Mechanical Behaviour Questions and Answers – Yield Strength

This set of Mechanical Behaviour Multiple Choice Questions & Answers (MCQs) focuses on “Yield Strength”.

1. What is yield strength of Kevlar?
a) 1650 MPa
b) 1737 MPa
c) 3620 MPa
d) 3757 MPa

Explanation: Kevlar has very high yield strength (3620 MPa). Its tensile strength is 3757 MPa. Tool Steel has yield strength 1650 MPa and maraging steel has 1737 MPa.

2. What is yield strength of annealed titanium?
a) 130 MPa
b) 138 MPa
c) 450 MPa
d) 830 MPa

Explanation: Titanium in the annealed state has lower yield strength (450 MPa). Its strength can be raised from alloying up to 830 MPa. Yield strengths of annealed iron and nickel are 130 MPa and 138 MPa respectively.

3. What is SI unit of yield strength?
a) N
b) N/m2
c) Nm2
d) g/cm2

Explanation: Yield strength is the stress at which material begins to deform. So yield strength has units of stress (N/m2). While g/cm2 is a CGS unit.

4. Strain offset of _____ is commonly used.
a) 0.002
b) 0.004
c) 0.006
d) 0.008

Explanation: For materials, which don’t show a definite yield point, an offset yield point is arbitrarily defined. It is generally set on 0.002. The plastic deformation corresponding to this point is very low.

5. _____ is the maximum stress that can be applied to the material without causing plastic deformation.
a) Tensile strength
b) Fatigue strength
c) Compressive strength
d) Yield strength

Explanation: Stress lower than yield strength causes elastic deformation in the material. Up to this point, Hooke’s Law is followed. Material attains its original shape after removal of stress.
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6. As the temperature of a material is lowered, the yield stress _____ and the fracture stress _____
a) Increases, increases
b) Increases, decreases
c) Decreases, increases
d) Decreases, decreases

Explanation: Lower temperature retards dislocation mobility thus increases yield stress. Fracture stress of material decreases because the mobility of atoms decreases. Hence the tendency of brittle fracture increases.

7. With decreasing grain size, yield strength of material ________
a) Increases
b) Decreases
c) Remains constant
d) First increases then decrease

Explanation: Yield strength of materials increases with a decrease in grain size. It may be explained using the Hall-Petch equation. But in nano range of grain size, yield strength of material decreases with a decrease in grain size.

8. Which material shows the yield point phenomenon?
a) Copper
b) Aluminium
c) Steel
d) Silver

Explanation: The main reason behind yield point phenomenon is the presence of interstitial atoms. These atoms segregate around dislocations so additional stress is required for plastic deformation. Copper, aluminium and silver have no interstitial atoms while steel contains C interstitial atoms.

9. Which factor deteriorates yield strength?
a) Cold working
b) Annealing
c) Work hardening
d) Grain refinement

Explanation: Annealing may be defined as a process of heating above recrystallization temperature. In it, the number of dislocations decreases thus yield strength decreases. Work hardening, cold working and grain refinement improve yield strength of the material.

10. Yield point phenomenon creates problems in deep drawing operations of sheet Steels.
a) True
b) False

Explanation: Yield point phenomenon causes problems in deep drawing. During deep drawing Steel becomes thinner in local areas. It generates depressions on sheet which are undesirable.

11. Which one of the following, gives the relation between work hardening and yield strength of material?
a) Δσy = Gbρ
b) Δσy = Gb√ρ
c) Δσy = G√ρb
d) Δσy = Gρ√b

Explanation: Equation Δσy = Gb√ρ shows the relation between dislocation density generated through work hardening and yield stress. Here, Δσy is changed in yield strength, b is a magnitude of burgers vector, G is the shear elastic modulus and ρ is a density of dislocations after work hardening. This equation shows that as dislocation density increases yield strength of material increases.

12. Work hardening causes a decrease in yield strength of the material.
a) True
b) False

Explanation: Loading of steel beyond its yield point and then unloading work hardening. It increases the value of yield strength. But it simultaneously decreases ductility of metal.

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