What does stress and strain Invariants mean? In simple terms I do unerstand that parameters that do not depend on cordinate system are called invariants. like principal stress and strains are invariants. But often i come across, First invariant(perhaps relating to volumetric strain) and second invariant (perhaps reltated to deviatoric strain). what are these quantities? what is the physical meaning of deviatoric strain? its invariant and volumetric strain invariant?
I would like to have a physical insight of this. would anyone please help me.