bullet1 Properties
bullet2 Mechanical

bullet3 Rigidity

This is a measure of a materials ability not to deflect under an applied load (stiffness).  For Example although steel is very much stronger than cast iron the latter material is preferred for machine beds and frames because it is more rigid and less likely to deflect with consequent loss of alignment and accuracy.

For a given load the cast iron bar deflects less than the steel bar because cast iron is a more rigid material .  However, when the load is increased the cast iron bar will break, whilst the steel bar under the same load merely deflects a little further but does not break .  Thus a materials which is rigid is not necessarily strong .  In fact more often than not rigid materials are found to be brittle. click here for a simple explanation and picture