K immo Kärkkäinen

Abstract

Non-propagation of surface cracks in constant-amplitude loading has been widely observed empirically. The phenomenon is linked to the well-known short crack problem; short cracks have been observed to propagate at a faster rate compared to equivalent long cracks. It is also known that crack closure reduces crack driving force, but needs a certain crack propagation length to become effective. On top of this, cracks initiating at a notch are affected by the notch plasticity field, which can delay crack closure saturation. Considering these findings, it appears that the phenomena of crack closure and crack arrest are linked.

Plasticity-induced crack closure is intrinsic to Mode I crack propagation in metals. Present work attempts to determine the role of plasticity-induced crack closure in non-propagation behavior. The purpose is to gain understanding of mechanisms affecting fatigue crack propagation and non-propagation.

In present work, extensive elastic-plastic finite element analysis of near-threshold propagation of short cracks nucleating from surface defects is presented. As non-propagation behavior is typical for surface defect initiated fatigue cracks, the modeled test specimen geometry is a circular cylinder with a microscopic hemispherical pit representing a surface defect. Existing three-dimensional finite element model allows for the detailed studying of fatigue crack propagation and plasticity-induced crack closure.

It is shown that the delayed development of plasticity-induced crack closure seems to significantly promote crack non-propagation behavior in surface defect initiated cracks. Additionally, differences in plasticity-induced closure along the transition from plane stress to plane strain contribute to the shape of the growing 3D crack.

Session