Subject: Machine Design -I

Topic: Basic properties of Machine design and theories of failure

Difficulty: Medium

Fatigue cycles:

There are two regions of this S-N curve namely

a) Low-cycle fatigue and

b) High-cycle fatigue.

The difference between these two fatigue failures is as follows:

1. Any fatigue failure when the number of stress cycles are less than 1000, is called low-cycle fatigue. Any fatigue failure when the number of stress cycles are more than 1000, is called high-cycle fatigue.
2. Failure of studs on truck wheels, failure of setscrews for locating gears on shafts or failures of short-lived devices such as missiles are the examples of low-cycle fatigue.
3. The failure of machine components such as springs, ball bearings or gears that are subjected to fluctuating stresses, are the examples of high-cycle fatigue.

Factor affecting Endurance limit: (i) Surface finish Factor:

The surface of the rotating beam specimen is polished to mirror finish. The final polishing is carried out in the axial direction to smooth out any circumferential scratches. This makes the specimen almost free from surface scratches and imperfections.

(ii) Size Factor:

The rotating beam specimen is small with 7.5 mm diameter. The larger the machine part, the greater the probability that a flaw exists somewhere in the component. The chances of fatigue failure originating at any one of these flaws are more. The endurance limit, therefore, reduces with increasing the size of the component

(iii) Reliability Factor:

The laboratory values of endurance limit are usually mean values. There is of tests are conducted even using the same material and same conditions. The standard deviation of endurance limit tests is 8% of the mean value. The reliability factor Kc depends upon the reliability that is used in the design of the component. The greater the likelihood that a part will survive, the more is the reliability and lower is the reliability factor.

(iv) Modifying Factor to Account for Stress Concentration:

The endurance limit is reduced due to stress concentration. The stress concentration factor used for cyclic loading is less than the theoretical stress concentration factor due to the notch sensitivity of the material. To apply the effect of stress concentration, the designer can either reduce the endurance limit by (Kd) or increase the stress amplitude by (Kf). We will use the first approach. The modifying factor Kd to account for the effect of stress concentration is defi ned as,