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Coverage is defined as the measure of the original surface area subsequent to being propelled with shot. Coverage is the least checked and/or understood characteristic of Shot Peening.
However, achieving 100% coverage in the prescribed area is crucial in the performance of the Shot Peened part. Without adequate coverage you might not achieve the compressive residual stress that is you are trying to achieve uniformly over the area that requires this treatment method. In short without proper coverage you may not achieve the added life or strength you are working to induce into the part.
Over the years there have been various methods designed to verify coverage. Below are some various methods to check Shot Peen Coverage.
One of the oldest methods to check coverage was evaluating the Almen Strip Saturation Time. This was accomplished by running the Almen Strips through the shot peen machine a second time and measuring the difference on the Almen Gage. If the difference was less than .002", then the process was defined as achieving 100%.
Another method is to apply a blue dye prior to peening the part. This allows for a visual examination of Shot Peening process. The percent of coverage was calculated by the amount of dye remaining on the part.
The most common method is the use a Fluorescent tracing material whereas; the part is covered with the Fluorescent material and is allowed to dry. The part is then checked under a ultra violet (black) light to assure complete uniform coating. The part is then processed through the Shot Blast chamber and re-examined under the black light. The remaining fluorescent material is measured to determine the degree and percentage of coverage.