On the development of a metallographic method to determine the strain distribution ahead of a crack tip

A metallographic method was used to determine the strain distribution ahead of a blunted crack tip under plane strain (midsection) and plane stress (surface) conditions as well as an advancing sharp crack tip under plane stress conditions. The material used was an HSLA steel in two treatments: after controlled rolling, which produced a fine-grained structure with grains 8 μm in diameter; and in a condition that thermally simulated a heat-affected zone structure (a coarse-grain size, 33 μm in diameter).The strain distribution ahead of a blunted crack tip can be expressed as ϵ = 0.14 ((XCTOD)(−0.64) where X is the distance from the moving crack tip, and CTOD is the Crack Tip Opening Displacement, for the steel tested under both plane strain and plane stress conditions.It was determined that the strain distribution ahead of an advancing sharp crack tip can be expressed by ϵ = 0.75exp(−38X).