Almen strips N, A and C: which one to use, and which shot peening standard applies to your manufacturing process
Updated: Oct 5
Almen strips come in three thicknesses — N, A and C — and the one you use depends on the intensity you are measuring. What follows is what they are, how to choose between them, and which standard binds you depending on who your customer is.
Shot peening is a heavily governed process, and most of that governance comes from aviation. In industrial manufacturing (AKA mass production) the same standards work as guidance rather than law — more on that here.
What Almen strips are and how the measurement works
Almen strips are the specimens used to measure peening intensity. The strip goes under the media jet, bends, and the arc it takes is the intensity by definition — measured in inches, and converted to millimeters by the chart below.
What the arc measures is the residual plastic deformation the media is inducing: the peening effect, and through it the fatigue life improvement on the part.
The same strips measure the intensity on any part. Where you put them under the jet decides what you learn, and that depends on the part itself and on which side you decided to improve. Value goes where it is needed: sometimes the part is peened all round, sometimes only on the face most likely to fail.
The Almen effect
The Almen effect is the measurable convex deformation of a standardised strip under a high-velocity stream of media. The impact plastically deforms the outer layer; the deeper layers resist the stretching, and the strip bows toward the blast stream. That curvature — the arc height — is a direct proxy for the intensity of the process.
How the strip is mounted and read
The strip is bolted to an Almen holder with four screws, and the holder goes on the fixture in the place the part would occupy — same distance from the wheel, same angle, same exposure. A strip read in a position the part never sees measures a process the part never gets.
Reading happens on an Almen gauge: the strip sits on four hardened balls, a dial indicator touches the centre of the convex face. The strip is measured before peening and again after, and the arc height is the difference between the two readings. Skipping the first reading assumes the strip came out of the box perfectly flat, which the standard does not.
Almen strip saturation and shot peening coverage
Saturation and coverage are the two conditions a successful peening process has to meet, and they are not the same condition. Saturation is verified on the Almen strips; coverage is what guarantees the parts have actually been peened, and it is bought with exposure time.
Almen strip saturation
Saturation is the point where more time almost stops adding intensity. It is read from the saturation curve — arc height against exposure time — and defined as the earliest point at which doubling the exposure produces no more than a 10% further increase in arc height. The arc height at that point is the peening intensity, the number that goes on the drawing.
Saturation of the Almen strip is what turns the result into a number you can check and repeat tomorrow, and therefore into a part lifetime performance you can promise and guarantee.
Shot peening coverage
A stream of media cannot be aimed, so the process has to deliver one certainty without measuring it point by point: that saturation has been reached everywhere on the surface exposed, not just where the strip was sitting. The answer comes from the saturation rule itself — double the exposure time, and you are covered.
How the curve is built and why the doubling works is covered in step B of this series.
That is what an Almen strip is and what it measures. Which of the three you put under the jet is the next question.
Which Almen strip to use: N, A or C
The Type A strip is the industry default and the one to start from. If the arc height comes back below 0.004" / 0.10 mm — under 4A — the strip is too stiff to read the process with any sensitivity, and you switch to N. If it goes past 0.024" / 0.61 mm — over 24A — the strip has run out of range, and you switch to C.
There is a shortcut that works from the process rather than from the measurement. Glass bead and ceramic media almost always land in N territory; cast steel shot and cut wire in A; only heavy peening with large shot reaches C. Starting from the media saves a round of testing, but the measurement still decides.
Almen strip dimensions and material
All three strips share the same footprint: 3.000" / 76.2 mm long, 0.747" / 18.9 mm wide, SAE 1070 cold rolled spring steel, hardened to 44–50 HRC.
What changes is the thickness: 0.031" / 0.79 mm for the N, 0.051" / 1.30 mm for the A, 0.094" / 2.39 mm for the C. That is the whole reason the three exist. A thin strip bends under a stream a thick strip barely registers; a thick strip still has room to bend when the thin one has curled as far as it will go. Choosing the strip is choosing the measuring range, nothing more.
