Mechanical Basic Interview Question and Answers Part 4

Basic of Mechanical Engineering Interview, viva ,Oral Question and Answers Part 4

Mechanical Engineering Interview Technical questions and answers with explanation for Job interview, competitive examinations and entrance tests.

91. Which reactor produces more fissionable material than it consumes ?

Breeder reactor.

92. Which reactor uses natural uranium as fuel ?

Gas cooled reacator.

93. Which reactor uses heavy water as moderator ?

CANDU.

94. Which reactor requires no moderator ?

Breeder reactor.

95. Which reactor uses primary coolant as fluoride salts of lithium, beryllium, thorium and uranium ?

Molten salt breeder reactor.

96. Why an increase in area is required to produce an increase of velocity in case of supersonic flow ?

Increase in area for increase in velocity for supersonic flow is required because the density decreases faster than velocity increases at supersonic speeds and to maintain continuity of mass, area must increase.

97. Under what circumstances would there be an increase in pressure in a divergent nozzle ?

For subsonic flow at inlet section of a diffuser a lower velocity and higher pressure will exist at the exit section. For supersonic isentropic flow at the inlet section a higher velocity and lower pressure will exist at the exit but if a shock wave occurs in the diffuser then a higher pressure will exist at the exit.

98. Why water can’t be used as refrigerant for small refrigerating equipment ?

The refrigerant should be such that vapour volume is low so that pumping work will be low. Water vapour volume is around 4000 times compared to R- for a given mass.

99. Which parameter remains constant in a throttling process ?

Enthalpy.

100. What is the difference between isentropic process and throttling process ?

In isentropic process, heat transfer takes place and in throttling process, enthalpy before and after the process is same.

101. What is the difference between isotropic and anisotropic materials ?

If a material exhibits same mechanical properties regardless of loading direction, it is isotropic, e.g., homogeneous cast materials. Materials lacking this property are anisotropic.

102. What are orthotropic materials ?

It is a special class of anisotropic materials which can be described by giving their properties in three perpendicular directions e.g. wood; composites.

103. What is view factor ?

View factor is dependent upon geometry of the two surfaces exchanging radiation.

104. What properties need to be considered for applications calling for following requirements :

(i) rigidity
(ii) strength for no plastic deformation under static load
(iii) strength to withstand overload without fracture.
(iv) wear resistance
(v) reliability and safety.

(i) Rigidity—Elastic modulus and yield strength
(ii) Strength (for no plastic deformation under static loading)—yield point
(iii) Strength (overload)—Toughness and impact resistance
(iv) Wear resistance—Hardness
(v) Reliability and safety—Endurance limit and yield point.

105. Explain the effects of alloying chromium and nickel in stainless steel.

Addition of nickel and chromium increases the tensile strength and increase in resistance to corrosion takes place.

106. Mention two types of dislocations.

Dislocation refers to a break in the continuity of the lattice. In edge dislocation, one plane of atoms gets squeezed out. In screw dislocation the lattice atoms move form their regular ideal positions.

107. What are the principal constituents of brass?

Principal constituents of brass are copper and zinc.

108. What is Curie point ?

Curie point is the temperature at which ferromagnetic materials can no longer be magnetised by outside forces.

109. Specific strength of materials is very high when they are in fibre size but lower when they are in bar form Why ?

Crystal structure has ordered, repeating arrangement of atoms. Fibres are liable to maintain this and thus have high specific strength. As size increases, the condition of ordered and repeating arrangements can’t be guaranteed because of several
types of defects and dislocations and thus the specific strength gets lower.

110. What is the percentage of carbon in cast iron ?

2.5%.

111. Which element is added in steel to increase resistance to corrosion ?

Chromium.

112. Whether individual components in composite materials retain their characteristics or not?

yes.

113. An elastomer is a polymer when its percentage elongation rate is ?

Greater than 100%.

114. If percentage elongation of a material is more than 200%, it is classed as ?

Rubber.

115. Why is it that the maximum value which the residual stress can reach is the elastic limit of the material ?

A stress in excess of elastic limit, with no external force to oppose it, will relieve itself by plastic deformation until it reaches the value of the yield stress.

116. Why fatigue strength decreases as size of a part increases beyond around 10 mm?

Perfection of material conditions is possible at lower sizes and as size increases, it is not possible to attain uniform structure
of the material.

117. Distinguish between creep and fatigue.

Creep is low and progressive deformation of a material with time under a constant stress at high temperature applications. Fatigue is the reduced tendency of material to offer resistance to applied stress under repeated or fluctuating loading condition.

118. While normal carburising and nitriding surface treatments increase fatigue strength, excessive treatment may decrease the fatigue strength. Why ?

Normal carburising/nitriding treatments increase volume due to phase transformation at Surface and introduce residual compressive surface stress and thus increase the fatigue strength. By excessive treatment the high compressive stresses are introduced but these are balanced by high in¬ternal tensile stresses of equal value and the subsurface fatigue cracks may develop in the regions of high tensile stress and lead to early fatigue failure.

119. List at least two factors that promote transition from ductile to brittle fracture.

Manner of loading, and the rate of loading promote transition from ductile to brittle fracture. A machine member may have ductile failure under static loading but may fail in brittle fashion when the load is fluctuating. Similarly a material may evidence ductile failure under tensile loading at ordinary testing speed but if load is applied at a high velocity then failure may be brittle.

120. Which theories of failure are used for (a) ductile materials, and (B) brittle materials ?

For ductile materials, theories of failure used are maximum shear stress theory, and maximum energy of distortion theory; while for brittle materials, theory of maximum principal stress, and maximum strain are used.