The Influence of the Grain Structure Size on Microstructurally Small Cracks

Abstract

Material ageing and fatigue effects play a significant role in safe operation of nuclear power plants. Particular issues are the initialization and propagation of microstructurally small cracks which can represent a significant proportion of a component’s life time. These cracks are still not well understood, one of the reason being that a number of microstructural features have a significant influence on such cracks: crystallographic orientations of grains, grain boundaries, inclusions, voids, material phases, etc. Microstructural features away from the crack have smaller effect on the crack tip parameters. Crack length also plays a role. For a long crack the influence of the surrounding microstructural features will be smaller than for a small crack. A crack of sufficient length can be modeled using classical fracture mechanics methods. The question is when do we reach this point? To try to answer this question we create a model containing a large number of randomly sized, shaped and oriented grains with a crack inserted into a surface grain. Random grain structure is modelled using a given Voronoi tessellation. We then extend the crack up to 7 grains in length and estimate the standard deviation of crack tip opening displacements (CTOD) due to the random crystallographic orientations of the grains surrounding the crack. To account for the deformation mechanism at the grain size scale anisotropic elasticity and crystal plasticity constitutive models are employed. We show that even at crack length equivalent to 7 grains the standard deviation of the crack tip opening displacements due to the surrounding crystallographic orientations is still 7.3% and that from engineering point of view a crack with length of less than 10 average grain sizes can not be taken as independent of the surrounding microstructural features.