  All systems are expected to be designed to fulfil their functions over their
requested lifetime. Nevertheless, failure of a system may occur, and this is
unfortunately, true also for vacuum systems. From a mechanical point of view,
buckling, leak by fatigue crack propagation, rupture of a fixed support under
unbalanced vacuum force... may happen. To well understand and anticipate the
behaviour of a structure, it is mandatory to first understand the mechanical
behaviour of materials as well as their failure modes. This is presented in a
first part of the document. Then, the structural behaviour of a vacuum system
and its relationship with the material properties is discussed. Finally,
guidelines for the material selection for a given application are roughed out.
