Hi, I need some help here. I'm an MEng Materials with no full background in engineering. So I'm in a bit over my head. I'm trying to understand the concept of hydrostatic stresses and effective strain in connection to worn surfaces. I am told I just need to know the equations but then I'm a bit confused. I'm trying understand how equivalent strain is calculated from the shear angle of the interface θ. In other words how this equation came about E (Z) = (square root of 3/3) * tan theta Can anyone please guide/help me? Thanks.