Posted by bjb in Tx from Ne on July 24, 2018 at 13:10:18 from (147.1.235.122):
In Reply to: Re: My experiment... posted by showcrop on July 24, 2018 at 10:43:57:
The load needs to open the crack tip (in this case the leading edge) which gets developed in three ways.
1. Tension loading radially due to the spinning blades (no moment, but still a fatigue load). The stress range in this case is 0 to full stress at a particular location at the full speed of the blades. The number of stops and starts is your number of cycles. Note in this setup, tension is not constant; it is zero at the free edge (tips) and maximum near the bolt.
2. Varying load due to the blades hitting air (wind resistance); hitting grass; or in extreme cases hitting say a small tree. This will cause a bending moment on the blades, which also causes tension in the leading edge due to this moment (note that when the mower is loaded the speed changes the tension as mentioned in part 1 so the stress range due to this mowing "moment" is amplified). In theory, ever blade of grass creates a small stress range, but you also could look at the speed of the mower due to changes in resistance and create stress range and number of cycles that way as well.
3. Bending and tension caused by vibration due to the out of balanced blades. Here the number of cycles is directly related to the frequency of the vibration.
A few other points: The way fatigue design works is the previously mentioned S-N curves (Stress verses cycle) curves, plots the maximum number of cycles at any given stress range. Using the previous equation for damage, you can see how much life as a % is used at any stress range and then add the damage together to estimate the service life.
For most steels there is a minimum stress range to cause fatigue, however, with such large "cracks" in the serration and the expected hardening of the leading edge of the face, makes this a highly likely area for rapid fatigue damage.
There is a case of "low, cycle fatigue" where at extremely high stress ranges, the life goes from thousands of cycles, to tens of cycles. This would be the case if the mower were to be spinning fully and come to a dead stop by hitting a large unseen object. A few times will rapidly increase the crack growth.
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