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Quality Assurance of Shot Peening by Automated Surface and Subsurface Residual Stress Measurement

Shot peening is frequently used to produce compressive residual stress in the surface layer of components for fatigue life enhancement and suppression of stress corrosion cracking (SCC). Shot peening is controlled by monitoring Almen intensity. Almen intensity is determined from the arc heights produced in series of at least four Almen strips peened for progressively longer times on one side of the strips. There is, however, no simple relationship between the Almen intensity and the residual stress distribution produced in the 1070 steel Almen strip. Arc height in Almen strips is a function of the induced total strain energy, or the area under the residual stress-depth distribution. Furthermore, quite different residual stress distributions can produce the same Almen strip arc height. Shot peening to the same Almen intensity using different shot sizes will also generally produce different subsurface residual stress distributions. The depth and magnitude of compression developed in a component being shot peened, generally having mechanical properties very different from the Almen strip, cannot be determined simply from the response of a steel Almen strip identically peened. Therefore, the only reliable method of controlling shot peening of a component is by measuring the subsurface residual stress distribution.