Failure Analysis & Durability Improvement

Residual Stress

Home Page	Contact Us	About VFA Engineering		Glossary

Residual stress is the stress that is in a component when no load is applied. If the residual stress is large enough to be significant, it must be added to the applied stress to accurately calculate the fatigue life of a component For example, if the applied stress is cycled between 0 & 100 KSI, and there is a residual stress of 25 KSI, the actual stress range is 25 - 125 KSI instead of 0 - 100 KSI. This means the R-ratio is 0.2 (25/125) instead of 0.0 (0/100). Increasing the R-ratio lowers the fatigue life.

Residual stress is often a by-product of the manufacturing process. In castings residual stress can occur because of different cooling rates within the component after the metal is poured. Items that are mechanically formed generally have significant residual stresses. Consider a spring that is formed by wrapping wire around a mandrel. On the inner side of the coils the wire yields in compression during the winding process, which results in tensile residual stress. Conversely, the outer side yields in tension which leaves a compressive residual stress. Residual stress can be calculated by modeling the manufacturing process.

Detrimental residual stress can be removed by annealing, but residual stress can be purposely induced into a part to improve its fatigure life. The classic way to do this is with shot-peening, but laser shock peening (LSP) can now be used to induce tensile yielding on the surface, leaving a compressive residual stress. This does leave some slight residual tensile stress below the surface, but since most fatigue failures originate at the surface, this is usually a good trade.

VFA Engineering Group

Failure Analysis & Durability Improvement

Contact Info
Ed Pope
317-750-3414
Ed@Failure-Analysis-Durability.com

URL
http://Failure-Analysis-Durability.com/residual-stress.htm

About Us