Almen Strips FAQ

Electronics Inc. Almen strips are available in various grades based primarily upon flatness (pre-bow) and hardness. The ideal Almen strip would have zero pre-bow and would have exactly HRc hardness of 47. The three most common pre-bow tolerances are shown in the following table. The hardness requirements are halved for aerospace applications. The thickness tolerance ± .001 inch is common for automotive applications and half of that tolerance is allowed for aerospace applications. The hardness tolerance of HRc 44-50 is standard for automotive applications and 45-48 HRc is required for aerospace applications.

Note: Grades 3, 2, 1, and 1-S are service marks of Electronics Inc. The Almen strip type (N, A, or C) is determined by its thickness.

Length: The standard length of the Almen strip is 3.00 inches with a common tolerance of ±.015 inch. Some specifications have different tolerances and not all specifications use the 3.00 inches as the nominal size. For all practical purposes, 3.00 inches can be used as the "length" of the Almen strip. Experiments with strips that are masked by .5 inch on each end and then peened do not show any difference in performance. This implies that the exact length of the Almen strip is not critical. However, it would be unwise to use strips that are shorter than the 3.00 inches since the impacting shot would then be corrupting the ends of the Almen holder and this would destroy the flat surface of the holder. Placing subsequent strips onto the holder might result in clamping the strip and causing it to bend while it is being peened. This bending action is a form of "stress" peening and will affect the final curvature of the strip and should be avoided.

Width: The standard width of the Almen strip is .75 inch with a common tolerance of ± .005 inch. Some specification have different tolerances and not all specifications use the .75 inch as the nominal width but for all practical purposes you can use .75 inch as the width of the Almen strip. Although no experiments are known to have been conducted to determine the influence of strip width on performance, it is assumed to be negligible.

Thickness: The thickness of the Almen strip determines its type, namely "N" = .031 inch, "A" = .051 inch or "C" = .0939 inch. It is because of the thickness differences that each type performs at the different intensity levels. The "A" strip is deemed to be the "standard" strip and is valid for use in the range of .004 inch and .024 inch. Below the .004 inch "A" intensity, the thinner "N" strip should be used. Above the .024 inch "A" intensity, the "C" strip should be used. The most common tolerance for thickness is ± .001 inch but some specifications require ± .0005 inch tolerance. The Almen strip is very sensitive to thickness variations, as can be seen by the relationship between the three strip types. An "A" strip deflection of .005 inch would result in approximately .015 inch deflection of an "N" strip. This indicates that the strip will increase its curvature by .010 inch per .020 inch change in thickness (from .051 inch thick to .031 inch thick). The standard tolerance for thickness, ± .001 inch, would therefore allow approximately ± .002 inch variation in expected curvature reading. For automotive applications this may be acceptable but for aerospace applications it would be deemed excessive. Therefore the ±.0005 inch thickness tolerance, which would translate into a ± .001 inch curvature tolerance, is required for aerospace applications.

Flatness: The initial pre-bow, or flatness, of the Almen strip acts like a latent bias in the measuring system. A strip that has a pre-bow of .001 inch will read .001 inch higher curvature than a strip with zero pre-bow. If the initial pre-bow value is negative than the resulting curvature will be diminished by the same amount. These readings are an approximation depending upon many factors, such as whether or not the strip has been peened on the concave or convex side and the nature of the residual stresses in the strip prior to its use. A compensation scheme may be used to negate some of these effects by taking a net reading of the strip curvature. The compensation practice should be limited to strips that exhibit less than .0005 inch pre-bow. Trying to compensate for excessive pre-bow above .0005 inch has not proven to be reliable.

Hardness: The hardness value of the Almen strip will affect its performance. Almen strips were tested at the low end of hardness HRc of 44 and high end of hardness HRc 50 and the results indicate that the curvature arc height varied by .0006 inch (total range) on the "A" strip at an intensity of .005 inch "A" and also at .010 inch "A". The aerospace grade strips, with hardness range limited to 45-48 HRc, would therefore limit this effect to .0003 inch (total) variation.

A's - 34 lb. (1,000 pcs/box)
N's - 21 lb. (1,000 pcs/box)
C's - 31 lb. (500 pcs/box)