Toughness
Toughness:
When a huge external force is applied on a metal, the metal will experience fracture. Toughness is the ability of a metal to resist fracture. The ability of a material to resist fracture due to high impact loads, is called toughness.
Hardness:
Hardness is the ability of a metal to resist permanent change of shape caused by an external force.
Strength:
When external forces are applied on a metal, breaking (brittle metal) or yielding (ductile metal) may occur. Strength is the property of a metal by which it resists external force without breaking or yielding.
double
One side of the nominal size
In a unilateral system of tolerance, the tolerance is allowed on One side of the nominal size. Unilateral Tolerance is concerned with variations in single direction whether plus or minus.