טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentGerstman Ztvi
SubjectPrediction of Two Level Fatigue Life by a Micromechanic
Model
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Emeritus Eli Altus


Abstract

A simple formula for two level fatigue life by a micromechanic model is proposed. It is based on a single parameter -   the slope of the basic logarithmic S-N curve.

The model is a generalization of previous studies (Altus, 1989) on one level loading. The interaction between a broken chain (element at micro scale) and its neighbor as the main source of fatigue damage progression is considered. The general idea is to use statistical micordamage parameters such as local probabilities of failure and neighbor interactions and relate them to macro response. A physical explanation for material behavior under H-L fatigue loading is found, without any empirical relations as in other models. Loading order effects are derived directly from the statistical nature of the micromechanic damage mechanism, leading to a macro behavior which fits experiments very well.

An analytic solution is obtained, which improves previous numerical results. The discrete algebraic equations are transformed into a nonlinear second order differential equation having a simple, power-law analytic solution for one stress level, known as Basquin law. A three stage damage growth for the two level loading is obtained, yielding a simple analytic equation in the form of a generalized Miner Rule. Physical insight that is absent in other models is obtained.

Two Magnesium alloys (AZ31 & AM50) were fatigue tested for validation and the results obtained were good. Behavior of human bones (hip), were found to be also well predicted by the model. Another subject which was checked is the change of endurance limit after initial cycling. The model showed qualitatively similar results when compared to the SAE 1020 fatigue tests.