# Mechanical Behaviour Questions and Answers – Fatigue Strength

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

1. _____ of material can be defined as its ability to resist a fluctuating or repetitive stress.
a) Creep strength
b) Fatigue strength
c) Impact strength
d) Tensile strength

Explanation: Material’s resistance to repetitive stress is known as fatigue strength. Creep is a time dependent deformation. Impact strength is the resistance of material towards fracture under impact loading.

2. What decreases fatigue strength?
a) Shot peening
b) Case hardening
c) Inclusions
d) Grain refinement

Explanation: Surface smoothness enhances fatigue strength of a material. Inclusions reduce surface smoothness. Shot peening, case hardening and grain refinement improve tensile strength and fatigue strength.

3. What is the approximate ratio of fatigue strength to the tensile strength?
a) 1:2
b) 1:3
c) 1:4
d) 2:1

Explanation: Fatigue strength is roughly half of the tensile strength. Thus the ratio is 1:2. So the factors which improve tensile strength also improve fatigue strength.

4. Which material doesn’t show fatigue limit?
a) Titanium alloys
b) Aluminium
c) Stainless steel
d) High Strength Steel

Explanation: Steels and titanium alloys show fatigue limit. It means that there is a stress level below which fatigue failure doesn’t occur. Aluminium doesn’t show fatigue limit.

5. What is the maximum endurance limit of aluminium?
a) 97 MPa
b) 130 MPa
c) 170 MPa
d) 290 MPa

Explanation: Maximum endurance limit of aluminium is 130 MPa. Steels have high endurance limit of 290 MPa. Irons and coppers have limits 170 and 97 MPa respectively.
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6. Fatigue fracture consists _____
a) Cleavage
b) Strains
c) Striations
d) Cracks

Explanation: Fatigue fracture is characterized as striations. Striations are found on crack surfaces. These are caused by an action of tensile and cyclic stress.

7. Which steel would you choose for Cyclic stress application?
a) Martensitic stainless steel
b) Ferritic stainless steel
c) Austenitic stainless steel
d) Duplex steel

Explanation: Ferritic and martensitic Steels are more notch sensitive. Austenitic Steel shows less notch sensitivity. That’s why it is more fatigue resistant.

8. _____ decrease fatigue life of a structure.
a) Round holes
b) Smooth transitions
c) Fillets
d) Square holes

Explanation: The shape of a structure greatly affects the fatigue life. Square holes can initiate cracks. While round holes, fillets and smooth transitions can improve fatigue strength.

9. When fatigue occurs above __ cycles, it is called high cycle fatigue.
a) 10
b) 103
c) 106
d) 108

Explanation: When fatigue takes place above 103 cycles, it is referred as a high cycle fatigue. In this case, the material is subjected to lower loads. Elastic deformation takes place.

10. What is term referred to fatigue failure resulting from the strains caused by expansion and contraction in thermal cycling?
a) Low cycle fatigue
b) High cycle fatigue
c) Corrosion fatigue
d) Thermal fatigue

Explanation: Thermal fatigue occurs due to the combination of thermal expansion and low thermal conductivity. The strain raised is proportional to temperature difference. Austenitic stainless steels are sensitive to thermal fatigue.

11. Cast materials have lesser fatigue life than wrought materials.
a) True
b) False

Explanation: Cast materials show poor ductility. So these show less resistance to crack propagation. Hence these show poor fatigue strength.

12. High fatigue strength materials are the better choice than corrosion resistant materials.
a) True
b) False

Explanation: Corrosion generated crakes act as notches. These lower the fatigue life of the material. That’s why corrosion resistant materials are preferred over materials which only have high fatigue strength.

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