Interview Questions - Design of Machine Elements

What are the types of fracture?
The two types of fracture are
  • Ductile fracture
  • Brittle fracture
Distinguish between brittle fracture and ductile fracture.
In brittle fracture, crack growth is up to a small depth of the material.
In ductile fracture, a large amount of plastic deformation is present to a higher depth.

Define stress concentration and stress concentration factor. 
Stress concentration is the increase in local stresses at points of rapid change in cross-section or discontinuities.
Stress concentration factor is the ratio of maximum stress at critical section to the nominal stress.

Explain the size factor in endurance strength.
The size factor is used to consider the effect of the size on endurance strength. A large size object will have more defects compared to a small one. So, endurance strength is reduced.
If K is the size factor, then
Actual endurance strength = Theoretical endurance limit x K

Explain Griffith theory. (Or) State the condition for crack growth. 
A crack can propagate if the energy release rate of the crack is greater than crack resistance.

What are the modes of fracture?
The different modes of fractures are:
  • Mode I (Opening mode) - Displacement is normal to crack surface.
  • Mode II (Sliding mode) - Displacement is in the plane of the plate.
  • Mode III (Tearing mode) - Out of plane shear.
What are the factors to be considered in the selection of materials for a machine element?
While selecting a material for a machine element, the following factors are to be considered
  • Required material properties
  • Manufacturing ease
  • Material availability
  • Cost
What are various theories of failure?
The failure theories are:
  • Maximum principal stress theory.
  • Maximum shear stress theory.
  • Maximum principal strain theory.
List out the factors involved in arriving at a factor of safety. 
The factors involved in arriving at a factor of safety are:
  • Material properties
  • Nature of loads
  • Presence of localized stresses
  • Mode of failures
Give some methods of reducing stress concentration. 
Some of the methods are:
  • Avoiding sharp corners.
  • Providing fillets.
  • Use of multiple holes instead of a single hole
  • Undercutting the shoulder parts.


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