Chapter 5 REVIEW solution of Design of Machine Elements By V B Bhandari 3Ed

REVIEW QUESTION

1.     What is stress concentration?

Localization of high stress due to abrupt changes.

2.     How will you account for stress concentration in design?

·         Photo elasticity.

·         Grid methods.

·         Electrical-strain gauge method.

3.     What is stress concentration factor?

The ratio of the highest stress to reference stress.

4.     What are the causes of stress concentration?

·         Variation in the material properties.

·         Load application.

·         Abrupt changes.

·         Discontinuities.

·         Scratches.

5.     What are the methods of reducing stress concentration?

·         Additional notches & holes.

·         Change thickness.

·         Fillet Radius

Etc.

6.     What is fluctuating stress? Draw stress stress-time curve?

When the component is subjected to fatigue or fluctuating load, the stresses induced are called Fluctuating Stresses.

 

7.     What is repeated stress? Draw stress stress-time curve?

Stress varying from zero to a maximum value of same nature.

8.     What is reversed stress? Draw stress stress-time curve?

Cyclic stress varies from one value of tension to the same value of compression.

9.     What is fatigue failure?

A time delayed fracture under cyclic loading.

10.    What are the machine components that fail by fatigue?

Transmission Shafts, connecting rods, rears, vehicle suspension spring & ball bearings. Etc

11. What is difference b/t failure due to static load & fatigue failure?

In static loading the component fails due its relative strength in compressive, tension and shear and it also depends on the type loading.

while

Fatigue failure is dependent fracture under cyclic loading.

12. What is endurance limit?

A threshold before changing state.

Or

The highest stress that a material can withstand for an infinite number of cycles without breaking.

13. What is fatigue life?

The number of stress cycles that a specimen sustains before failure.

14. What is S-N curve?

It is a plot of the magnitude of an alternating stress versus the number of cycles.

15. What is low cycle fatigue?

A fatigue failure when the number stress cycles are less than 1000.

16. Give practical example of low cycle fatigue failure?

·         Failure of studs on truck wheels.

·         Missiles

Etc.

17. What is high cycle fatigue?

A fatigue failure when the number stress cycles are more than 1000.

18. Give practical example of high cycle fatigue failure?

·         Springs

·         Ball bearings

·         Gears

Etc.

19. What is fatigue stress concentration factor?

The ratio of fatigue strength of the smooth member to that of the notched member.

20. What is notch sensitivity?

The developing notches Under fluctuating loading due to the stress concentration.

21. What is notch sensitivity factor?

The measure of how sensitive a material is to notches.

22. What are the factors that affect endurance limit of the machine part?

·         Surface finish factor.

·         Size factor.

·         Reliability factor.

·         Modifying factor to account for stress concentration.

23. What is surface finish factor?

The endurance limit of the material under fluctuation loading depends upon the surface conditions. 

14.            What is S-N curve?

It is a plot of the magnitude of an alternating stress versus the number of cycles.

15.            What is low cycle fatigue?

A fatigue failure when the number stress cycles are less than 1000.

16.            Give practical example of low cycle fatigue failure?

·         Failure of studs on truck wheels.

·         Missiles

Etc.

17.            What is high cycle fatigue?

A fatigue failure when the number stress cycles are more than 1000.

18.            Give practical example of high cycle fatigue failure?

·         Springs

·         Ball bearings

·         Gears

Etc.

19.            What is fatigue stress concentration factor?

The ratio of fatigue strength of the smooth member to that of the notched member.

20.            What is notch sensitivity?

The developing notches Under fluctuating loading due to the stress concentration.

21.            What is notch sensitivity factor?

The measure of how sensitive a material is to notches.

22.            What are the factors that affect endurance limit of the machine part?

·         Surface finish factor.

·         Size factor.

·         Reliability factor.

·         Modifying factor to account for stress concentration.

23.            What is surface finish factor?

The endurance limit of the material under fluctuation loading depends upon the surface conditions.

24.            What is size factor?

To consider the effect of the size on endurance strength. A large size object will have more defects compared to a small one ,So endurance strength is reduced.

25. What is reliability factor?

It is the probability that a product will continue to work normally over a specified interval of time, under specified conditions.

26. What is modifying factor to account for stress concentration?

It is equal to the inverse of fatigue stress concentration factor.

27. What is miner’s equation?

It states that there are different stress levels and the number of cycles to failure under Fatigue.

ɑ₁ + ɑ₂ + ɑ₃…..+ ɑ_n  = 1

28. Where do you use miner’s equation?

To determine the life of the component subjected to different stress levels.

29. What is the Goodman line?

A straight line joining S_e on the ordinate to the S_ut on the abscissa.

30. What is the Soderberg line?

A straight line joining S_e on the ordinate to the S_yt on the abscissa.

31. Explain the modified Goodman diagram for bending stress?

The Goodman line is modified by combining fatigue failure with failure by yielding.

The BFC would be safely within the Goodman line but would yield on the first cycle. The portion BF OF the line is vulnerable & need correction. This is the reason of modifying Goodman line.

32. Explain the modified Goodman diagram for torsional shear stress?

Up to a certain point, the torsional mean has no effect on the torsional endurance limit. The area OABC represents the region of safety. It is not necessary of construct a fatigue diagram for the fluctuating torsional shear stress because AB is parallel to X axis.

33. What is Gerber curve?

A parabolic curve joining S_e on the ordinate to the S_ut on the abscissa.

34. What is difference between Gerber curve , Goodman & soderberg lines?

Gerber curve has more chances of failure. While Goodman line lesser chances of failure. But Soderberg has the least chances.