Split Sleeve Cold Expansion
FTI's Split Sleeve Cold Expansion™ system is a cost effective solution to problems associated with fatigue cracks in holes in metal structures. Split Sleeve Cold Expansion is accomplished by pulling a tapered mandrel, pre-fitted with a lubricated split sleeve, through a hole in aluminum, steel or titanium. The function of the disposable split sleeve is to reduce mandrel pull force, ensure correct radial expansion of the hole, preclude damage to the hole, and allow one-sided processing.
The process works by imparting beneficial compressive residual stress around the hole. The action of drawing the mandrel through the starting hole causes a radial plastic flow of material and results in an annular zone of residual compressive stresses that extend up to one diameter beyond the edge of the hole as shown below.
The sleeve allows for one-sided processing and shields the hole from frictional forces generated by the high interference of the expansion mandrel. The residual stresses created by cold expansion significantly increase fatigue life by reducing the stress intensity factor and crack growth life by reducing the applied stress ratio at the hole. The magnitude of the peak residual compressive circumferential stress is about equal to the compressive yield stress for the material. The compressive stress zone spans one radius to one diameter from the edge of the hole, for diameters up to 1/2 inch for most materials. A balancing zone of tensile stresses lies beyond the circumferential compressive stress zone.
Benefits of Split Sleeve Cold Expansion
- In most cases, at least 3:1 fatigue life improvement
- Arrests small crack growth
- Cost effective alternative to redesign
- Adds no weight to structure
- Simple, one-sided operation
- Works on all common aerospace materials
- Applicable in both production and rework
- Can be automated
Cold Expansion to Size
Cold Expansion to Size is the same general practice as standard Split Sleeve Cold Expansion, but the process eliminates final reaming of the hole. Cold Expansion to Size is material dependent, and the tooling is designed according to the properties of the material. By knowing how different material reacts to the radial stresses of Split Sleeve Cold Expansion, we can predict the final hole size. Cold Expansion to Size is ideal for production work, but has also been used in rework situations. Also, Cold Expansion to Size works well when incorporated in automated drilling/fastener insertion systems.
Countersink Cold Expansion
Yes, Cold Expansion can be applied to holes requiring a countersink. The tooling is specifically designed to simultaneously cold expand the countersink and straight portion of the hole. Countersink Cold Expansion is applicable in both production and rework.
The Split Sleeve Cold Expansion Tooling System uses the same Puller Unit and PowerPak as many of our other product families, however, there are variations in the mandrels, sleeves, nosecaps, and other durable tooling. A complete Cold Expansion system includes (but is not limited to):
• Cutting Tools
• Mandrel & Nosecap
• Split Sleeves
• Check Gages (combination & mandrel)
• Puller & PowerPak
Below are the basic steps in the Split Sleeve Cold Expansion System. Download our Cold Expansion Installation Poster (11x17") or for detailed specifications, including starting hole diameter, tooling sizes, and more - please download FTI Specification 8101, available in our Technical Library.