Which is also why they are not interchangeable mid-process. Changing strip type changes the scale you are reading on, and a number taken on one scale does not mean the same thing on another.
Strip type | Thickness | Range | Media |
N — Normal | 0.031" / 0.79 mm | below 0.004" / 0.10 mm / 4A | glass bead, ceramic bead |
A — Standard | 0.051" / 1.30 mm | 0.004"–0.024" / 0.10–0.61 mm / 4A–24A | cast steel shot, cut wire |
C — Heavy | 0.094" / 2.39 mm | above 0.024" / 0.61 mm / 24A | large shot, heavy peening |
Converting between Almen strip types
The working rule in the shop is 1A ≈ 3N and 1C ≈ 3A. Useful for a sanity check, and for talking to a customer who specifies a strip you are not running.
It is not a conversion the standard recognises. SAE J443 does not declare the three types equivalent, and the ratios drift at the ends of each range — exactly where you would want to use them. In the overlap zones, where the current strip is near the edge of its range and the next one is just coming into its own, the only safe move is to run the test on both and record both numbers.
If the customer's specification names a strip type, that is the strip you measure on. A converted figure is not a measurement.
Which shot peening standard you actually have to follow
Which document binds you depends on who buys the part. Aerospace work comes with a specification named on the drawing and an audit behind it. General industry has standards too, but they arrive as reference rather than requirement.
What follows is the whole landscape, from the SAE core outward to the national standards and the prime contractor specifications. You do not need all of it. You need to know which line of it has your name on it.
SAE J2441 — Shot Peening.
Current revision June 2015 (first issued 2000). The core SAE process standard: it sets the engineering requirements for peening a part's surface by impingement of metallic shot, glass beads or ceramic shot, in order to induce a compressive residual stress layer and raise fatigue strength and resistance to stress-corrosion cracking. Where the AMS documents govern the aerospace side, J2441 is the general-industry reference for the process itself. SAE J2441
SAE J442 — Test Strip, Holder, and Gage for Shot Peening.
Current revision May 2022 (first issued 1952). This is the standard behind the Almen strip itself: it defines the requirements for the equipment and supplies used to measure peening arc height — the test strip, its holding fixture and the gage that reads the bend. It fixes what the strip must be so that a measurement is comparable anywhere; how the strip is then used to read intensity is covered by J443 and J2597. SAE J442
SAE J443 — Procedures for Using Standard Shot Peening Almen Test Strip.
Current edition November 2024. The companion to J442: where J442 defines the strip, J443 tells you how to use it — how to build the saturation curve, read peening intensity from it, and monitor machine repeatability. It also fixes which strip to use for which range: type A for arc heights of 0.004"–0.024" / 0.10–0.61 mm / 4A–24A, type N below that, type C above. This is the procedure that turns a bent strip into a number you can specify and control. SAE J443
SAE J2597 — Computer Generated Shot Peening Saturation Curves.
Current edition September 2017 (a revision is in progress). It defines how to build and read the saturation curve by software rather than by hand, calculating intensity within a fixed tolerance band. A manual curve is subjective and hard to reproduce; J2597 gives a consistent, archivable method. J443 recommends it but does not force it — computer-generated curves are the expected practice in a modern facility, yet manual construction remains acceptable. SAE J2597
SAE J2277 — Shot Peening Coverage Determination.
Current edition January 2023 (first issued 2003). Where intensity tells you how hard, coverage tells you how much of the surface has actually been hit. J2277 gives the procedures to measure it and to relate coverage to the part's exposure time in the media stream. It also makes an important point often missed on the floor: the effect depends on coverage in both directions — too little leaves gaps where a crack can start, too much can itself harm fatigue strength and component life. SAE J2277
SAE ARP 7488 — Peening Design and Process Control Guidelines.
