Mechanical properties of engineering materials

Mechanical properties of engineering materials

Materials are characterized by their properties. They may be hard, ductile or heavy. Conversely, they may be soft, brittle or light. The mechanical properties of materials are the properties that describe the behavior of the material under the action of external forces. They usually relate to elastic and plastic behavior of the material. Mechanical properties are of significant importance in the selection of material for structural machine components.

Mechanical properties

Strength: strength is defined as the ability of the material to resist, without rupture, external forces causing various types of stresses. Depending upon the type of stresses induced by external loads, strength is expressed as tensile strength, compressive strength or shear strength.

Elasticity: elasticity is defined as the ability of the material to regain its original shape and size after the deformation, when the external forces are removed. During elastic deformation, the atoms of the metals are displaced from their original positions but not to the extent they take up new positions. Therefore, when the external force is removed, atoms of the metal return to their original positions and the metal takes back its original shape. Steel is perfectly elastic within a certain elastic limit.

Plasticity: plasticity is defined as the ability of the material to retain the deformation produced under the load on a permanent basis. During plastic deformation atoms of the metal are permanently displaced from their original positions and take up new positions.

Stiffness or Rigidity: stiffness or rigidity is defined as the ability of the material to resist deformation under the action of an external load. All materials deform when stressed, to a more or less extent. Modulus of elasticity is the measure of stiffness.

Resilience: resilience is defined as the ability of the material to absorb energy when deformed elastically and to release this energy when unloaded. A resilient material absorbs energy with in elastic range without any permanent deformation. Resilience is measured by a quantity, called modulus of resilience, which is the strain energy per unit volume that is required to stress the specimen in a tension test to the elastic limit point.

Toughness: toughness is defined as the ability of the material to absorb energy before fracture takes place. In other words, toughness is the energy for failure by fracture. Tough materials have the ability to bend, twist or stretch before failure takes place. Toughness is measured by a quantity called modulus of toughness.

Malleability: the ability of the material to deform to a greater extent before the sign of crack, is called malleability, when it is subjected to a compressive load. The term ‘malleable’ comes from a word meaning ‘hammer’, and in narrow sense, it means the ability to be hammered out into thin sections. Malleable metals can be rolled, forged or extruded because these processes involve shaping under compressive force.

Ductility: the ability of a material to deform to a greater extent before the sign of crack, when it is subjected to tensile force, is called ductility. In other words, ductility is the permanent strain that accompanies fracture in a tension test. Ductile metals can be formed, drawn or bent because these process involve shaping under tension.

Brittleness: brittleness is the property of a material which shows negligible plastic deformation before fracture takes place. Brittleness is the opposite of ductility. A brittle material is that which undergoes little plastic deformation prior to fracture in a tension test.

Hardness: hardness is defined as the resistance of the material to penetration or permanent deformation. It usually indicates resistance to abrasion scratching, cutting or shaping. Hardness is an important property in the selection of material for parts which rub on one another such as pinion and gear, cam and follower, rail and wheel and parts of ball bearing.