From generic towards a micromechanical fatigue model

AbstractFatigue life formulas are still based on phenomenological models which adopt simple relations directly from experiments for different loading conditions and use fitted material parameters. The combination of enormous complexity of fatigue damage processes and simple, macro appearance of the formulas (usually power laws), are the source of Generic Fatigue Models (GFM). GFMs rely on minimal, but coherent, micro-details which are independent of the specific micro structure. Such a model has been developed, connecting analytically the S-N power law and endurance stress in terms of statistical strength distributions of material microelements and their neighbors.This paper describes two types of generalizations of the basic GFM: a.) Two level (H-L and L-H) loading, in which a history dependent micro-damage evolution law is proposed, and b.) Multiaxial fatigue response by a simple 2D truss. Emphasize is on minimal parameters and capability of analytical predictions, in which every “material constant” has a physical or micro-geometrical meaning. The theoretical generalizations are compared with experimental data from the literature and show that the predictions are coherent with main experimental features

Modeling plasticity by non-continuous deformation

Plasticity and failure theories are still subjects of intense research. Yet, no compre-hensive theory has been achieved and the Molecular Dynamic method is far from providing a general model which contains only the essentials. This study is motivated by the observation that the continuum assumption in plasticity that neighbour material elements remain neigh-bours at all-time are physically impossible, since neighbour detachments, local slips and neighbour switching must operate, i.e., Non-Continuous Deformation (NCD). Material micro-structure is modelled by a set of point elements (particles) interacting with their neighbours. Each particle can detach from its neighbours and/or attach to a new neighbour during defor-mation. Simulations on two dimensional specimens subjected to uniaxial compression loading and unloading were conducted. Each specimen contained 100 particles with stochastic hetero-geneity controlled by a “disorder” parameter λ. It was found that a. the macro response is typ-ical to elasto-plastic behaviour; b. The number of detachments is linear with plastic energy; c. The number of attachments is linear with the residual strain, and d. Volume is preserved under plastic deformation. Rigid body displacement of local ensemble of elements was also ob-served. Higher disorder coefficient λ decreases the macro elastic modulus and increases the plastic energy