Hill Engineering Blog

Cold expansion

Fatigue is one important failure mode that guides the design and engineering of aircraft structure. As we have discussed previously aircraft are often manufactured using rivets and fasteners, which require drilling many holes in the structure during assembly. The holes act as stress concentrations, which tend to be locations where fatigue cracks are found. Compressive residual stresses act to hold cracks shut and result in improved fatigue performance. This residual compressive stress can provide substantial benefits in terms of performance, safety, cost, and inspection requirements. To take advantage of the benefits of compressive residual stress, cold expansion is often applied to aircraft fastener holes.

A typical cold expansion process involves placing a protective sleeve on the inside of a hole and then pulling an oversized, tapered mandrel through the hole. The material near the hole is plastically deformed resulting in a state of residual compressive stress near the hole.

A typical cold expansion process will produce near yield-strength level compressive residual stress at the edge of the hole, which decays with distance from the hole edge, crossing zero at a distance of 0.5 to 1.0 times the hole diameter.

We at Hill Engineering take pride in our expertise in technology areas related to cold expansion. Our ERS-toolbox® software is used to predict compressive residual stress from cold expansion. Our contour method technology is useful for measurement of residual compressive stress from cold expansion. Our fatigue analysis capability is used to forecast the performance of the structure during service after cold expansion.

Please contact us for more information about cold expansion and how Hill Engineering can help support the sustainment of your structure.

Compressive residual stress near a cold expanded fastener hole (measured by the contour method).