The Effect of Inherent Tolerances in the Almen Test Strip on Shot Peening Process Reliability

Author:  R.S. Simpson, D.O. Clark & G.L. Chiasson
Source:  The Shot Peener magazine, Vol 02/No 1, March 1988
Doc ID:  1988099
Year of Publication:  1988
Abstract:  
Defining current allowable tolerances for Almen Test Strips per applicable U.S. Military and industry specifications, and how they affect shot peening process reliability and reproducibility, is the purpose of this paper. The Almen strip has historically been and currently is the only available non-destructive measurement instrument utilized to represent the amount of aggregate energy transfer to a workpiece resulting from the shot peening process. Since there is currently no available means of shot peen process non-destructive test performed directly on a workpiece, the importance of the almen strip continues to remain high. An area of much recent discussion in technical circles concerned with the shot peening process has been what quantitative and qualitative information the Almen strip can provide, and what it can and cannot measure in terms of process effectiveness. While important, there is another concept which needs considerably more definition than is currently available in technical literature. This is Almen test strip reliability and succinct quantitative definition of how reliably the Almen strip, as currently specified in applicable U.S. Military and industry specifications. As in any process measurement, the cumulative acceptable tolerances of gauges and measurement instruments are of critical importance. Commonly specified production Almen intensity ranges are +/- 2A. Not uncommon is +/- 1A. During research conducted by the authors, several phenomena associated with the Almen strip have been identified as affecting its capacity to reliably measure processes specified to a +/- 2A or smaller intensity range.