Separating "Intensity" from "Coverage" or "Cycle Time" has been one of our biggest challenges. Our SAE committee decided to remove references to coverage from SAE J443 “Procedures for Using Standard Shot Peening Test Strip” and create a new document SAE J2277 entitled "Shot Peening Coverage" issued in January of 2003.
First, let's discuss the problem and then the solution. Our goal is to impart a uniform compressive stress profile on the surface of a part that is subjected to cyclic fatigue and hence improve its fatigue life or load capability. The depth of the effective compressive stress is determined by the peening intensity. Peening intensity is controlled by shot size hardness, velocity etc. and this results in stress zones at a particular depth. It would seem reasonable to therefore insure that the entire part or critical area is peened with sufficient impacts so that the compressive stress is achieved. So, to control the process we must accomplish two things: determine intensity and determine coverage. As you will see below it is necessary to first determine intensity and then establish the correct peening cycle time.
There is presently no “Intensity Meter” that allows us to gage the shot particle intensity so we have to rely upon the Almen method of imparting stresses to a test coupon. By making dents on one surface of the test strip we can force the strip to curve. It will continue to curve as it accepts more dents. As this process continues the curving diminishes and stabilizes. By mapping this process onto a graph of strip curvature (arc height) versus exposure time we can witness the resulting relationship. This graph has become known as the Almen Intensity Saturation Curve, or saturation curve. But having the curve was in itself not sufficient to provide daily process control. It was up to Almen to devise a means of “interpreting” the value of the saturation curve. This leads to the 10% rule that states, “Intensity is the first point on the saturation curve whose value, when the time is doubled, increases by no more than 10%. Once the peening machine is operated at this condition we can proceed to determining the proper cycle time.
If you were to examine a single dent on the Almen strip we would observe that (approximately) a .009-inch dent diameter occurs after impact with shot at a .010 inch “A” intensity. Accumulation of enough of this size dent on the Almen strip causes it to curve as exhibited by the saturation curve. But what happens if we hit another target material that is different from the Almen strip? The Almen strip is SAE 1070 cold rolled steel with hardness range of 44-50 HRc. If you were to substitute a much harder part, perhaps a carburized gear with hardness of 60 HRc you would see a much smaller dent. If you were to substitute a much softer part, such as aluminum with hardness equivalent to 30 HRc then we would see a much larger dent. Therefore to completely cover the hard gear would take a lot more dents and longer exposure time then it would for soft aluminum.
So, my question to you (or your friends) is “Why would you use the Almen strip denting characteristics to set the machine cycle time? If you want complete coverage of your part wouldn’t this imply complete denting of the intended target, not the Almen strip? The Almen strip is used ONLY for intensity determination. Period.
