Plastic deformation of constrained systems: Boundary effects and their implications for strain gradient plasticity

Classical plasticity fails to capture the material response observed when small volumes of material are subjected to inhomogeneous plastic deformation. The decreasing size of many engineering devices has lead to a strong interest in accurate modeling of the material behavior of small volumes. By including a strain gradient term in the governing field equations of classical plasticity, strain gradient plasticity (SGP) models have been developed which include an internal length scale in order to capture size effects. While there has been considerable work on developing various SGP models, experimental investigations have not been undertaken to explore which models describe the microscopic behavior accurately.; The present study proposes to address this issue by conducting a set of deformation experiments that directly measure the material response on the micron scale. Specifically, sapphire/aluminum/sapphire layered sandwich beams are deformed in simple shear using the asymmetric four-point bend test configuration. After deformation, X-ray micro-diffraction experiments using synchrotron radiation are conducted to measure lattice rotations in the deformed aluminum layer, which can be related to the strain. Because simpler strain gradient plasticity models predict a uniform strain field near the interface, and more complex models allow for the presence of a strain gradient very close to the boundary (called a boundary layer), these experiments will be able to determine which model best describes the true material behavior. After 6% deformation, samples containing 50 mum aluminum foils show 10--15 mum thick boundary layers of reduced lattice rotation present at both interfaces, indicating the simpler type of SGP model inadequately describes the microscopic material behavior.; In order to explore the effect of deformation volume and degree of constraint, two additional sets of samples were fabricated, one with a 100 mum metal layer and second with 280 mum. The crystal orientation of the aluminum is found to be the parameter that most heavily influences the material response, clouding the exact effect of changing volume and relaxing constraint. This result was not anticipated from current modeling efforts, indicating additional modification (of the models) is necessary.