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Ductile and brittle materials
A ductile material is one such as copper which may be drawn out into a wire.
A brittle material is one like cast iron which will stretch and then break.

The graph shows a ductile mayterial under stress:
(a) OP is a straight line - in this region Hooke's law is obeyed.
(b) P is the limit of proportionality – up to P strain is proportional to stress.
(c) E is the elastic limit - up to E, if the load is removed the material will return to its original length (although the stress may not be proportional to the strain up to this point).
(d) Y is the yield point - between E and Y the material becomes plastic, that is, if the load is removed the material will contract but all the extension is not recoverable. The material follows the dotted line YS on the graph during contraction and the remaining extension is known as a permanent set.
(e) Z - after this point none of the extension is recoverable.
(f) B - this is the breaking stress beyond which the material will break.

 
 
 
© Keith Gibbs 2010