# Mechanical Behaviour Questions and Answers – Stress Cycles

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

1. Failure of metals under fluctuating stress is known as __________ failure.
a) Creep
b) Fatigue
c) Dynamic
d) Tensile

Explanation: Failure of metals under fluctuating stress is known as fatigue failure. The stress varies with time. Examples are a shaft, bridge etc.

2. ________ % of engineering failures are fatigue failures.
a) 10
b) 30
c) 50
d) 90

Explanation: Fatigue is a severe problem in engineering. Around 90% of failures are fatigue failures. So its study is necessary.

3. Fatigue failures occur on stresses _________
a) Above tensile strength
b) Above fracture strength
c) Below elastic limit
d) Below tensile strength

Explanation: Fatigue failure occurs in dynamic loading condition. These stresses may be below the elastic limit.

4. What is a type of fatigue failure?
a) Rupture
b) Brittle fracture
c) Ductile fracture
d) De-cohesive

Explanation: Fatigue failures are catastrophic in nature. These are very common. The final fracture occurs in a brittle manner.

5. What initiates the crack in fatigue?
a) Cyclic stress
b) Tensile stress
c) Plastic strain
d) Compressive stress

Explanation: Cyclic stress initiates the crack. Tensile stress causes the crack to grow. It grows through localized plastic strain.
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6. What is the example of repeated stress cycle?
a) Axles
b) Rotating shafts
d) Cantilever

Explanation: Axles, rotating shafts and cantilever are examples of alternating stress. Machine carrying load is an example of repeated stress cycle. In this the mean stress is positive.

7. The zone ________ is the area on which the final fracture occurs.

a) O
b) R
c) S
d) I

Explanation: The zone S is the area on which final fracture has occurred. It occurs when the cross section of the component is reduced.

8. Which region is smoother and more lustrous?

a) R
b) S
c) O
d) G

Explanation: Region R is smoother and more lustrous. It is with a comparison to region S. Region S is much rougher in texture.

9. How many zones of failure are shown in car rear axle surface fracture?
a) 1
b) 2
c) 3
d) 4

Explanation: The fractured surface of a motor car rear axle shows 3 zones of failure. Zone I covers half section and beach markings.

10. Which zone covers beach markings?
a) I zone
b) II zone
c) III zone
d) IV zone

Explanation: Zone I covers half section and beach markings. In zone II, crack propagation is rapid. Zone III covers a fractured surface.

11. Fatigue cracks are easily detectable.
a) True
b) False

Explanation: Fatigue cracks are usually fine. These are difficult to detect. It leads to catastrophic failure.

12. Fatigue failure only occurs if cyclic stress, tensile stress and plastic strain, all are present.
a) True
b) False

Explanation: Fatigue fracture is the simultaneous action of cyclic stress, tensile stress and plastic strain. If anyone of these three is absent, fatigue doesn’t take place.

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