Material Science - Short answer questions from AMIE exams

Why is cast iron used as a material for machine tool beds?
Cast iron has good machinability, stable dimensions and has got the property of damping vibrations in machines tools.

When does Hook’s law appear to fail?
Hooke’s law states that within the elastic limit, for elastic deformation, stress is proportional to strain. This does not hold in the case of elastomers like rubber whose stress-strain behaviour is not linear in their elastic range. 

Define the term "dislocation".
It is a type of line imperfection in a crystal in which positions of planes of atoms are missing or misaligned. These may be edge type or screw type. 

What are the units of the surface energy of a grain boundary?
Ergs/cm2 or dynes/cm.

When is grain boundary shear possible in metals?
Grain boundary shear is possible only at temperatures above 0.5 Tm where Tm is the melting point of the metal expressed in absolute scale.

Give two examples of heat-resisting alloys.
Researchers have discovered that tantalum carbide and hafnium carbide materials can withstand scorching temperatures of nearly 4000 degrees Celsius.

If a tensile force of 10 N is applied to a specimen of length 100 mm, width 10 mm and thickness 1mm, what is the tensile stress developed in the specimen?
1 N/mm2

Name the property by virtue of which the material can absorb maximum energy before the fracture takes place.
The maximum energy absorbed before fracture is known as toughness.

What is the compressive strength of a material?
Compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce the size, as opposed to Tensile strength which withstands loads tending to elongate.

Name the heat treatment process used for duralumin alloy.
Duralumin is annealed.

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