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It is well known that plating and other coatings processes can cause reduction of Fatigue Resistance in metal parts. Shot peening prior to coatings processes will restore most, or all, of the lost fatigue properties. To assure repeatable and reliable results from the shot peening process, the shot must be controlled. The most critical characteristics of peening media are size and shape. Improperly sized shot will give too little, too much or inconsistent layers of compressive stress. Improperly shaped shot may cause surface damage and fatigue properties lower than in the unpeened condition. For the past forty years, the methods for measurement of shot size and shape have not changed. Shot size is measured by sieve analysis and shape is measured visually by an inspector. These inspection techniques lack speed, accuracy and repeatability and are subject to a significant amount of human error.

Many aerospace components undergo a finishing process during their manufacture. Such finishing processes include chromium, nickel, zinc and cadmium plating on steels; anodize on aluminum alloys; and various coatings on titanium and nickel base alloys. Certain of these finishing processes result in loss of fatigue strength of the part in question. Loss of fatigue strength caused by chromium and nickel plating amounting to 60% has been measured. Shot peening is frequently used on fatigue critical parts to regain fatigue properties which may have been lost due to a particular finishing operation. Shot peening is a process which bombards the surface of a part with small spherical balls. The cold working effect of the peening balls leaves the surface of the part in a state of residual compressive stress. The beneficial compressive stresses induced by shot peening are effective in increasing fatigue properties of the component.