The Two-Way Relationship Between Residual Stress and Fatigue/Fracture

Most mechanicians are aware that residual stresses affect fatigue and fracture. Few are aware of the full extent of the effects residual stress have on myriad aspects of those and other structural processes AND of the reverse relationship: fracture concepts are hugely intertwined with residual stress measurements. This talk first reviews the straightforward effect that residual stress has: it acts just like an external load and, depending on the sign, accelerates or retards failure processes. The talk then describes one exemplar less well-known effect: residual stress can make it very difficult to measure true material properties like fracture toughness. The talk then describes the influence of fracture concepts, most notably the mode I stress intensity factor K-I , on residual stress measurement. A short and elegant derivation reveals that incremental slitting measurement strain data can be converted to residual stress profiles using a K-I analysis and a conceptually simple calibration factor that is independent of the stress distribution. The talk next shows that several error sources for slitting and contour method stress measurements can be directly correlated with the K-I caused by residual stress. The talk then lays out Bueckner's superposition principle, the most powerful but misunderstood and mistrusted tool for analyzing stress fields around cracks. Originally developed for fracture mechanics, Bueckner's is used to calculate the K-I from residual stresses and to calculate calibration coefficients for hole drilling and slitting. More notably, the contour method for measuring residual stress is a direct experimental embodiment of Bueckner's principle. In fact, we review an experimental demonstration that misfit measurements between mating fracture surfaces can be used to calculate the residual stresses that existed prior to the fracture.