Properties of materials
One material can often be distinguished from another by means of physical properties,such as color,density,specific heat,coefficient of thermal expansion,thermal and electrical conductivity,magnetic properties,and melting point. Some of these,for example,thermal conductivity,electrical conductivity,and density,may be of prime importance in selecting material for certain specific uses.Those properties that describe how a material reacts to mechanical usage,however,are often more important to the engineer in selecting materials in connection with design.These mechanical properties relate to how the material will react to the various loadings during service.
Mechanical properties are the characteristic responses of a material to applied forces.These properties fall into five broad categories:strength,hardness,elasticity,ductility,and toughness.
(1) Strength is the ability of a material to resist penetration and abrasion.Bridges girders,elevator cables,and building beams all must have this property.
(2) Hardness is the ability of a material to resist penetration and abrasion.Cutting tools,files,and drills must resist abrasion,or wear.Armor plate,crushing machinery,and metal rolls for steel mills all must resist penetration.
(3) Elasticity is the ability to spring back to original shape.Auto bumpers and all springs should have this quality.
(4) Ductility is the ability to undergo permanent changes of shape without rupturing.Modern,deep-formed auto bodies and fenders and other stamped and formed products must have this property.
(5) Toughness is the ability to absorb mechanically applied energy.Strength and ductility determine a material's toughness.Toughness is needed in railroad cars,automobile axles,hammers rails and similar products.