On the role of microstructure in ductile failure
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Failure in structural materials occurs initially by localization of deformation, and subsequently through a process of nucleation, growth and coalescence of voids. Predicting material failure requires a careful investigation of the different stages of damage evolution at the multiple scales. The main objective of this thesis is to explore the evolution of damage and to correlate this with the deformation of the material at the continuum and microstructural levels. This is accomplished through macroscopic measurements of strain evolution using digital image correlation and microscale measurements of strain and damage using optical and scanning electron microscopy. Three materials with different microstructure were examined. In oxygen-free, high-conductivity copper, a high-purity material without appreciable second phase particles, strain levels in the order of three were observed in the material without any trace of damage. Failure was observed to be triggered by plastic instability in the form of shear bands and the emergence of a prismatic cavity that grows in a self-similar fashion by an alternating slip mechanism. In Al 6061-T6, a material with a dispersion of second phase particles at a volume fraction of about 0.01, nucleation of damage does not appear until plastic strain levels of 0.5 to 1.0. Once damage in the form of particle fracture or decohesion at the interface initiates, subsequent failure follows by the void nucleation, growth and coalescence; but, dominated by the fluctuations in the distribution of second phase particles, final separation occurs in a highly localized layer of material on the order of the grain size, corresponding to a small increase in the overall strain. In nodular cast iron, a material with an initial porosity of about 0.10, growth of voids was observed initially, but this was terminated by a transition of the deformation into a localized region. Phenomenological models based on strain-to-failure and micromechanical models based on a mechanistic description of the microscale deformation are evaluated in light of the above examination of failure in these three classes of materials.