Cold Expansion — BACC 5060 — Fatigue Life +300%

Cold
Working

Processes — Cold Working

Cold expansion of fastener holes to introduce compressive stresses around the bore. Multiplies fatigue life by at least 3×. BACC 5060 compliant process for primary aerospace structures.

Applicable standards

BACC 5060ABP1-4100MIL-HDBK-516

Related industries

How it works

Cold working — or cold hole expansion — consists of pulling through a fastener hole a mandrel of slightly larger diameter, creating a controlled radial plastic deformation (typically 3% to 6% of the nominal diameter). This deformation introduces a high-intensity, deep residual compressive stress field around the hole (several millimetres radially).

Hole edges are the classic fatigue-crack initiation site on aerospace structures. By installing a compressive field there, cold working significantly delays crack initiation and propagation: fatigue life is multiplied by 3 to 10 depending on the alloy and loading mode. The process is specified by Boeing (BACC 5060) and adopted across the civil aerospace industry.

Two techniques coexist: split sleeve (Fatigue Technology — disposable split sleeve between mandrel and hole, preserves surface finish) and split mandrel (West Coast Industries — split mandrel expanding inside the hole, faster in production).

Typical applications

Fastener holes on primary aircraft structure

Wing spars, fuselage frames, reinforcements — original cold-working applications, historically developed for the Boeing 707.

Maintenance returns — life extension

On in-service aircraft, cold working is applied during scheduled visits to extend the operational life of critical holes.

Repair after fatigue-crack detection

Stop-hole + cold working: proven repair technique to block the propagation of an initiated crack.

Offshore and railway structural holes

Non-aerospace applications: offshore platforms, rail cars, civil-engineering bridges.

Military structural repair

Fighter aircraft, helicopters — cold working within Service Life Extension programs.

New serial production manufacturing

Production-line integration on modern aerospace lines (A350, 787, F-35).

Parameters by material

Material	Expansion rate	Preferred technique
Aluminum 2024-T3	3.5-4.5%	Split sleeve or split mandrel
Aluminum 7075-T6	3.8-4.5%	Split sleeve preferred
Titanium Ti-6Al-4V	3.0-3.8%	Split sleeve mandatory
Stainless A286	2.5-3.2%	Split sleeve
Inconel 718	2.0-2.8%	Split sleeve + adapted lubrication

The chosen expansion rate depends on the material's ductility and the part's thickness. Too low a rate does not generate useful residual stresses; too high a rate initiates cracks at the hole edge. BACC 5060 tolerances are tight (±0.3%). On thick parts (> 8 mm), a post-expansion ovality check is mandatory.

Frequently asked questions

Does cold working change the final hole diameter?+

Yes. Expansion leaves a residual diameter increase of about 0.5% to 1.5% depending on the alloy. The initial hole sizing accounts for this expansion to reach the intended final diameter — typically a post-cold-working finishing ream.

Cold working or shot peening — what's the functional difference?+

Cold working specifically targets the hole edge with a stress depth of several millimetres — the most effective treatment against fatigue initiated at the bore edge. Shot peening treats the overall surface with a depth of a few hundred microns. The two are complementary: cold working + shot peening can multiply fatigue life by 10.

Can cold working be performed on composite?+

No, cold working is a process for ductile metals. On metal/composite assemblies, cold working is applied only to the metallic part (before or after assembly depending on access).

What checks after cold working?+

Final diameter (BACC 5060 tolerance), straightness/ovality (on thicknesses > 5 mm), visual inspection for absence of cracks initiated by exceeding the ductility threshold, full operator/sleeve batch/tooling traceability.

Is cold working applicable to all diameters?+

Typically 3 mm to 40 mm for standard tooling. Beyond 40 mm, special tooling exists but ROI must be assessed case by case. Below 3 mm, shot peening remains more relevant.

ColdWorking