The Vanishing Influence of Microstructure on Growing Short Cracks

Microstructural features play a prominent role in the initialization and propagation of microstructurally short cracks. These features include crystallographic orientations of grains, grain boundaries, inclusions, voids, material phases, etc. Their influence is expected to vanish both with increased distance from the crack tip as well as with increased crack length. Current engineering fracture mechanics techniques do not account for these features and can therefore only be applied to sufficiently long cracks. From the engineering point of view it is therefore quite useful to have an estimate of this length. In this paper we build upon the authors’ previous work to estimate decreasing influence from the surrounding microstructural features with increasing crack length. To achieve this, a model containing a large number of randomly sized, shaped and oriented grains is employed. The grain structure is modeled using a Voronoi tessellation. A series of cracks with lengths from one to several grain lengths are inserted into the model, extending from a grain at the surface towards the interior of the model. The crack tip opening displacements (CTOD) are estimated and statistically analyzed for a series of random crystallographic orientation sets assigned to the grains adjacent to the crack. Anisotropic elasticity and crystal plasticity constitutive models are employed at the grain size scale. A rapid decrease of the CTOD scatter is observed with increasing crack length showing a clear tendency to stabilize at standard deviation of about 5% at the effective crack length of about 6 average grain sizes (0.3 mm in AISI 316 stainless steel. A crack with the length exceeding about 10 average grain sizes can be safely taken as a macroscopic crack.