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Re: My experiment...


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Posted by bjb in Tx from Ne on July 24, 2018 at 05:47:05 from (147.1.235.122):

In Reply to: Re: My experiment... posted by Eldon (WA) on July 23, 2018 at 16:02:03:

An 1/8" notch is a large initial crack when it comes to fatigue. With a presume hardened blade that could spell disaster. The things that affect fatigue life:
1. Size of the initial crack (in this case 1/8") The larger the initial crack the less "Life" it has. At some point, the crack length will go from a ductile (slow growth) to brittle fatigue (think glass)
2. The type of material, usually the higher the strength (or in this case hardened) the lower the "toughness" and the quicker a crack will grow (there is less plasticity at the crack tip so it grows faster)
3. The stress range (note this is not absolute stress). The fatigue will be worse if the nominal stress is only 30% of failure, but the stress range is 20% of yield, than if it is 70% of yield with only 10% stress range. (Sr)
4. The number of cycles (Nc) (This can be caused by vibration (high number of cycles) or by impact (low number or cycles)
5. The Stress Concentration Factor (SCF) (the peak stress near the crack divided by the nominal stress). This probably is somewhere around 3 to 5 for your blades.
6. The material exponent on the Stress Range. For steel it is usually in the range of 3-5.
7. The total number of cycles expected at a given stress range (Tc) which is depended on the initial crack size

The total life (as a percentage) at a given stress range = (SCF * Sr) ^ 3 * Nc / Tc (If you set this equation equal to 1 then you can back out the EXPECTED number of cycles at a given stress range until failure.)

Notice the exponential effect of the stress range (and SCF) when it comes to fatigue.

My recommendation would be to change the blades so they don't fail catastrophically.

bjb
Texas Structural Engineer

PS In some specialty high strength metals, the critical crack (before it becomes brittle and fails) is only thousandths of an inch.


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