Date of Award
Doctor of Philosophy (PhD)
Civil and Environmental Engineering
George Z. Voyiadjis
A micro-mechanical damage model for high cycle fatigue loading based on thermo-dynamical principles is developed for uni-directional continuous fiber reinforced metal matrix composites. The model uses a micro-mechanical based theory to predict the fatigue damage evolution in the individual constituents of the composite material, namely the fibers and the matrix, from which the overall fatigue damage evolution in the composite is obtained. The micro-mechanical analysis is performed for each individual constituent using stress and strain concentration tensors based on the Mori-Tanaka method. A fatigue damage criterion based on thermo-dynamical principles is developed and applied to each of the constituents. Fatigue damage evolution equations are derived for the individual constituents and appropriate damage model parameters are established which reflect the physical behavior of the constituents with respect to damage evolution during the fatigue life of the material. The developed model is implemented into a numerical simulation code which is then used to simulate several fatigue tests for a uni-directional metal matrix composite system. High cycle fatigue loading is only modeled here, which is characterized by elastic deformations at the macroscale. The fatigue loading is applied as a uni-axial normal stress in the fiber direction in the form of a sinusoidal wave. A parametric study is conducted to show the influence of the model parameters on the damage evolution process in the constituents. These parametric studies show that the model is able to capture different dominating failure modes in the composite material. Finally, several complete fatigue simulations are performed. Results from these simulations are presented in the form of damage evolution curves for the individual constituents as well as the overall composite material. The obtained results show qualitative good behavior with respect to the physical behavior observed in fatigue experiments. The failure mode for the investigated composite material system is captured and displayed properly. Furthermore, a comparison with available experimental data is made for the number of cycles to failure obtained from the simulations. This comparison is shown in the form of a Wohler diagram and satisfactory agreement is observed.
Echle, Rainer, "A Micro-Mechanical Fatigue Damage Model for Uni-Directional Metal Matrix Composites." (1997). LSU Historical Dissertations and Theses. 6417.