Verification of Cold Working and Interference Levels at Fastener Holes

The USAF recognizes that benefits to cold working fastener holes include significantly increased life of a structure subjected to fatigue loading. However, full realization of this potential from a maintenance and engineering standpoint is hindered by the fact that there is currently no adequate field technique for determining how well the material structure immediately surrounding the hole has responded to the cold working process. Thus, there is a distinct need be able to: 1. Determine the success of a cold working treatment at a fastener hole, 2. Quantify the results of the treatment in a robust crack growth and residual life analysis, 3. Monitor the evolution or relaxation of the residual stress state as usage on the component accumulates, and 4. Understand the effectiveness of the residual stress field in the presence of a crack. Our team’s approach strives to provide depot-supportable measurement of cold work residual stresses using NDI techniques currently in the USAF maintenance infrastructure. We also aim to provide the ASIP community with the life assessment tools (based on AFGROW) and criteria needed to capture the benefits of cold working in life extension and to guard against stress relaxation, the influence of cracks, and inadequate processing.