I've been doing some work on continuum mechanics and I can't find a good explanation for "stress free boundary conditions". I'm doing the question below:
A solid linearly elastic isotropic circular cylinder of length $h$, radius $a$ and constant density $r$ is suspended with the axis vertical. The cylinder is slightly deformed due to its own weight. The $X_3$ axis corresponds to the axis of the cylinder. The lower end of the cylinder lies in the plane $X_3 = 0$ with the origin at its centre. The lateral surface of the cylinder $X^2_1 +X^2_2 - a^2 =0$ and the lower face $X_3 =0$ are stress free. State the stress free boundary conditions on these surfaces.
I can draw the cylinder but I'm very lost on what to write for the boundary conditions? I'd prefer to understand it over just knowing the answer flat out please (:
Thank you!