It is the ability of a material to absorb energy while being deformed before breaking. In every piece, prior to breaking or fracture, a pre-cracking phenomenon occurs, hence toughness is defined as the resistance to crack propagation of the material.
The toughness obtained ina a piece will depend on the chemical composition, the forming processes prior to its carrying out and the heat treatment if it has been made. Any defect or internal discontinuity that a piece may present will be detrimental to its toughness