Design of Machine Elements: Basic Interview Question

Question No.01
Define Ductility.
Answer: It is the property of the material enabling it to be drawn into wire, with the application of
tensile force. It must be both strong and plastic. It is usually measured in terms of percentage
elongation and reduction in area, (e.g.) Ni, Al, and Cu.

Question No.02
Define fatigue.
Answer: When a material is subjected to repeated stress, it fails at stresses below the yield point
stress; such type of failure of the material is called fatigue.


Question No.03
Define: Factor of safety.
Answer: The ratio between maximum stresses to working stress is known as factor of safety.
Factor of safety = Maximum stress /Working stress

Question No.04
Define endurance limit.
Answer: Endurance limit is the maximum value of completely reversed stress that the standard
specimen can sustain an infinite number (106) of cycles without failure.

Question No.05
What is impact load?
Answer: If the time of load application is less than one third of the lowest natural period of vibration
of the part, it is called an impact load.

Question No.06
What are the various phases of design process?
Answer: The various phases of design process are:
1. Recognition of need.
2. Definition of problem
3. Synthesis
4. Analysis and optimization
5. Evaluation
6. Presentation

Question No.07
What is the use of Goodman & Soderberg diagrams?
Answer: They are used to solve the problems of variable stresses.

Question No.08
What are the factors affecting endurance strength.
Answer: Factors affecting endurance strength are
1. Load
2. Surface finish
3. Size
4. Temperature
5. Impact
6. Reliability

Question No.09
What are the types of variable stresses?
Answer: types of variable stresses are:
1. Completely reversed or cyclic stresses
2. Fluctuating stresses
3. Repeated stresses

Question No.10
Differentiate between repeated stress and reversed stress.
Answer: Repeated stress refers to a stress varying from zero to a maximum value of same nature.
Reversed stress of cyclic stress varies from one value of tension to the same value of compression.

Question No.11
What are the types of fracture?
Answer: The two types of fracture are
1. Ductile fracture
2. Brittle fracture

Question No.12
Distinguish between brittle fracture and ductile fracture.
Answer: In brittle fracture, crack growth is up to a small depth of the material.
In ductile fracture large amount of plastic deformation is present to a higher depth.

Question No.13
Define stress concentration and stress concentration factor.
Answer: 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

Question No.14
Explain size factor in endurance strength.
Answer: 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


Question No.15
Explain Griffith theory. (Or) State the condition for crack growth.
Answer: A crack can propagate if the energy release rate of crack is greater than crack resistance.

Question No.16
What are the modes of fracture?
Answer: The different the modes of fractures are:
1. Mode I (Opening mode) – Displacement is normal to crack surface.
2. Mode II (Sliding mode) – Displacement is in the plane of the plate.
3. Mode III (Tearing mode) – Out of plane shear.

Question No.17
What are the factors to be considered in the selection of materials for a machine element?
Answer: While selecting a material for a machine element, the following factors are to be
considered
1. Required material properties
2. Manufacturing ease
3. Material availability
4. Cost

Question No.18
What are various theories of failure?
Answer: The failure theories are:
1. Maximum principal stress theory.
2. Maximum shear stress theory.
3. Maximum principal strain theory.

Question No.19
List out the factors involved in arriving at factor of safety.
Answer: The factors involved in arriving at factor of safety are:
1. Material properties
2. Nature of loads
3. Presence of localized stresses
4. Mode of failures

Question No.20
Give some methods of reducing stress concentration.
Answer: Some of the methods are:
1. Avoiding sharp corners.
2. Providing fillets.
3. Use of multiple holes instead of single hole
4. Undercutting the shoulder parts.


Question No.21
Explain notch sensitivity. State the relation between stress concentration factor and notch
sensitivity.
Answer: Notch sensitivity (q) is the degree to which the theoretical effect of stress concentration is
actually reached.
The relation is, Kf = 1 + q (Kt-1)

Question No.22
What are the factors that affect notch sensitivity?
Answer: The factors effecting notch sensitivity are:
1. Material
2. Notch radius
3. Size of component
4. Type of loading
5. Grain Structure

Question No.23
What are the different types of loads that can act on machine components?
Answer: different loads on machine components are:
1. Steady load.
2. Variable load.
3. Shock load
4. Impact load.

Question No.24
Define machinability
Answer: It is the property of the material, which refers to a relative ease with which a material can
be cut. It is measured in a number of ways such as comparing the tool life for cutting different
material

Question No.25
What is an S-N Curve?
Answer: An S- N curve has fatigue stress on ‘Y’ axis and number of loading cycles in ‘X’ axis. It is used
to find the fatigue stress value corresponding to a given number of cycles.

Question No.26
What is curved beam?
Answer: In curved beam the neutral axis does not coincide with the centroidal axis.

Question No.27
Give some example for curved beam.
Answer: C frame, crane hook


Question No.28
What is principle stress and principle plane?
A plane which has no shear stress is called principle plane the corresponding stress is called principle
stress.

Question No.29
Write the bending equation.
Answer: The bending moment equation is, M/I = f/y= E/R,
Where,
M – Bending moment (M is in N-mm)
I - Moment of inertia about centroidal axis (I is in mm⁴)
f – Bending Stress (f is in N/mm²)
y - Distance from neutral axis (y is in mm)
E - Young’s modulus (E is in N/mm²)
R - Radius of curvature (R is in mm)

Question No.30
Write the torsion equation.
Answer: The torsional equation is, T/J = q/r = GӨ/L
Where,
T – Torsional moment (T is in N-mm)
J - Polar moment of inertia (J is in mm⁴)
q – Shear stress in the element (q is in N/mm²)
r - Distance of element from centre of shaft (r is in mm)
G- Modulus of Rigidity (G is in N/mm²)
Ө – Angle of twist (Ө is in radians)
L – Length of the shaft (L is in mm)