Mechanical Properties of Material

  • Understand the importance of studying material properties while studying material behaviour.

  • Define mechanical properties of a material such as strength, hardness, ductility, elasticity, etc.

  • To relate mechanical properties with the behaviour of materials under load.

  • Yield point In stress-strain curve, if the stress is too large, the strain deviates from being proportional to the stress. The point at which this happens is the yield point because there the material yields, deforming permanently (plastically).

  • Yield stress Hooke’s law is not valid beyond the yield point. The stress at the yield point is called yield stress, and is an important measure of the mechanical properties of materials. In practice, the yield stress is chosen as that causing a permanent strain of 0.002.

  • Strength is the measure of the ability of the material to with stand or to resist external forces. Higher the strength of the material, higher will be the resistance offered by the material to the applied external force. For example a wooden bar will break easily and iron bar will not break easily because the resistance offered by the iron bar is very high. Depending upon the type of loading the strength may be tensile, compressive, shear, torsional and bending.

  • Tensile strength When stress continues in the plastic regime, the stress-strain passes through a maximum, called the tensile strength (\(s_{TS}\)).

  • Malleability is the ability of a metal to be hammered into thin sheets. It is the ability of a solid to bend or be hammered into other shapes without breaking. Examples of malleable metals are gold, iron, aluminum, copper (to a degree) and lead. When a piece of hot iron is hammered it takes the shape of a sheet.

  • Ductility The ability to deform before braking. It is the opposite of brittleness. Ductility can be given either as percent maximum elongation \(e_{max}\) or maximum area reduction.

  • Brittleness is the property of the material because of which the material fails or breaks or fractures or ruptures without any appreciable amount of deformation, before failure.In other words, lack of ductility is brittleness. An elongation of less than 5% is often taken to indicate a brittle material. Example of brittle material: Chalk, glass, cast iron etc., it will not stretch and bend before breaking.

  • Toughness is a measure of the amount of energy a material can absorb before failure or fracture takes place. The toughness of a material is expressed as energy absorbed (Nm) per unit volume of material (m\(^3\)) or Nm/m\(^3\).Toughness is related to impact strength i.e. resistance to shock loading and it is desirable in the components subjected to shock and impacts such as gears, hammer, anvil. The area under the stress and strain curve of brittle and ductile material represents the toughness of materials.

  • Resilience is the capacity to absorb energy elastically. The energy per unit volume is the area under the strain-stress curve in the elastic region.

  • Elasticity is the ability of a material to restore back to its original shape and size, after it has been subjected to tensile, compressive, torsional or shear stresses.

    • Modulus of Elasticity E is defined as the ratio of tensile stress to strain. It is determined by a tensile test.

    • Modulus of rigidity G is defined as the ratio of shear stress and strain and it is determined by a torsion test.

    • Bulk modulus K defined as the ratio of pressure and volumetric strain.

    • Poisson’s ratio \(\nu\) is the negative ratio of transverse to axial strain.

  • Plasticity is the property of the material because of which the material is able to retain (maintain) the deformed shape even after removal of deforming forces. Clay, polyethylene, etc.

  • Hardness is the resistance to plastic deformation (e.g., a local dent or scratch). Scratch hardness is the measure of how resistant a sample is to fracture or permanent plastic deformation due to friction from a sharp object, whereas indentation hardness measures the resistance of a sample to material deformation due to a constant compression load from a sharp object.

  • Stiffness is also called as the rigidity and is the ability of the material to resist the deflection or Elastic deformation. Stiffness is generally related to the springs, thicker springs are stiffer.

Solved Examples

Solved Example:

50-1-01

Toughness of a material is equal to area under _______ part of the stress-strain curve. (GPSC AE CE 2018- Part B)

Solution:
Toughness refers to the capacity of a material of absorb energy prior to failure. Its value is equal to the entire area under the stress-strain curve.

Correct Answer: C

Solved Example:

50-1-02

The hardness is the property of a material due to which it:

Solution:
Hardness is the ability of a material to resist deformation, which is determined by a standard test where the surface resistance to indentation is measured. The most commonly used hardness tests are defined by the shape or type of indent, the size, and the amount of load applied.

Correct Answer: A

Solved Example:

50-1-03

For a ductile material, toughness is a measure of:

Solution:
For a ductile material, ability to absorb energy upto fracture.

Correct Answer: B

Solved Example:

50-1-04

Mechanical properties include all of the following, except:

Correct Answer: D

Solved Example:

50-1-05

Ductility of a material can be defined as:

Correct Answer: C

Solved Example:

50-1-06

The property of a material which enables it to resist fracture due to high impact loads is known as: (JKSSB JE ME2015)

Correct Answer: D

Solved Example:

50-1-07

Resilience of a material is important, when it is subjected to:

Correct Answer: D

Solved Example:

50-1-08

Malleability of a material can be defined as: (ISRO VSSC Tech. Asst. Mech Feb2015)

Correct Answer: A

Solved Example:

50-1-09

Isotropic materials are those which have the same: (SSC JE ME March 2017)

Correct Answer: A

Solved Example:

50-1-10

A material is known as allotropic or polymorphic if it:

Correct Answer: B

Solved Example:

50-1-11

Which of the following represents the allotropic forms of iron:

Correct Answer: D

Solved Example:

50-1-12

What is the amount of energy required to fracture a given volume of material?

Correct Answer: A

Solved Example:

50-1-13

What mechanical property of a material which is a time-dependent permanent strain under stress? (AFCAT EKT Mech 2016- Set 02)

Correct Answer: C

Solved Example:

50-1-14

What is the maximum stress below which a material can theoretically endure an infinite number of stress cycles?

Correct Answer: C

Solved Example:

50-1-15

What is the resistance of a material to plastic deformation?

Correct Answer: A

Solved Example:

50-1-16

The property of a material which allows it to be drawn into a smaller section is called: (RPSC Lecturer 2014)

Correct Answer: B

Solved Example:

50-1-17

The property of a material by virtue of which it can be beaten or rolled into plates is called: (UPSSSC JE Mech 2015- Paper 2)

Correct Answer: A