# Mechanical properties of materials

Mechanical properties of materials:

1. Elasticity:The tendency of a material to regain its original dimensions (size and shape) upon the removal of load or force. Eg. Steel is more elastic than rubber. The ratio between tensile stress and tensile strain or Compressive stress and compressive strain is called young’s modulus of Elasticity.

Young’s Modulus of elasticity = E = Stress/Strain = σ / ε

Modulus of rigidity or shear modulus is the ratio of shear stress to the shear strain.

Modulus of Rigidity or Shear Modulus = Shear stress/shear strain

Bulk or Volume modulus of elasticity is the ratio of normal stress to the volumetric strain.

Bulk Modulus = Normal stress/Volumetric strain

1. Plasticity:The tendency of a material to permanently deform when subjected to external load beyond the elastic limit.
2. Toughness:The ability of a material to absorb energy in plastic deformation till the point of fracture is known as toughness. Toughness is indicated by the total area under the stress strain curve up to the fracture point. Eg. Copper has higher toughness than Cast iron.

Modulus of toughness = ½ (Ultimate Tensile strength + Yield strength) X (elongation strength) X elongation

1. Resilience:The ability of a material to absorb energy under elastic deformation and to recover this energy upon removal of load is termed as resilience. Resilience is indicated by the area under the stress strain curve till the point of elastic limit.

Modulus of Resilience = (Yield strength)2 / 2 Modulus of elasticity

1. Yield strength:The ability of a material to resist plastic deformation and represents the stress below which    the deformation is entirely elastic in nature. The magnitude of yield strength for a metal is a measure of resistance to plastic deformation.
2. Tensile strength or Ultimate tensile strength:It is the ratio of maximum stress that a material can withstand without being fractured to the original area of cross section of the material. Ultimate tensile strength or tensile strength is the highest point in a stress-strain curve.
3. Impact strength:Ability of material to resist or absorb energy before it fractures during plastic deformation. It is closely associated with toughness with the difference that toughness takes into account both the strength and ductility of the material. Ductile materials have higher impact strength than brittle materials. Impact strength can be measured by two methods. (a). Izod Test (b). Charpy test
4. Ductility:The ability of a material to be drawn into wire is known as ductility. It is a tensile property and it is the capacity of a material to undergo deformation without being fractured. It cab be measured as the percent (%) elongation or percent area reduction.

% elongation = [(lf – l0) / l0] X 100

% area reduction = [(A0 – Af) / A0] X 100

Where lf is the length at the point of fracture

l0 is the original length

Af is the original cross section area

A0 is the cross sectional at the point of fracture

1. Malleability:The ability of a material to be formed into sheets by hammering or rolling is called malleability. It is a compressive property. e.g. Gold is the most malleable metal.
2. Brittleness:It is the tendency of a material to crack when it is subjected to deformation. It is opposite to ductility and malleability. e.g. Cast iron is a brittle material.
3. Creep:It is a deformation of a material due to the constant load for a long period of time. It is time and temperature dependent property of material. It takes place in three stages. i.e. primary, secondary and tertiary. a). First stage of creep known as primary creep occurs at relatively low temperature and the creep rate decreases with time. b). Second stage of creep known as secondary creep in the range of 0.4 to 0.7 Ti.e. is the absolute melting temperature is a period of constant creep rate and hence referred to as steady state creep. It is the most important part of the creep curve for engineering applications. c). The third stage of creep known as tertiary creep occurs beyond 0.7 Thas an accelerated rate and results in fracture of the material.
4. Fatigue:When a body is subjected to repeated and fluctuating load it tends to develop a characteristic behavior under which failure occurs which is referred to as fatigue.
5. Hardness:The resistance offered by a material to indentation is referred to as hardness of that material. It is the property by virtue of which it can resist abrasion, scratching and penetration. Hardness can be measured by the following tests. (a). Rockwell hardness test  (b). Brinell Hardness test  (c). Vicker’s hardness test.
6. Harden ability:It indicates the degree of hardness that can be imparted to a material by the process of hardening. It deals with the depth and distribution of hardness that can be induced in that particular material which can be increased by addition of alloying elements.
7. Wear Resistance:Wear is the unintentional removal of the material from the surface of the body. Wear is of two types. 1). Abrasive wear 2). Adhesive wear.  Wear resistance is the ability of a metal to resist this unintentional removal.