Questions and answers from 11 to 20 on Material Science and Engineering.
11. Which element is added in steel to increase resistance to Corrosion ?
12. Whether individual components in composite materials retain their characteristics or not
13. An elastomer is a polymer when its percentage elongation rate is _____ ?
Greater than 100%.
14. If percentage elongation of material is more than 200%, it is called as .... ?
15. Why is it that the maximum value which the residual stress can reach is the elastic limit of the material ?
A stress more than the elastic limit, with no external force to oppose it, we 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 10mm?
The perfection of material conditions is possible at lower sizes, and as size increases, it is not possible to attain a 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 carburizing and nitriding surface treatments increase fatigue strength, excessive treatment may decrease the fatigue strength. Why ?
Normal carburizing/nitriding treatments increases volume due to phase transformation at the surface and introduce residual compressive surface stress and thus increases fatigue strength. By excessive treatment, the high compressive stresses ae introduced but these are balanced by high internal elastic stresses of equal value, and the surface fatigue cracks may develop in the regions of the high tensile stress and lead o early fatigue failure.
19. List at least two factors that promote the transition from ductile to brittle fracture.
The manner of loading and the rate of loading promote the 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 the load is applied at a high velocity then failure may be brittle.
20. Which theories of failure are used for
(a) ductile materials
(b) brittle materials ?
For ductile materials, theories of failure used are maximum shear stress theory and maximum energy of distortion theory;
For brittle materials, the theory of maximum principal stress and maximum strain are used.