What are stress-free boundary conditions? (Continuum Mechanics) 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!
consider a hallow cylinder, such that it is opened in upper surface and closed in lower surfaces, now we can see what is stress free boundary? the opened upper surface of cylinder has imaginary boundary so that we are only assume, but sensation is not possible, that is called stress free boundary, and now the lower surface has a boundary so that possible to sense, that is called rigid boundary
consider a hallow cylinder, such that it is opened in upper surface and closed in lower surfaces, now we can see what is stress free boundary? the opened upper surface of cylinder has imaginary boundary so that we are only assume, but sensation is not possible, that is called stress free boundary, and now the lower surface has a boundary so that possible to sense, that is called rigid boundary