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Bending of a Rotating Shaft

December 21, 2013

Bending of a Rotating Shaft

 

Ah fuck i just checked and that was one of the books i lent out. well this image does a pretty good job of showing what i mean. so the way to look at that is at a certain frequency you can plot the point of failure. So the blue line charted on that picture is for a single frequency. So if you know the stress thats being cycled at that frequency you can use this chart to find the cycles until failure. The dotted line you see on the chart is the asymptote, meaning that below a certain stress the object could cycle and infinite number of times (in theory). Now as you change the frequency that line moves, it gets stretched in different directions for a lack of better words. this means the asymptote, and the rate at which it approached it changes. i cant really find a good image on this since i dont have my book, but just image a bunch of lines on the chart, each one for a different frequency. Now this is the part where my memory gets hazy, but the failure rate to frequency connection is actually counter intuitive at first. more people would expect higher frequencies to fatigue the material faster because higher frequencies have more energy in sounds, and light. but think of it this way, the average stress of a cycle for a single period is the exact same. whether its 1Hz or 1,000,000Hz the average value wont change because its the same shape, just longer/shorter. This means lower frequencies will provide the same amount of “damage” per unit time, but a lower frequency lasts longer per cycle. a 1Hz wave would put stress on the material for 1 second per cycle, while a 2Hz wave would put stress on the material for 0.5 seconds per cycle. so in reality lower frequencies actually affect the material more. i hope this helped, especially since i couldnt provide good pictures for it.

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