Material Science Objective Type Questions (AMIE Winter 2017)

Material Science Objective Type Questions (AMIE Winter 2017) 

(2 marks each)

Define a Burger Vector.
The Burgers vector associated with a dislocation is a measure of the lattice distortion caused by the presence of the line defect. The diagram shows the convention for measuring the Burgers vector. A circuit is made around a dislocation line in a clockwise direction (top picture) with each step of the circuit connecting lattice sites that are fully coordinated. This circuit is then transferred to a perfect lattice of the same type. Because of the absence of a dislocation within this circuit, it fails to close on itself, and the vector linking the end of the circuit to the starting point is the Burgers vector, b = QM.

The Burgers vector defined in this way is a unit vector of the lattice if the dislocation is a unit dislocation, and a shorter stable translation vector of the lattice if the dislocation is a partial dislocation.

Shear modulus, G (kN/mm²), obeys proportionality with an elastic modulus, E(kN/mm²). If E = 100 kN/mm² and Poisson’s ratio, υ = 0.25, calculate the value of G.
The Poisson’s ratio (v), Young’s modulus (E), and Modulus of rigidity (G) are bound by the following relation:
2G(1 + υ) = E
Therefore, when υ = 0.25, the ratio G/E = 0.4
G/100 = 4
Hence G = 400 kN/mm²

Define anelasticity and viscoelasticity.
Viscoelastic materials are those for which the relationship between stress and strain depends on time or, in the frequency domain, on frequency. Viscoelastic materials have properties that depend on the strain rate.

Anelastic solids represent a subset of viscoelastic materials: they have a unique equilibrium configuration and ultimately recover fully after removal of a transient load. After being squeezed, they return to their original shape, given enough time. The transient strain is recoverable after the load or deformation is removed. The timescale for recovery may be very short, or it may be so long as to exceed the observer's patience or even lifetime.

State Griffith theory.
The Griffith theory states that a crack will propagate when the reduction in potential energy that occurs due to crack growth is greater than or equal to the increase in surface energy due to the creation of new free surfaces. This theory is applicable to elastic materials that fracture in a brittle fashion.

What is the fatigue limit of a material?
It is the highest stress that a material can withstand for an infinite number of cycles without breaking. Also called also endurance limit.

Define the terms (a) Curie temperature and (b) Remanence of a material.
Curie point, also called Curie Temperature, the temperature at which certain magnetic materials undergo a sharp change in their magnetic properties.

Remanence is the magnetic flux that remains in a magnetic circuit after an applied magnetomotive force has been removed.

What is meant by mobility?
Mobility, in solid-state physics, measurement of the ease with which a particular type of charged particle moves through a solid material under the influence of an electric field.

Define hardenability? State the factors affecting the hardenability.
Hardenability is a way to indicate a material’s potential to be hardened by thermal treatment. Hardenability describes how deep the steel may be hardened upon quenching from high temperature. Factors affecting are:

  • Increasing % of carbon increases hardenability
  • Every common alloying element except cobalt increases hardenability
  • Increased grain size increases hardenability due to a reduction in pearlite nucleation sites

What is TD nickel?
TD Nickel is a recently developed alloy containing 2 volume percent thoria and the balance nickel (Ni - 2ThO2). This alloy shows promise as a structural material for use in the temperature range from 1000 to 1300 °C. It has excellent thermal stability, high thermal conductivity, and a high melting point. The material has sufficient ductility for simple cold-forming operations and can be machined in the same manner as stainless steel.

What is vulcanization of rubber?
Vulcanization of rubber is a process of improvement of the rubber elasticity and strength by heating it in the presence of sulfur, which results in three-dimensional cross-linking of the chain rubber molecules (polyisoprene) bonded to each other by sulfur atoms.

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