What is Almen intensity?

Almen intensity measures the kinetic energy transferred to the surface during shot peening. Developed by John Almen in 1945, the method relies on measuring the arc height of a standardised strip exposed to the shot stream: the greater the arc height, the higher the intensity.

The three types of Almen strip

N strip (Narrow)

Thickness 0.79 mm (0.031 in). Used for low intensities, typically below 4A. Common on thin parts, aluminium alloys and low-stress applications.

A strip (Almen)

Thickness 1.29 mm (0.051 in). The most widely used strip in aerospace — intensity range 4A to 24A. Standard reference AMS 2430.

C strip (Coarse)

Thickness 2.39 mm (0.094 in). Used for high intensities beyond 20A equivalent, typically > 8C. Heavy roto peening applications, cold-working hole expansion.

Measurement and conversion

Arc height is measured with a calibrated Almen gauge. Intensities are expressed as F30A (0.30 mm arc height on an A strip) or F12C (0.30 mm equivalent arc height on a C strip). Approximate equivalence rule: 1A ≈ 0.35N ≈ 3.5C.

The saturation curve

Saturation is reached when doubling the exposure time increases the arc height by no more than 10%. This is the AMS 2430 qualification criterion. Building the curve typically requires 5 to 8 different exposure points.

Common operational pitfalls

• Strip contamination by oil or media residue — distorts the measured arc height.
• Gauge wear — periodic calibration on a reference block is mandatory.
• Poorly built saturation curve — the declared intensity is not reproducible on the actual part.
• Uncontrolled nozzle-to-strip distance — the parameter with the strongest influence on scatter.

AMS 2430 / AMS 2432 compliance

AMS 2430 (Shot Peening) and AMS 2432 (Computer-Monitored Shot Peening) require intensity qualification before any part is processed. The operator must be able to produce a saturation curve valid at the date of the operation, a gauge calibration record and media traceability. NADCAP reinforces these requirements during audits.

Kronos Engineering recommendations

We regularly audit customer installations showing 10 to 20% deviation between declared intensity and actual intensity. Our independent audits (all equipment brands) identify this drift before it is picked up in a NADCAP audit. Contact us for a pre-audit of your shot peening processes.

Free tool: our free online Almen saturation curve calculator — N/A/C strips, SAE J443 curve construction, audit pitfalls and NADCAP checklist.

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