Shot peening, flap peening or cold expansion: which to choose?
Shot peening, flap peening or cold expansion: compare treated areas, depth, controls and use cases to pick the right fatigue life treatment.
A fatigue crack almost always starts at the surface or at the edge of a hole, where local tensile stress peaks. Shot peening, flap peening (also called flapper peening) and cold working (also called cold expansion) share the same goal: introducing a layer of compressive residual stress that delays crack initiation and slows crack growth. They get there by three different routes, and they are not interchangeable. This comparison is written for the methods or structures engineer — at an OEM, an MRO or an airline — who has to select the right fatigue life enhancement process.
Shot peening: the surface of a whole part
Shot peening propels calibrated media (steel, ceramic or glass) at high velocity onto the surface. Each impact plastically deforms a thin layer of metal; the elastic material beneath holds it in compression. The affected depth is in the range of a few tenths of a millimetre and depends directly on shot size, intensity and material: a common rule of thumb is that it is of the same order as the media diameter.
The process is controlled by two quantities. Almen intensity, measured with the strips, holder and gauge defined by SAE J442 and determined from a saturation curve per SAE J443 (saturation is the point where doubling exposure time raises arc height by no more than 10%). Coverage, the share of the surface actually impacted, determined per SAE J2277. The reference specifications are AMS 2430 (automatic shot peening) and AMS 2432 (computer-monitored shot peening), supplemented by each manufacturer's own specifications. Shot peening is mostly performed in the shop, in an enclosed cabinet, which makes it the natural choice for new parts and removed parts: spars, fittings, discs, blades, springs and landing gear components.
Flap peening: shot peening that comes to the aircraft
Flap peening — also known as flapper peening or rotary flap peening — uses a portable rotary tool whose flaps carry bonded shot. As they spin, the flaps strike the surface and produce the same near-surface compression mechanism as shot peening, with no loose media to contain or recover. It is governed by AMS 2590 (Rotary Flap Peening of Metal Parts), with flap assemblies defined by AS2592.
Its strength is mobility: it treats a limited area directly on the aircraft, in the hangar or on the line, typically after corrosion or damage has been removed by blending, when the repair manual calls for the compressive layer to be restored. Control remains that of peening — intensity and coverage — with one particularity: arc heights obtained with the magnetic strip holder must be converted before the saturation curve is plotted. The process requires a trained operator, since consistency depends on rotational speed, angle and traverse. It suits accessible surfaces and radii that are not too tight; it does not replace a cabinet for a complete part.
Cold working: compression around fastener holes
Cold working — or cold expansion — treats a hole, not a surface. In FTI's split sleeve process, a tapered mandrel fitted with a lubricated split sleeve is pulled through the hole. This expands the hole wall beyond its yield point; once the surrounding material springs back, a ring of compression remains around the hole. According to FTI, this zone extends from about one radius to one diameter beyond the hole edge (for holes up to 1/2 inch in most materials), with peak hoop stress close to the material's compressive yield strength. The compression is therefore much deeper, but localised around the hole.
Control is not based on Almen strips but on geometry: starting hole diameter checked with a gauge, mandrel and sleeve matched to the diameter, applied expansion — a few per cent of the diameter — followed by final reaming to the functional size. A worn mandrel leads to insufficient expansion, hence systematic tooling checks. At Boeing the reference is BACC 5060; elsewhere the FTI specification or the manufacturer's own applies. The manufacturer states a fatigue life improvement of at least 3:1 in most cases. The tooling is portable and the process is used both in production and in repair. Kronos Engineering is an official FTI partner for split sleeve cold expansion.
Comparison table
| Criterion | Shot peening | Flap peening | Cold expansion |
|---|---|---|---|
| Principle | Loose shot propelled in a cabinet | Rotary tool with bonded-shot flaps | Tapered mandrel and split sleeve pulled through the hole |
| Treated area | Surfaces, whole part or masked areas | Local, accessible surface | Bore and surroundings of a hole |
| Compressive depth | A few tenths of a mm (order of shot diameter) | Near-surface layer, depending on intensity achieved | About one radius to one diameter around the hole (FTI data) |
| Where | Shop | Shop or on-wing | Shop or on aircraft, portable tooling |
| Process control | Almen intensity (SAE J442/J443), coverage (SAE J2277) | Converted Almen intensity, coverage | Starting hole gauging, applied expansion, final ream |
| References | AMS 2430, AMS 2432, OEM specifications | AMS 2590, AS2592 | FTI specification, BACC 5060, OEM specifications |
| Typical case | New part, removed part, landing gear | Local repair after blend-out | Fastener holes in structure |
How to choose, case by case
On-wing repair of a corroded area
Once corrosion has been blended out, the ground surface has lost its original compressive layer. If the repair manual calls for peening and the part cannot be removed, flap peening is the answer: work in the hangar or on the line, with no cabinet, and intensity and coverage checked on site. If the area includes a fastener hole, cold expansion of the reworked hole can be added to the surface treatment.
Fastener holes in structure
Holes are stress raisers that surface peening does not treat effectively in depth. Cold expansion is designed for them: wing joints, frames, stringers and fittings, in production as well as in repair after oversizing. It is the process of choice whenever cracking initiates at a hole edge.
New part in production
For a machined part to be treated over its whole surface or on areas defined on the drawing, cabinet shot peening — automatic per AMS 2430 or computer-monitored per AMS 2432 — gives the best repeatability. If the part has critical holes, cold expansion is performed in addition, in the sequence set by the manufacturer's process routing.
Landing gear
Ultra-high-strength landing gear steels are fatigue-sensitive and often plated. Shop shot peening, carried out before electroplating such as chrome in overhaul routings, remains the rule for removed parts. Flap peening only comes in for local rework permitted by the manufacturer's data.
Frequently asked questions
Is flap peening equivalent to shot peening?
The mechanism is the same and control is also based on intensity and coverage, but it falls under its own specification, AMS 2590. It only replaces shot peening where the repair document allows it.
Can a hole be peened instead of cold expanded?
Hole peening exists, but it only produces a near-surface layer. Cold expansion creates a far larger compressive zone around the hole; where the drawing or repair calls for cold working, there is no equivalence.
What evidence of conformity do we receive?
For peening: intensity records, saturation curve and coverage verification. For cold expansion: starting hole and tooling checks, then final size. Everything is documented within the customer's NADCAP framework, either directly or through approved subcontractors.
One partner for all three processes
Kronos Engineering's engineers have been performing special processes since 2009 and master all three treatments, in the shop and on site, in more than 50 countries, with a 24/7 AOG service. Tell us about the part, the material and the area to be treated: we will help you select the right process and reply to your quote request within 12 business hours. Contact us.