CAREER: Multi-Scale Experiments of Fracture in Elastic-Plastic Materials

点击翻译按钮获取中文摘要

AbstractThe research and educational program consists of developing a multi-scaleexperimental program to study the mechanics of fracture in elastic-plasticmaterials. The research is intended to complement and guide multi-scalesimulations of fracture in elastic-plastic materials. The experiments willfocus on understanding the behavior of cracks that exist along the grainboundary of symmetric tilt bicrystals of either pure aluminum or purecopper. In such a specimen there is a well known, but poorly understood,phenomenon known as directional dependence of fracture. If a crack isintroduced along the grain boundary, the amount it grows macroscopicallydepends upon the direction within the grain boundary that it propagates. Ajudicious choice of crystallographic orientation and loading techniqueswill ensure that the mechanical properties at the macroscopic length scaleare identical for both cracks, yet the amount of macroscopic growth willbe different. Thus the explanation for the directional dependence mustdepend upon deformation mechanisms at the smaller length scales as well asthe interaction of the deformation mechanisms across length scales.In the experiments, cracks will be introduced and propagated in oppositedirections within the grain boundary of symmetric tilt bicrystals. Thestructure of the asymptotic deformation fields will be measured underplane strain conditions to investigate a certain type of straindiscontinuity, known as kink shear discontinuity, that is predicted bytheory and observed in some materials under certain conditions. Theexistence of kink shear discontinuities depends critically upon plasticconstitutive relations at very small length scales. The next set ofexperiments will be to measure the directional dependence of fractureunder both a monotonic and cyclic loading to document the degree of crackgrowth in different directions along a grain boundary. The final set ofexperiments will measure the lattice curvature of the crystal close to thecrack tip to ascertain the density of geometrically necessary dislocationswhich play an important role in strain gradient plasticity. All theexperiments will be simulated with the discrete dislocation plasticitytechnique in collaboration with other researchers.The educational component is to reach out to secondary schools in theneighborhoods around Columbia University, particularly in Harlem, todevelop a science module that is suitable for students in their final twoyears of high school with the goal of inspiring them to continue theireducations. The module will demonstrate that materials are made ofdiscrete atoms by discussing the phenomenon of diffraction. To do so, thestudents will first gain intuition into the diffraction phenomenon byidentifying and matching symmetries in diffraction gratings and therelated diffraction patterns that are created with a standard laserpointer. Then the crystallographic aspects of face-centered cubic metalsand the symmetries of a cube will be introduced. Finally the students willbe asked to identify the two-fold, three-fold, and four-fold rotationsymmetries in separate Laue back reflection x-ray diffraction patterns andcorrelate the patterns with the crystallographic orientations that exhibitthe same sets of symmetries. Thus the students will identify thesymmetries inherent in crystals without having to understand any of thedetails of the diffraction process.