Edition January 2018. This is the guidance that used to live inside AMS 2430 and was split out into its own Aerospace Recommended Practice. It gives the design and process considerations that go with AMS 2430 or 2432: recommended intensity ranges for various materials, how to define the peening area, how to obtain coverage on hard-to-reach features, and pre- and post-peening handling. It is a recommended practice, not a requirement — but it is where the default engineering judgement now sits, and where explicit customer requirements always take priority over it. SAE ARP 7488
SAE J3020 — Medical Device Shot Peening.
Edition 2014. A dedicated standard for peening medical devices — implants and similar — where controlled compressive residual stress is used to enhance a material's mechanical properties and fatigue behaviour. It carries the same intensity-and-coverage logic as the aerospace documents but with the traceability and material constraints of the medical field. It is the reference when the peened part goes into a human body rather than an engine.
NADCAP AC7117 — Surface Enhancement / Shot Peening Audit Criteria.
Current revision D. This is not a technical standard but the aerospace accreditation criteria for the peening process, run by PRI (Performance Review Institute). Where AMS 2430 and 2432 say how to peen, AC7117 is the checklist an auditor uses to certify that a shop actually does it right — with supplementary checklists for the different techniques: AC7117/1 computer-controlled, /2 automated, /5 manual. For a subcontractor peening flight-critical parts, Nadcap accreditation to AC7117 is effectively the entry ticket: without it, the aerospace primes will not place the work. NADCAP AC7117
AMS 2430 — Shot Peening.
Current revision U, April 2018. The main aerospace process specification. Until revision U it was titled "Shot Peening, Automatic" and covered only automated peening; U dropped "Automatic" and brought manual peening and batch (tumble/barrel) peening into scope, as Appendices A and B. It also moved the old process-guidance notes out of the document and into ARP 7488. Automated peening remains the default: manual or batch may be used only when the purchase order explicitly authorizes it. This is the specification most aerospace customers call out on the drawing. AMS 2430
AMS 2432 — Shot Peening, Computer Monitored.
Current revision E, 2022. It sits one level above AMS 2430: the process is the same, but every key parameter — shot flow, air pressure, wheel speed, process time, part-to-nozzle position — is monitored in real time by computer against defined limits, with automatic shutdown if any drifts out of tolerance. AMS 2430 remains an integral part of it, so the two are used together, and peening to 2432 meets or exceeds 2430. This is the specification reached for on the most critical parts — rotating aero-engine components and parts working at the limit of their material — where every single piece must be proven, not just sampled. AMS 2432
AMS 2590 — Rotary Flap Peening of Metal Parts.
Current revision C, 2022. A portable variant of peening that uses bonded-shot rotary flap assemblies rather than a blast stream. The principle is the same as conventional peening, but intensity is read with a magnetic Almen strip holder. It is used for local re-peening of blemished areas, for straightening and reshaping, and for cosmetic restoration of already-peened parts. Because it is a hand-held process, it needs explicit customer approval when used in place of manual or automated shot peening — which is exactly why it belongs in a discussion of where the rules bend. AMS 2590
MIL-S-13165 — Shot Peening of Metal Parts (cancelled).
The original US military specification for shot peening, to induce compressive residual stress and improve resistance to fatigue, stress-corrosion cracking and galling. It was adopted by SAE as AMS-S-13165, then cancelled in December 2007 and superseded by AMS 2430; the Department of Defense withdrew its adoption in May 2009. It is listed here because it still appears on legacy drawings and older specifications — but wherever it is called out, the current requirement to meet is AMS 2430. SAE AMS-S-13165
ASTM B851 — Automated Controlled Shot Peening prior to Nickel or Chromium Plating, or as a Final Finish.
Current edition B851-04, reapproved 2020 (active). It targets a specific problem: nickel and chromium plating lowers a part's fatigue strength, so this specification requires automated controlled peening before plating to put the surface back into compression and offset that loss — or as a final finish in its own right. It uses cast steel, conditioned cut wire or ceramic shot, excludes hand and rotary flap peening, and is not for brittle materials. It is the ASTM counterpart of ISO 12686, and the reference when a plated part must keep its fatigue performance. ASTM B851
ISO 12686 — Automated Controlled Shot-Peening of Metallic Articles.
