A structure must be both strong and stable. If it is not strong it will break (or snap or crack or crush) and collapse. This can happen when the material is not strong enough to take the stress coming on it.
If a structure is not stable it will move or fall or slide; even if the material of the component parts has not broken (or failed).
The body of an unstable structure may remain intact without any yielding, crushing, cracking etc. But it may move as a whole (rigid body displacement). This movement can be one of the following
- Over tuning
- Rotating about a horizontal or vertical axis.
Strength is a material property. If the stress in the structure exceeds the limiting stress at any part of the structure, this condition is a state of failure.
Buckling or crippling of a compression member in a structure is also a form of failure. Theoretically this is classified as instability. In most cases of buckling (of a compression member) a member suddenly switches from compression mode to bending mode. This leads to enormous stresses and consequent failure by developing high bending compressions and bending tensions. So structures likely to fail by buckling of components are borderline cases between unstable structures and weak structures.