If someone were to ask you to define the difference, what would you said? (to justify modeling a material one way or the other) In an elasto-plastic solid you see permanent deformation after the yield strength is breached as a function of ... stress, right? (in the von Mises regime). If the same material is modeled as visco-elastic, does any "yield strength" even apply, or do you see some visco-elastic strain over time (and under some load of course, with intervals of deviatoric stress) and thus capture some "extra" details (I don't think "creep" is important because we are talking about the Maxwell treatment). I have been combing the web and YouTube for a CLEAR and TOTAL distinction between the two and I only find tidbits. (if you care, the material we are discussing is Earth's crust, heated in the area of a magma chamber, which applies intervals of "excess pressure" to the crust around it when it is being actively filled with magma from below and expanding as gases exsolve).