Edition 1999, last confirmed current in 2022. The international reference for controlled peening of metallic parts before nickel or chromium plating, or as a final finish, using cast steel, conditioned cut wire, ceramic shot or glass beads. Its emphasis is repeatability: it describes the methods needed to keep the process parameters rigidly under control so the result is the same part after part. Like its ASTM twin B851, it excludes hand and rotary flap peening and is not for brittle materials. This is the document to cite when an international, rather than national, designation is required. ISO 12686
ISO 26910-1 — Springs, Shot Peening: General Procedures.
Current edition 2023 (second edition, replacing 2009). The international process standard for peening springs, the counterpart at ISO level of Japan's JIS B 2711. It sets the general requirements for peening springs to raise their fatigue and stress-corrosion resistance through compressive residual stress, covering peening mode, machine category (centrifugal vs air-blast), Almen strip classes, and pre- and post-peening treatment. Where ISO 12686 covers peening before plating, this is the ISO document for the spring maker. ISO 26910-1
JIS B 2711 — Springs, Shot Peening.
Edition 2013, active, with Amendment 1 issued in 2026. The dedicated Japanese standard, focused on the application where peening earns its keep most in that market: springs. It specifies peening applied to springs to raise their resistance to fatigue, stress-corrosion cracking and delayed fracture, mainly by introducing compressive residual stress at the surface. Narrower in scope than the American or ISO documents — it is a product standard for springs rather than a general process spec — but it is the reference a Japanese spring maker works to. JIS B 2711
JB/T 11552 — Shot Blasting and Peening Strengthening, Technical Requirements (China).
Edition 2013. The Chinese process standard for peening, issued as a machinery-industry (JB/T) standard. It fixes the terminology, the process parameters, the media, the technical requirements and the inspection methods, all aimed at building compressive stress at the surface to raise fatigue strength or stress-corrosion resistance. It is complemented by a metallic-abrasive series covering the media grades. It is the reference a Chinese plant works to, and in substance it tracks the same ISO and SAE logic. JB/T 11552
IS 7001 — Shot Peening of Steel Parts (India).
Issued by the Bureau of Indian Standards (Spring sectional committee, TED 21), superseding the earlier IS 7377 and maintained in current form. Unlike the Russian or Korean situation, India has its own national process standard for peening steel parts. It goes as far as recommending Almen ranges and coverage — for example a 90% minimum coverage, and intensity bands such as 10A to 20A for medium and light springs, 7C to 15C for heavy-duty — with the exact intensity agreed between maker and purchaser. It is the reference an Indian spring or component maker works to. IS 7001
Boeing — BAC 5730, Shot Peening.
The Boeing process specification for shot peening. It is the document a supplier must work to when peening Boeing airframe and structural parts — layered on top of the AMS framework and, where it differs, taking priority over it. Like all prime specs it is proprietary: you receive it through the Boeing supplier relationship, not off a public shelf. Boeing BAC 5730 — proprietary, via Boeing
Airbus — ABP 1-2028 and AIPI 03-10-001, Shot Peening.
Airbus works through two documents: ABP 1-2028 covers the peening media, and AIPI 03-10-001 covers the peening process itself (rectification of metallic parts by shot peening). A supplier peening Airbus parts must meet both, on top of the AMS/ISO base. Proprietary: obtained through the Airbus supplier channel, not publicly. Airbus ABP 1-2028 / AIPI 03-10-001 — proprietary, via Airbus
Rolls-Royce — RPS 428, with media specs CSS 119 / CSS 120.
