Short crack growth and internal stresses

Abstract

Short crack growth behavior has been examined using the two parametric approach developed earlier by the authors. It is commonly accepted that there is a lack of similitude in the description of the short crack growth behavior. Contrary to this understanding, it is shown that there is no anomaly in the short crack growth behavior. The apparent anomaly arises because of ignoring: (1) the second parameter associated with the threshold K∗max; and (2) the existence of internal stresses in the crack tip field where the short cracks nucleate and grow. In the case of short cracks nucleating from notches the internal stresses can be pre-existing or can be generated in situ if they form at a free surface. In the latter case, formation of intrusions and extrusions at the persistent slip bands are the precursors for the crack nucleation providing the necessary internal stresses. Thus short cracks grow under a total force, consisting of both internal and applied stresses, satisfying the same two thresholds for long cracks. Examples are provided from the literature to illustrate the concepts. It is shown that the internal stresses decrease as a short crack grows out of the existing stress field. These internal stresses can be predicted using elastic or elastic-plastic continuum approximations. Application of this concept is extended to understand the role of: (1) residual stresses; and (2) the transformation induced internal stresses on the crack growth. Based on the analysis, we restate the similitude concept as: equal crack tip forces result in equal crack growth rates for the same crack growth mechanism, provided all the contributing forces are taken into consideration.