Identification of the Cap Model of Elastoplasticity of Non-Compact Media Under Compressive Mean Stress

A program of basic tests and a method for identifying a three-dimensional model of the elastoplastic behavior of an isotropic porous or powdery consolidated medium experiencing arbitrary quasi-static loading under compressive medium stress at room temperature are proposed. The medium under consideration under compressive medium stresses is compacted with increasing effective stress, which leads to a nonlinear change in elastic modules, hardening and dilatancy (coupling of shear and volumetric components of deformations) in the yield region. To describe this behavior, the cap model of DiMaggio and Sandler, which is present in application software packages, is considered. As basic tests, the free and constrained compression of a cylindrical sample is considered according to a special program containing the stages of loading and unloading with a sequential increase in the amplitude voltage. Samples with a given porosity for free compression tests are manufactured using a tight compression test rig. According to the initial slope of the discharge curves, the values of the elastic modulus for free and constrained compression are determined in a certain range of porosity changes, according to which the Poisson’s ratio is determined. The five constants of the cap model are correctly and explicitly determined by the deformation curve of the material under constrained compression over a wide range of changes in axial deformation (and density), the flow stress under free compression of the sample at some density, and the assumption that the coefficient of transverse deformation in the yield region is equal to the Poisson’s ratio. The elastic and plastic constants were determined according to the test data of powdered paraffin grade T1 with a fraction of 0.63 mm. The corresponding model is applicable for numerical simulation of extrusion processes and mold filling for casting by melting models, processes for manufacturing blanks of non-melting polymer composites by powder technology, stamping sealing elements from flexible graphite and other pressure treatment processes of non-compact media.