A Master of Science Thesis in Civil Engineering submitted by Reem Al-Himairee entitled, "Characterization and Modeling of Damage in Steel at Different Strain Rates," submitted in January 2011. Available are both soft and hard copies of the thesis.
The structural behavior of steel, as many structural materials, changes during deformation processes under complex mechanical loadings. The damage process may start at some point in the deformation process in the form of micro-cracks and micro-voids leading at its latest stage to the development of macro-cracks and consequently material failure. Therefore, systematic understanding of the ductile failure mechanism due to accumulation of plastic deformation is needed to enable proper structural design and hence provide better serviceability. In the last two decades, the micro-structural concepts to define material failure have received wide attention as a better alternative to classical mechanics methodologies. The main objective of this research is to better understand damage initiation and evolution throughout the deformation process at different strain rates. The proposed study relies on a continuum damage mechanics approach that involves characteristic parameters to describe the accumulation of plastic strain and damage under different strain rates. The work has been divided into experimental, theoretical, and simulation phases. The experimental phase involves testing under monotonic uniaxial tensile loading to evaluate the tensile ductile damage behavior. The obtained material parameters are then used as the basic data in the simulations that are performed afterwards. Moreover, the damage is determined through two techniques: reduction in elastic modulus through loading-unloading curves and area measurements using Scanning Electron Microscope (SEM). The theoretical phase proposes a new energy based model that captures the damage dissipation potential. The model has been confirmed theoretically by applying the proposed formula to the data available in the literature. Finally, this model has been implemented as a new user defined material in the finite element analysis software ABAQUS where damage is quantified. The results from the experiments and the model are then compared. In this context, a new damage identification procedure is presented and different aspects of it are particularly addressed. The results of this study show that a simpler model can be utilized for damage assessment in steel based on the following conclusions: The rate of loading is a main sensitive parameter that affects damage, as it has increased significantly with increasing loading rate. Towards higher loading rates, the damage grows in a faster mode. Damage is highly a nonlinear process indicating the steel is pushed closer to a complete state of fracture with the accumulation of strain. The comparisons indicate good agreement between the experimental and the applied energy based model results. The results using elastic energy equivalence yields more conservative values of damage than the strain equivalence hypothesis. The agreement between parameters that are measured by the new approach and those found in the literature are good. The finite element analysis has shown a good correlation with the model predictions.