Rolls-Royce runs its peening under RPS 428 (Rolls-Royce Process Specification), backed by its own media specifications CSS 119 and CSS 120. A shop peening Rolls-Royce engine parts works to these rather than to AMS alone — and where they tighten AMS, the Rolls-Royce document wins. Proprietary: supplied through the Rolls-Royce approval route. Rolls-Royce RPS 428 / CSS 119-120 — proprietary, via Rolls-Royce
General Electric — P11TF3 (with sub-specs P11TF3-S19, P11TF8-S11).
The GE Aviation peening specification, with detail sub-specs such as P11TF3-S19 and P11TF8-S11 defining the media and process variants. A supplier peening GE engine components qualifies to these documents, layered on top of AMS 2430/2432. Proprietary: held through the GE supplier and S-1000 approval system. GE P11TF3 — proprietary, via GE Aviation
Pratt & Whitney — PWA 6, Shot Peening.
The Pratt & Whitney process specification for shot peening of engine parts. Like the other engine primes, PWA 6 sits on top of the AMS framework and governs where it differs. A supplier peening P&W components qualifies to it directly. Proprietary: obtained through the Pratt & Whitney supplier approval route. Pratt & Whitney PWA 6 — proprietary, via P&W
Safran — DMR 71-110, Shot Peening.
The Safran group peening specification (used across its engine and equipment divisions). A supplier peening Safran parts works to DMR 71-110 on top of the AMS/ISO base, with the Safran document taking priority where it departs from them. Proprietary: held through the Safran supplier channel. Safran DMR 71-110 — proprietary, via Safran
Dassault — DGQT series, Shot Peening and Forming.
Dassault Aviation works to its own DGQT process documents — covering shot peening as well as related forming operations (for example DGQT for ultrasonically activated forming and for machined-part finish). A supplier on Dassault work qualifies to these rather than to a public standard alone. Proprietary: obtained through the Dassault supplier route. Dassault DGQT — proprietary, via Dassault
Shot Peening Standards in manufactiring
The problem peening solves — metal fatigue — was first met on the railway axles of nineteenth-century England, where shafts broke under loads far below the material's static limit. It became a discipline in twentieth-century America, tied to aviation: an aircraft needs its components to perform and to stay light, and a fatigue failure in flight costs lives. That is why the peening world is so heavily normed — the weight of the standards above is the weight of aviation safety.
Outside that world runs a parallel one: industrial manufacturing, where most of peening's benefits can be obtained without following the whole imposing framework. Here the standards act less as law and more as guidance and orientation. The process has to be built around the real needs and the real targets, so that it is genuinely an improvement and genuinely worth the money for the company applying it.
A word on how peening is measured
There is one more thing that shapes all of this: how you measure peening. In the end you measure the arc of an Almen strip bent by the process, to a standard. It is not a direct measurement — it grew up empirically, as a way to put a number on intensity and coverage when nothing else could. Decades later, systems arrived that measure the residual stress far more precisely, but they carry an abnormal cost that keeps them rare. So the Almen strip, indirect as it is, remains the working language of the whole field.
Why the rest of the world leans on the American body
The main framework is fundamentally American, then branches into national and international variants and, finally, into the specifications of individual manufacturers — all tied to the aviation branch — each stricter than, or amending, the common base. Most of the world simply took advantage of this large, mature body of work: local standards often do little more than point back to the American ones. That is why there is less separate national depth than you might expect — the Russian GOST refers to the AMS documents, the Chinese GB does the same, and so on. It is the sensible choice: use work already done, done well and validated, rather than spend everything needed to develop a parallel path that would have added little.
The point of all this
You do not have to comply with every standard here. What matters is knowing which one your customer and your industry impose, which apply as a rule and which only as a guideline, and — where the standard allows it — which points can reasonably be amended for a production reality that aviation never had to face.
Media used in Shot Peening
The standards that define the shot itself — cast shot, cut wire, sizes and hardness (SAE J441, J444, J827, the AMS 2431 series, VDFI 8001) — are on the blasting materials page, since they belong to the media, not the process.
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3) Almen strips types and how they should be used to validate. this post.
