A new finite element procedure for fatigue life prediction and high strain rate assessment of cold worked Advanced High Strength Steel

This dissertation presents a new finite element procedure for fatigue life prediction and high strain rate assessment of cold worked Advanced High Strength Steel (AHSS). The first part of this research is related to the development of a new finite element procedure from an energy-based fatigue life prediction framework previously developed for prediction of axial, bending and multi-axial fatigue life. The framework for the prediction of fatigue life via energy analysis consists of constitutive laws which correlate the cyclic energy to the amount of energy required to fracture a material. In this study, the energy expressions that construct the new constitutive laws are integrated into a minimum potential energy formulation to develop new finite elements for fatigue life prediction for structural components subjected to axial, bending and multi-axial cyclic loads. The comparison of finite element method (FEM) results to the existing experimental fatigue data verifies the new finite element method for fatigue life prediction. The final output of this finite element analysis is in the form of number of cycles to failure for each element. The performance of the fatigue finite element is demonstrated by the fatigue life predictions from Al 6061-T6 (Aluminum) and Ti-6Al-4V (Titanium Alloy). In addition to developing new fatigue finite elements, a new equivalent stress expression and a new finite element procedure for multiaxail fatigue life prediction are also proposed. In order to develop the new equivalent stress equation, energy expressions that construct the constitutive law are equated in the form of total strain energy and the distortion energy dissipated in a fatigue cycle. The resulting equation is further evaluated to acquire the equivalent stress per cycle using energy based methodologies. The new procedure is applicable to biaxial as well as multiaxial fatigue applications. The second part of this research is related to the development of LSDYNA material model for vehicle crash simulation based on high strain rate assessment of cold worked AHSS. The performance of a vehicle during a crash is an important subject in automobile research. In order to simulate an actual crash using software like LSDYNA, it is desirable to have accurate stress/strain data for materials. The material models available in the literature ignore the effect of cold working on the material and present data only for flat sheets. In this research, the cold worked AHSS with curved cross-section is tested at strain rates of 1000 (in/in)/s and the data is used to develop a corresponding LSDYNA material model for vehicle crash simulation.