What is the Almen test and how is it performed?

The Almen test is the standardized method for measuring shot peening process intensity. It was developed by J.O. Almen in 1930 and is standardized by SAE J441, J442 and J443. It is the only internationally accepted method for verifying and certifying the parameters of a shot peening process.

The test principle is based on the deformation of a standard test piece — the Almen strip — when subjected to the flow of spherical particles under process conditions. The strip is manufactured in special steel under strict hardness and flatness tolerances. When the abrasive impacts one face, the induced compressive stresses cause the strip to curve toward the impacted side. The height of that arc, measured with the Almen gauge (precision dial indicator), is proportional to the process intensity.

Almen strips are grouped into three types according to their thickness: type N (0.79 mm) for low intensities, type A (1.30 mm) for medium intensities — the most common in industry — and type C (2.29 mm) for high intensities.

The test procedure consists of fixing the strip in the standard holder, exposing the active face to the abrasive flow for the defined process time, removing the strip and measuring the arc height with the Almen gauge. To determine the saturation point — the process acceptance criterion — tests are carried out at different exposure times. The saturation point is reached when doubling the exposure time produces an arc height increase of less than 10%.