flaw versus a typical 0.050-inch radius flaw). This results in a benefit often referred to as ôpartial credit.ö In the case of A-10 and T-38, for example, critical fastener holes are the primary driver for structural inspections, accounting for approximately 70% to 90% of the fatigue critical locations on the airframe. The "partial credit" is driven by the lack of Quality Assurance (QA) tools to ensure the correct processing occurred at the correct critical locations. This program will develop the necessary QA tools enabling more effective structural integrity management, extended inspection interval requirements, and increased aircraft mission capability and rates.