Visualizing force chain buckling in stress and displacement fields of granular assembly using photoelastic measurement

摘要

Recent advances in digital image analysis enable us to visualize the displacement field while the visualization of the stress field has not been intensively studied. In this study, the photoelastic measurement technique was employed to visualize the force transmission of a particle assembly. The quantitative measurement of the relative retardation using the compensator allows us to evaluate the particle stresses. A model assembly bounded by a steel frame was built by stacking bi-dimensional circular particles made of polycarbonate elastomer. Each particle was coated by a thin photoelastic sheet so that the force transmission among particles can be visualized. In a contacted particle, both magnitude and orientation of principal stress difference can also be measured via the photoelasticity technique. Overall displacement fields were constructed by using a digital imaging process on sequential images of moving particles. The model assembly was subjected to incremental surface loading, which simulates a bearing capacity problem of the shallow foundation on granular soils. The different distributions of the contact stresses at the initial loading and near the failure were quantitatively compared. The particle stresses obtained from photoelastic measurement are not proportional to the external loads but increase exponentially when the external load approaches the critical value for failure. The pattern of internal stresses and displacements observed in the pre-failure elastic state disappears immediately after the buckling of confined force chains.