If the same shaft is rotating at some rpm under same load then it is experienced that the shaft is failed after working for a limited number of cycles.
Fatigue failure is a progressive failure leading from the point of stress concentration. Now question rises that what stress a component can resist for infinite number of cycle.
For that we perform an experiment on fatigue testing machine. The fatigue testing machine and standard test specimen are shown in Figure 7. A specimen of known yield and tensile strength is taken conforming to the relevant standards. To start the experiment we load the specimen in such a way that the stress induced is just less than the ultimate strength in tension. The shaft is rotated till failure and the number of stress cycles is noted.
Repeat the test by lowering the value of load. We see that the number of cycles for failure is increased. This procedure is continued and results are plotted on either log-log or semi-log graph paper. The curve so obtained is known as S-N curve as shown in Figure 8.
In the case of ferrous metals and its alloy, the S-N curve becomes horizontal after 10 6 7 will not be encountered irrespective of the number of stress reversals.
The strength corresponding to this knee is called endurance limit of fatigue limit. The ordinate of S-N curve shows the fatigue strength and strength at a particular number of cycle N.
The endurance limit obtained above is based on the standard experiment and knee was obtained which clearly identify the endurance limit or fatigue limit. The knee does not exist in S-N curves for all material of engineering importance. The materials which exhibit the knee are low strength carbon and alloy steels, some stainless steels, irons, titanium alloys and molybdenum alloys.
There are some other materials such as aluminum, copper, nickel alloy, high strength carbon and alloy steel and some stainless steels do not exhibit knee. Sometime we even do not have the experimental fatigue data, in such case following relations are used for calculation purposes:. Your email address will not be published.
Save my name, email, and website in this browser for the next time I comment. Skip to content. Figure 1: General case of fluctuating stress. Eq 1a.
Eq 1b. Figure 2: Various terms related to fluctuating stress. Eq 2a. Eq 2b. Eq 2c. Therefore tensile stresses occupy the first half and Compressive stresses occupy the second half of the stress distribution. Whereas, for fluctuating loading stresses , there comes a definite non zero mean stress value. Therefore, the stresses here can be purely tensile, purely compressive or Mix of both tensile and compressive.
For designing a component subjected to completely reversed stresses , there are further 2 sub conditions under which study is done. It mostly finds applications in steels. Depending upon the Value of the life N given to us, draw a vertical line passing through log 10 N on the abscissa, which further intersects AB at point F.
Draw a line FE parallel to abscissa. Then, we discuss how to estimate a fully adjusted endurance limit. Finally, a case study covering the root cause analysis of the fatigue failure of the Aloha Airlines flight will emphasize the dangers of fatigue failure.
Machine Design Part I. Enroll for Free. This Course Video Transcript. From the lesson Fatigue Failure - Part I In week 4, we will introduce critical fatigue principles, starting with fully revisable stresses and the SN Curve.
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