Mixed-mode fatigue crack growth using cohesive zone modeling

In order to capture nonlinear crack tip behavior and account for mixed-mode fatigue crack growth, a cohesive zone based fatigue crack growth model is proposed in conjunction with the Park-Paulino-Roesler (PPR) traction-separation relationship. The model clearly defines five stages during arbitrary fatigue loading: softening, unloading, reloading, contact, and complete failure. The cohesive traction-separation relationship is based on the PPR fracture potential, while the fatigue damage is accumulated by introducing two conjugate damage measures. One damage measure is associated with the rate of separation, while the other is related to the rate of traction (or local stress). Additionally, two model constants are introduced to control the normal contact condition, which may be associated with physical conditions such as crack closure, crack face roughness, and oxidation of fracture surface. Furthermore, computational simulations of the proposed fatigue crack growth model are performed for a simple mode-I test, double cantilever beam test, modified mixed-mode bending test, and three-point bending test. The observed computational results lead to stable and consistent fatigue crack growth.