Evaluating The Effectiveness Of Localized Masking To Prevent Changes In Residual Stress When Shot Peening

When shot peening components, it is common practice to mask certain localized regions where peening roughness is not desirable. For example, surfaces that have been highly machined such as bearing races or areas of interference-fit may require masking to protect them from the cold working effects of shot peening. Oftentimes shot peening the grip sections of fatigue test coupons can prevent cracking and failures due to fretting fatigue in the grip area but could affect the outcome of the test if applied to the gauge section of the coupons. Thus, maintaining an “as machined” surface roughness is the most common reason for masking an area when shot peening, where no-observable-dimples is the quality standard. In the aerospace industry, masking is also used on thin areas to prevent distortion or changes to the shape of the part. There are several techniques commonly used for masking. For high production parts, permanent masking tools are often made from rubber or abrasion resistant (UHMW) plastics whereas tape is commonly used when a lower quantity of parts is being shot peened. However, the Authors of this paper could not find any specifications in the literature that describe either what a suitable thickness of the tape should be, or how many layers of tape should be used for any given peening intensity and coverage. As common practice today, one “layer” of tape is typically used for lower peening intensities whereas two “layers” are normally used when peening at higher intensities. The question that this research aims to answer is, even though dimples from peening may not be not apparent, was there some level of compressive stress introduced in taped regions that could change the performance characteristics in presumably “tape-masked” regions? To answer this question, a comprehensive test plan was developed to map residual stresses using the X-ray Diffraction (XRD) in a series of 7050-T651 aluminum alloy coupons that were masked with various tape thickness’ and shot peened using various peening intensities. Regions that were shot peened with and without tape-masking were characterized, as well as transition regions since some modeling efforts predict that tensile residual stresses directly at the transition area could create a stress concentration. The following paper discusses the results obtained and conclusions derived from this research.