Contact of nonflat crack surfaces during fatigue

A model has been developed to predict crack opening and closure behavior for propagating fatigue cracks which are nonflat and undergo significant sliding displacements. Crack surfaces were characterized by a random distribution of asperity heights, density of asperities, and asperity radii. The propagating crack was subdivided into ligaments and each ligament was treated as a contact problem between two randomly rough surfaces. The far-field tensile stresses were varied in a cyclic manner for R = 0.1 and -1 loading conditions. The contact stresses at the minimal load were determined by analyzing the local crushing of the asperities. Then, upon loading the crack opening, stresses were computed when the contact stresses were overcome. The results of crack opening stresses were correlated with CTOD/sigma(0) where CTOD is the crack-tip opening displacement and ao is the average asperity height. The asperity effects on closure were compared with plasticity-induced closure results from the literature for identification of conditions when one mechanism dominates the other.