Constitutive Modeling of Material Damage for Fatigue Failure Prediction

This paper presents a constitutive modeling of material damage capable of characterizing fatigue damage and plastic damage for rate-independent materials under multiaxial loading with the theory of damage mechanics. First, an internal state variable known as the damage variable is introduced with two scalars to characterize material degradation due to the change of material microstructures under fatigue. Then, depending upon the level of applied stress in a material element, the damage accumulation is postulated to undergo two processes, the accumulation of fatigue damage and plastic damage. Two surfaces, fatigue damage surface and plastic damage surface, are proposed to quantify fatigue damage initiation and plastic damage initiation. The damage-coupled constitutive equations of elasticity and plasticity plus two corresponding damage evolution equations are formulated based on the irreversible thermodynamics. Finally, a failure criterion based on the overall damage accumulation is postulated to govern crack initiation and propagation in material elements. A test program to determine the necessary material parameters is also provided. These parameters are considered intrinsic material properties required as the finite element input for predicting damage accumulation under fatigue loading. As an example, experiments are carried out to measure a set of damage parameters for aluminum alloy 2024-T3. The measured parameters are then used to demonstrate systematically how the damage parameters can be effectively evaluated.