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Interview Questions Mechanical Engineering Part-1 :: Saudi Aramco standard

Mechanical Engineering Interview Questions: Part-1

1. 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.

2. 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.

3. What is view factor ?
View factor is dependent upon geometry of the two surfaces exchanging radiation.

4. 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.

5. 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.

6. 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 fom their regular ideal positions.

7. What are the principal constituents of brass?
Principal constituents of brass are copper and zinc.

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

9. 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.

Also Read : Mechanical Engineering All E-Books

10. What is the percentage of carbon in cast iron ?
2.5%.

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

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

13. An elastomer is a polymer when its percentage elongation rate is ?
Greater than 100%.

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

15. 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.

16. 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.

17. 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.

18. 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 internal 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.

Also Read : Mechanical Engineering All E-Books

19. 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.

20. 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.

21. What does thermal diffusivity of metals signify.
Thermal diffusivity is associated with the speed of propagation of heat into solids during changes in temperature with time.

22. For conduction of heat, the instantaneous rate of heat flow is product of three factors. What are these ?

  1. Area of the section of the heat flow path, perpendicular to the direction of heat flow.
  2. temperature gradient, i.e. change of temperature w.r.t. length of path.
  3. Thermal conductivity of material.

23. How convective heat transfer is effected and on what factors it depends ?
Convective heat transfer is effected between a solid and fluid by a combination of molecular conduction within the fluid in combination with energy transport resulting from the motion of fluid particles. It depends on boundary layer configuration, fluid properties and temperature difference.

24. Which is the common element between brass and bronze ?
Copper.

25. What does following alloy designation indicate FG 250 ?
Grey cast iron with tensile strength of 250 MPa.

26. How is ceramic defined ?
It is a solid formed by combination of metallic and non-metallic elements.

27. Give one example of metal classified as per structure as BCC, FCC, HCP and CCP.

  • BCC (body centred cubic) structure—Molybdenum
  • FCC (face centred cubic) structure—Aluminium
  • HCP (hexagonal closed packed) structure—Zinc
  • CCP (cubic dosed packed) structure-Copper.

28. What is the name of solid solution of carbon in alpha iron and delta iron ?
Ferrite and austenite respectively.

29. Explain the difference between pearlite and cementile ?
Pearlite is eutectoid mixture of ferrite and cementile. Cementite is chemical compound of iron and carbon.

Also Read : Mechanical Engineering All E-Books

30. Give one example each of the following proportion of materials dimensional, physical, technological and mechanical.
Roughness, enthalpy, toughness, and hardness respectively.

31. For which parts the Wahl factor and Lewis form factor used ?
For springs and gears respectively.

32. How oxygen can be removed from steel during melting? What are fully killed steels ?
Oxygen can be removed by adding elements such as manganese, silicon or aluminium which, because of their high affinity for oxygen, react with it to form non-metallic oxides which rise into the slag. Steels which have had most of their dissolved oxygen removed are called “fully killed steels”.

33. Hydrogen cannot be removed easily from molten steel. What harm hydrogen has on property of steel ?
Execessive hydrogen results in the formation of small fissures often described as hairline cracks or flakes in the steel. Large forgings in alloy steel are particularly sensitive to this phenomenon.

34. What is allotrope ? In what forms of cubic pattern, iron exists ?
Some elements exist in more than one crystalline form. Each form is known as “allotrope”. Iron exists in two forms of cubic pattern, namely body centered cubic (bcc) and face-centered cubic (fee).

35. What is the difference between alpha iron, delta iron and gamma iron ?
The bcc form of iron exists between room temperature and 910°C, and between 1400°C and the melting point at 1539°C. The lower temperature form is known as “alpha”-iron and the higher temperature form as “delta”-iron. The face-centered cubic form existing between 910°C and 1400°C is referred to as “gamma-iron”.

36. Metals, in general are of low strength and do not possess required physio-chemical and technological properties for a definite purpose. Alloys are therefore more than metals alone. Discuss the arrangement of atoms and structures of alloys.
Alloys are produced by melting or sintering two ore more metals, or metals and a non-metal, together. Alloys possess typical properties inherent in the metallic state. The chemical elements that make up an alloy are called its components. An alloy can consist of two or more components. The phase and structures of alloys describe the constitution, transformations and properties of metals and alloys. A combination of phases in a state of equilibrium is called a system. A phase is a homogeneous portion of a system having the same composition and the same state of aggregation throughout its volume, and separated from the other portions of the system by interfaces. For instance, a homogeneous pure metal or alloy is a single-phase system. A state in which a liquid alloy (or metal) coexists with its crystals is a two-phase system. Structure refers to the shape, size or the mutual arrangement of the corresponding phases in metals or alloys. The structural components of an alloy are its individual portions, each having a single structure with its characteristic features.

37. What is the difference between isotropic material and homogeneous material ?
In homogeneous material the composition is same throughout and in isotropic material the elastic constants are same in all directions.

38. Explain the difference between the points of inflexion and contraflexure.
At points of inflexion in a loaded beam the bending moment is zero and at points of contraflexure in loaded beam the bending moment changes sign from increasing to decreasing.

39. What is the difference between proof resilience and modulus of resilience ?
Proof resilience is the maximum strain energy that can be stored in a material without permanent deformation. Modulus of resilience is the maximum strain energy stored in a material per unit volume.

Also Read : Mechanical Engineering All E-Books

40. What is the difference between column and strut ?
Both column and strut carry compressive load. Column is always vertical but strut as member of structure could carry axial compressive load in any direction.

41. Explain the difference between ferrite, austenite and graphite ?
Ferrite is the solid solution of carbon and other constituents in alpha-iron. It is soft, ductile and relatively weak.
Austenite is the solid solution of carbon and other constituents in gamma-iron. It exists in ordinary steels at elevated temperatures, but it is also found at ordinary temperatures in some stainless steels.
Graphite has a hexagonal layer lattice. ‘

42. Explain the terms solid solution, eutectic, eutectoid and peritectic.

  • Solid Solution. When a homogeneous mixture of two (or more) atomic forms exists in solid state, it is known as solid solution.
  • Eutectic. A mixture of two (or more) phases which solidify simultaneously from the liquid alloy is called an eutectic. Alloys in which the components solidify simultaneously at a constant temperature the lowest for the given system, are called eutectic alloys.
  • Eutectoid. Eutectoid alloys are the alloys for which two solid phases which are completely soluble become completely insoluble on cooling before a certain temperature called eutectoid temperature.
  • Peritectic. A peritectic transformation involves a reaction between a solid and liquid that form a different and new solid phase. This three phase transformation occurs at a point called peritectic point.

43. What do you understand by critical points in iron, iron-carbide diagram ?
The temperatures at which the phase changes occur are called critical points (or temperatures).

45. Why PERT is preferred over CPM for evaluation of project ?
PERT is based on the approach of multiple time estimates for each activity.

46. What is the percentage of chromium in 18 : 4 : 1 IISS ?
4%.

47. What is stellite ?
It is a non-ferrous cast alloy containing cobalt, chromium and tungsten.

48. Which rays are produced by cobalt-60 in industrial radiography ?
Gamma rays.

49. What are killed steels and what for these are used ?
Killed steels are deoxidised in the ladle with silicon and aluminium. On solidification no gas evolution occurs in these steels because they are free from oxygen.

50. What is critical temperature in metals ?
It is the temperature at which the phase change occurs in metals.

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