Negar Akbarian

Grade:  Master

Thesis Title:

Implementation of Low Cycle Fatigue Damage with Isotropic and Kinematic Hardening.

Year: Sept. 2010- Oct. 2012.

Abstract:

Internal damage can caused due to the presence and development of microscopic cracks and cavities that eventually cause to part fracture. For most metallic materials, the damage behavior is a combination of brittle and ductile response and contribution of each mode is, to a significant extent, dependent on the temperature and loading rate. One of the most important mode of material deterioration in metal is fatigue damage. It is observed in mechanical components subjected to a large number of load and/or temperature cycles. Low cycle fatigue failures may occure when structures are subjected to heavy cyclic loadings which induce irreversible strains on small or large scale, giving rise to damage up crack initiation and propagation. Generally, in low cycle fatigue the number of cycles to rupture is low (lower than 104 cycles).

The main objective of this study is to evaluation of low cycle fatigue damage under cyclic loading. An elastic-plastic-damage-coupled constitutive model is implemented in ABAQUS via its user subroutine UMAT. This model is applied to low-cycle fatigue problems. In low cycle fatigue damage exists the plastic deformation in material. Damage mechanics models based on continuum damage mechanics (CDM) are the most useful tools in evaluation of fatigue behavior in ductile material. The metallic plasticity models exhibiting nonlinear isotropic hardening, nonlinear kinematic hardening (Chaboche - Marquis model) and ductile damage (Lemaitre - Chaboche model) is studied in detail, under isothermal and small strain conditions. The nonlinear kinematic hardening model - originally due to Armstrong and Frederick- has proven powerful in modelling cyclic plasticity for metals. Continuum Damage Mechanisms, which represents a local approach to failure, is one of the most promising tools used to predict macro - crack initiation and propagation. In this study, the coupled approach is used to solve the nonlinear equations. The material behavior is modeled by constitutive equations taking into account its progressive deterioration. The microscopic deterioration of a representative volume element is modelled by a continuous (in space and time) variable defined at macro - scale and referred to as the damage variable. The ultimate phase of the damage evolution is detected by a local criterion and corresponds to the failure of the representative volume element, and hence to a macro-crack initiation. The spatial evolution of the completely damaged zone corresponds to the propagation of the macro-crack. The evaluate the accuracy of the subroutine, the results are compared with the references. The results of low cycle fatigue life prediction of a gas valve membrane and fracture location are in a good agreement with experimental.

Keywords:

damage mechanics, low cycle fatigue, life prediction,cyclic loading.