Measuring Micromechanical Behavior for Polycrystalline Materials under Cyclic Loading
Understanding the crack initiation and propagation mechanisms of a polycrystalline material under cyclic loading remains a challenging problem. Complicated crystal stresses arising from single crystal anisotropy and complex grain and phase morphologies make the prediction of crack initiation and propagation at the grain scale difficult. In this work, oxygen free high conductivity copper specimens were cyclically loaded while high energy synchrotron x-ray diffraction experiments were performed to find the orientation dependent crystal stresses. From the x-ray diffraction data, the lattice strains and the peak widths were measured for several crystallographic planes oriented in many different directions at various points along a specimen?s life. Using the lattice strain measurements, crystal elastic strain distribution and stress distribution over orientation space were calculated. It was found that the evolution of the crystal stress distribution over orientation space with respect to specimen life is small but not negligible. The peak widths associated with the dislocation density and the distribution of elastic strain in a material also showed small changes with respect to specimen life indicating changes in the grain size scale. The contributions from changes in dislocation density and elastic strain distribution to the peak widths were decomposed and showed that while the elastic strain distribution over orientation space is complicated, its contribution to the peak width is small.
fatigue; synchrotron radiation; lattice strain; crystal stress; x-ray diffraction
dissertation or thesis