Does zero strain always imply zero stress?

Question

In solid mechanics, can I always assume that if an object undergoes no strain, then no stress is applied to it? I think it's true only because I can't seem to find a counter-example.

Answer 1

Zero strain implies zero internal stress but you can still have external stress or volume forces applied on it. The equation of motion of continuum mechanics are: $$\rho\partial_t^2u_i = C_{ijkl}\nabla_j\nabla_ku_{l} + f_i$$ with $u(\vec{x},t)$ the displacement to the equilibrium position, $C_{ijkl}$ the stiffness matrix, $\rho$ the mass density, $f_i = \nabla_j\tau_{ij}$ the volume forces derived from an applied external (as in not a consequence of Hooke's law and the natural elasticity of your solid) stress $\tau_{ij}$.

Answer 2

Zero strain does not always imply zero stress and visa versa. There are matterials that display stress-strain, $\sigma-\epsilon,$ hysteresis behaviour. In matterials like this, when you start loading them, they behave normally, i.e increasing the stress increases the strain. However, when you start to unload them (remove the load), instead of the stress becoming zero when the strain becomes zero, the matterial has some residual stress applied to it! Similary, if you repeat the cycle, although the stress becomes zero the strain retains a permanent value, i.e. the matterial remains permanently deformed! These are very interesting elastic properties of such matterials. Stress-Strain hysteresis phenomena are very well known and are discussed extensively in literature.