What is Almen intensity?

Almen intensity characterises the ability of a shot stream to plastically deform a surface during shot peening. Developed by John O. Almen at General Motors in the early 1940s (US patent 2,350,440, granted in 1944), the method consists of peening one face of a standardised steel strip clamped to a holder. Once released, the strip bows under the effect of the compressive layer introduced, and its arc height is measured: the greater the arc height, the higher the intensity.

Intensity is neither a direct measurement of kinetic energy nor a measurement of the residual stress in the part: it is a comparative, repeatable indicator of how the process is set. Two standards govern it: SAE J442 specifies the strips, the strip holder and the Almen gauge; SAE J443 specifies how to use them, including how to build the saturation curve.

The three types of Almen strip

N, A and C strips share the same plan dimensions (about 76 × 19 mm) and differ only in thickness, and therefore in their sensitivity to the shot stream.

N strip

Thickness 0.79 mm (0.031 in). SAE J443 recommends it when the intensity would be below 0.10 mm A (0.004 in A): low intensities, fine shot, glass beads, thin-walled parts.

A strip

Thickness 1.29 mm (0.051 in). The reference strip and by far the most widely used, particularly in aerospace. Range recommended by SAE J443: 0.10 to 0.60 mm A (0.004 to 0.024 in A).

C strip

Thickness 2.39 mm (0.0938 in). Recommended above 0.60 mm A (0.024 in A): high intensities produced by large shot on heavy steel parts. It has nothing to do with cold working by hole expansion (split sleeve, split mandrel): that mechanical process is not controlled by Almen intensity but by applied expansion and controlled hole diameters.

These ranges are recommendations. Where two strip types could be used, the customer specification or the drawing defines which one applies.

Measurement, notation and conversion

Arc height is measured with the Almen gauge specified in SAE J442, whose zero and accuracy are checked periodically against a flat block and reference blocks. An intensity is always stated as an arc height together with a strip type: for example 0.30 mm A, i.e. about 0.012 in A, often shortened to 12A in inch-based specifications (the value in thousandths of an inch followed by the strip letter). Specifications usually call for a range, for example 0.008–0.012 in A. An arc height without a strip letter is meaningless, and an "equivalent" intensity quoted on another strip type is not a standard notation.

Rule of thumb for a given shot stream: the thinner N strip bows about three times more than the A strip, and the A strip about three times more (3 to 3.5 is commonly quoted) than the C strip. An arc height of 0.30 mm N therefore corresponds to roughly 0.10 mm A, and 0.10 mm C to roughly 0.30–0.35 mm A. These ratios are approximate and non-linear: they help you choose the strip, never replace a saturation curve built on the specified strip.

The saturation curve

A series of strips is peened with identical settings and increasing exposure times (or numbers of passes), and arc height is plotted against time. Under SAE J443, the saturation point is the first point on the curve where doubling the exposure time (from T to 2T) increases arc height by no more than 10%; the intensity is the arc height at that point. AMS 2430 and AMS 2432 make this definition mandatory by calling up SAE J443.

SAE J443 requires at least four strips per curve, with increasing times that bracket the saturation point. Once the curve is established, intensity is verified with a single strip exposed at time T: its arc height must repeat the curve value within ±0.038 mm (±0.0015 in). Note that saturation applies to the strip, not to the part. Coverage of the part is a separate quantity, determined according to SAE J2277.

Common operational pitfalls

• Non-conforming or contaminated strips — initial flatness out of tolerance, oil or media residue: the measured arc height is wrong. A strip is used only once.
• Unchecked gauge — without periodic verification on a reference block, every reading drifts together and nobody notices.
• Poorly built saturation curve — too few points, or points badly spread around T: the declared intensity is not repeatable.
• Unrepresentative strip holder — it must reproduce the position, angle and stand-off of the critical area of the part; otherwise the measured intensity says nothing about the actual part.
• Uncontrolled stream parameters — nozzle distance and angle, air pressure or wheel speed, media flow rate, shot size and condition: each one shifts the curve.

AMS 2430 / AMS 2432 compliance

AMS 2430 (automatic shot peening) and AMS 2432 (computer-monitored shot peening) rely on SAE J442 and SAE J443: the specified intensity must be established by a saturation curve for each combination of settings before any part is processed, then verified periodically. During a NADCAP Surface Enhancement audit (AC7117 checklist), the auditor expects full traceability: saturation curves, strip certificates, gauge verification, media control and intensity verification records.

Kronos Engineering recommendations

A gap between declared intensity and the intensity actually achieved on the critical area usually comes from one of the pitfalls above, and it is often discovered late, during a customer audit. Our independent audits (all equipment brands) compare the actual process with the applicable specification: saturation curves, strip holder positioning, gauge and media condition. They let you correct this drift before it causes a failed NADCAP audit.

Put it into practice: enter your time / arc height pairs into our free Almen saturation curve calculator to get the saturation point and the intensity according to the SAE J443 10% rule. Curve not converging, or intensity out of range? Describe your process: we reply within 12 business hours.

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