J2597_201001 Computer Generated Shot Peening Saturation Curves

The purpose of this recommended practice is to provide an objective method of determining the intensity of peening from a saturation curve. SAE J443 provides instructions for use of the test strip and a review of that document illustrates the need for a standardized procedure.

Procedures for determining intensity have evolved slowly – as illustrated by the following summary: SAE J443 January 1952, Procedures for Using Standard Shot-Peening Test Strip states: “The gage reading corresponding with the point A where the curve flattens out is generally taken as the measurement of the intensity of that particular peening. In some cases, this point is difficult to pick out and requires some judgment.” This same description appears in the June 1961 edition of SAE J443. SAE J443 January 1984 states the following: “Saturation has been attained when the “knee” of the curve is passed and increasingly longer periods of peening time are required for a measurable increase in test strip arc height. The location of the knee, point A shown in Figure 1, can be defined as that point on the curve beyond which the arc height does not increase more than “X” percent when the peening time is doubled. An arc height increase of 20% for doubled peening time may be adequate for some applications. An increase of 10% for doubled peening time defines the knee for critical applications. A smaller percentage increase than 10% requires longer peening time to reach this “knee” in the curve.” The practice of determining the 10% increase was left to the artisan and they often referred to arc heights at double the exposure times until “10% or less” was discovered without focusing on the curve.

The universal availability of computers and curve-fitting capabilities now provides the mechanism for introducing an objective, unambiguous, procedure for intensity determination. Using a computer algorithm to draw a “best fit curve” and to derive a specified “intensity” will give substantial improvements in terms of process control accuracy and repeatability.