So, I was introduced tangentially to plastic deformation and hardening functions. I see we have three basic function ... and the linear and kinematic seem to be the most common defaults ... I see it is a measure of the strain of a material under stress. But how would you know which version to use for some material? Is it some tabled value like thermal conductivity or specific heat? For instance, I need to choose a "hardening function" for Earth's heated crust as it undergoes the pressure of an inflating magma reservoir. I cannot find such a function in the literature so how would I go about choosing some reasonable function? As an added note, this has to be applied in COMSOL multiphysics software which requires a "Isotropic tangent modulus" and a "kinematic tangent modulus" .... I cannot find ANY value for those parameters. I'm not even sure WHAT they are. Are these parts of the hardening function to describe the curvature of the line? Since mechanics and materials is not the mainstay of what I do, this is all new to me. Thanks for any help!