How to Prevent Airframe Stress Cracks with Cold Expansion Technology

What is metal fatigue?

Airframe components such as wings bend up and down several thousand times a year when the aircraft takes off and lands. This is called cyclic loading and can cause cracks to form in the metal. The weakest part of the metal is where the holes are drilled to hold fasteners, bushings, and fittings. These holes are usually the starting point for cracks to form.

Cold expansion shields the hole from these forces by creating what is called a “zone of residual compressive stress” around it (image below).

?What is 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 the 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.

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 the structure
• Simple, one-sided operation
• Works on all common aerospace materials
• Applicable in both production and rework
• Can be automated

Wencor is Fatigue Technology’s preferred strategic partner, supporting its Cold Working tooling “CB” product line for the commercial maintenance and repair market and is committed to being your stocking source for Fatigue products. For more information, contact us at [email protected] or www.wencor.com and follow us on LinkedIn.