OPTIMIZING THE THICKNESS OF MASKING MATERIALS REQUIRED TO PREVENT CHANGES IN RESIDUAL STRESS DURING 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 plastic deformation due to 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. 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 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 X-ray Diffraction (XRD) in a series of 7050-T7451 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.