Fatigue life predictions of metal structures based on a low-cycle, multiaxial fatigue damage model

A modeling strategy is developed accounting for the simulation of fatigue in metallic structures subjected to multiaxial, cyclic loading conditions. Cyclic plastic deformations are assumed to cause microcrack nucleation and growth, which in turn, lead to a deterioration of the mechanical and thermal properties. Microcrack nucleation and growth are modeled by a continuum damage mechanics formulation which accounts for a progressive loss of material integrity. The formulation utilizes a critical plane method for the prediction of microcrack nucleation. The emerging microcracks are assumed to be aligned with the critical plane orientation causing an anisotropic damage behavior. A second-order damage tensor is defined to approximate the damage behavior by an orthotropic material response. Damage evolution caused by microcrack growth is obtained from an empirical energy criterion based on the inelastic strain energy density. This way, the isotropic material response of a pristine material becomes orthotropic during damage evolution causing a direction dependent loss of material integrity. The continuum damage mechanics formulation in combination with the nonlinear Finite Element Method enables the simulation of structural deterioration caused by a propagating region of material failure. The approach is implemented into the Finite Element Method and extended by a cycle jump technique to reduce the computational time. The modeling strategy is exemplified on micrometer-sized cantilever beams under low-cycle fatigue conditions.