Mechanical Basic Interview Question and Answers Part 7

Basic of Mechanical Engineering Interview, viva ,Oral Question and Answers Part 7

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

174. Under what condition a convergent divergent nozzle required ?

When pressure ratio is greater than critical pressure ratio.

175. What is endurance limit and what is its value for steel ?

Endurance limit is the maximum level of fluctuating stress which can be tolerated indefinitely. In most steels this stress is approximately 50% of the ultimate tensile strength and it is defined as the stress which can be endured for ten million reversals of stress.

176. How the net work to drive a compressor and its volumetric efficiency behave with increase in clearance volume ?

Work remains unaltered and volumetric efficiency decreases.

177. What do you understand by sulphur print ?

Sulphides, when attached with dilute acid, evolve hydrogen sulphide gas which stains bromide paper and therefore can be readily detected in ordinary steels and cast irons. While sulphur is not always as harmful as is sometimes supposed, a sulphur print is a ready guide to the distribution of segregated impurities in general.

178. What is the different between brass and bronze ?

Brass is an alloy of copper with zinc; and bronze is alloy of copper with tin.

179. What is the effect of addition of zinc in copper? What is the use of 70/30 brass ?

By addition of zinc in copper, both tensile strength and elongation increases. The 70/30 brass has excellent deep drawing property and is used for making radiator fins.

180. What for admirality brass used ?

Admirality brass with 29% zinc and 1% tin has good corrosion resistance and is used for condenser and feed heater tubes. Aluminium is also added to brass to improve corrosion resistance.

181. What is the maximum use of magnesium ?

Magnesium is used to alloy with aluminium and as an additive for making SG (Spheroidal Graphite) iron.

182. What for zinc finds applications ?

Galvanizing consumes the largest proportion of zinc. Zinc is resistant to corrosion but is attacked by acids and alkalies. Zinc alloy.s are suited for making die casting since the melting point is reasonably low.

183. Which factors influence the type of fracture in failure of a material ?

Seven factors influencing type of failure are :

Type of material (inherent structure properties),
Manner of loading (Static versus dynamic),
Strain rate (static, dynamic, impact),
Stress distribution (discontinuity in material/shape),
temperature, and
surface treatment.

184. What is the name given to ratio of actual cycle efficiency and ideal cycle efficiency.

Efficiency ratio.

185. List two effects of manganese in plain carbon steels.

Manganese increases tensile strength and hardness. It decreases weldability.

186. Name the strongest and weakest type of atomic bonds.

Metallic bond is strongest and molecular bond also known as Vander Waals bond is weakest.

187. In which process internal energy remains constant ?

Isothermal process.

188. What is temper embrittlement in alloy steels and what are its effects ?

Embrittlement attack is usually intergranular in metals, i.e. cracks progress between the grains of the polycrystalline material. It imparts a tendency to fail under a static load after a given period of time in those alloy steels which are susceptible to embrittlement.

189. What are whiskers ?

Whiskers are very small crystals which are virtually free from imperfections and dislocations.

190. What is Bauschinger effect ?

According to Bauschinger, the limit of proportionality of material does not remain constant but varies according to the direction of stress under cyclic stresses.

191. What is the difference between heat capacity and specific heat of a material ?

The heat capacity of a material is the amount of heat transformed to raise unit mass of a material 1 degree in temperature.
The specific heat of a material is the ratio of the amount of heat transferred to raise unit mass of a material 1 degree in temperature to that required to raise unit mass of water 1 degree of temperature at some specified temperature.
For most engineering purposes, heat capacities may be assumed numerically equal to;specific heats.

192. Explain the rule to find specific heat of aqueous solutions.

For aqueous solutions of salts, the specific heat can be estimated by assuming the specific heat of the solution equal to that of the water alone. Thus, for a 15% by weight solution of sodium chloride in water, the specific heat would be approximately 0.85.

193. What do you understand by latent heat ? Give four examples of latent heats.

For pure substances, the heat effects accompanying changes in state at constant pressure (no temperature change being evident) are known as latent heats. Examples of latent heats are : heat of fusion, vaporisation, sublimation, and change in crystal form.

194. Define the terms free energy and free enthalpy. What is their significance and importance ?

Free energy (or Helmholtz function) is defined as/= u -Ts.
It is equal to the work during a constant-volume isothermal reversible nonflow process.
Free enthalpy (or Gibbs function) is defined as g = h – Ts (where u = internal energy, h = enthalpy, T = temperature, s = entropy)
Gibbs function is of particular importance in processes where chemical changes occur. For reversible isothermal steady-flow processes or for reversible constant-pressure isothermal nonflow processes, change in free energy is equal to net work.

195.Which parameter remains constant in isochoric process ?

Volume.

196. What is polytropic process ? Under what conditions it approaches isobaric, isothermal, and isometric process ? In which reversible process no work is done ?

A polytropic process is one that follows the equation pun = constant (index n may have values from – oc to + oo. This process approaches isobaric when n = 0, isothermal when n = 1, and isometric when n = <x>. No work is done in isometric process.