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. BENEFIT: AP/ES continues to focus on the research and development of analytical processes that simulate fatigue from nucleation, through short crack growth, to structural failure of components. This advanced service life assessment capability places APES into the unique position to pursue comprehensive solutions to problems across many air vehicles platforms and multiple industries that need to assess and maintain the integrity of their structure. APES efforts have improved fatigue analytical models that account for surface integrity effects in the nucleation phase from constituent particles, fretting, effects of many types of corrosion, and the interaction of widespread or multiple site damage (WFD or MSD). APES successes have been leading the industry in life prediction approaches that are presently being pursued by others in the AFRL and other DoD organizations. APES models are readily integrated into or linked with the existing life prediction methodologies and codes, such as FASTRAN and AFGROW. The addition of residual stress influences and the time-dependent nature of residual stress relaxation to crack growth modeling will be a powerful augmentation to the toolbox available to us as consulting engineers and to our customers. The keen interest in these holistic structural integrity processes has already provided our business with close links with various military and civilian aviation organizations in the US and Canada. A key element of the proposed program is the concurrent development and validation of non-destructive inspection techniques that can be directly integrated into existing USAF maintenance programs using equipment already in the USAF inventory. The advantage that would be afforded to APES and other project participants, which includes the multiple aircraft customers and agencies, is that the integration process is established along with the technical solution. The proposed approach thus enables rapid and cost effective incorporation into the particular customer’s infrastructure and provides opportunities for APES to grow as a consulting engineering business, which is our forte. The work proposed here directly supports the needs of at least two USAF ASIP offices and, if successful, could provide a direct path to commercialization in Phase III.