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Old 08-06-2018, 06:49 PM
Farmersamm Farmersamm is offline
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Default Yield vs. Fatigue

I don't imagine there's any engineers on here, but anyways...……….

Lever attached to an axle. All of the flex is mid span on the lever. The moment connections are highly reinforced. But ya gotta be an absolute moron to figure there ain't a ton of stress on the connection to the axle.

Combined flex is probably additive. Flex on the lever, and torque flex on the axle.

Total flex is probably about 3/16". Annoying, but maybe liveable. That's the killer. It's hard to see inside the steel.

You can make everything rigid, but if it ain't necessary...…….why bother.

You can hang enough steel on anything, but in the end...is it warranted. I can make anything into a boat anchor, but that ain't fabrication.
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Old 08-06-2018, 06:54 PM
Farmersamm Farmersamm is offline
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It's what separates the rednecks from the builders I guess. It's a fine line. When the bumper hitch weighs more than the pickup truck, ya gotta do a reality check

Anyways, this was a good part of my day. Standin' lookin' thinkin'
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Old 08-06-2018, 06:58 PM
Farmersamm Farmersamm is offline
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How many cycles, how many cycles, how many cycles...…………...The question for the ages. I can't lay my hands on any puter models to figger it out, so it's all in the noodle.

Just musing, no reply necessary.
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Old 08-06-2018, 07:13 PM
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Yield and fatigue are different concepts. So I was told many moons ago.
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Old 08-06-2018, 07:26 PM
Farmersamm Farmersamm is offline
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Yield and fatigue are different concepts. So I was told many moons ago.
How so? When do you approach the point where too many cycles are too much. I know civil engineers take this into account Generally deeper beams are the answer......which has to take yield into account
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Old 08-06-2018, 07:28 PM
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Are we talking about the amplitude of flex? I'd like to know the cutoff point in terms of overall movement.
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Old 08-06-2018, 07:32 PM
Farmersamm Farmersamm is offline
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More correctly...…..frequency of movement, and limits of movement.

The structural guys generally spec (so I've been told) deeper beams to keep flex to a comfortable level for the occupants of a building, but that level is generally less than what's acceptable in an industrial loading application where people aren't involved.
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Old 08-06-2018, 07:42 PM
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https://www.engineeringtoolbox.com/s...it-d_1781.html
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Old 08-06-2018, 08:27 PM
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Quote:
Originally Posted by Farmersamm View Post
I don't imagine there's any engineers on here, but anyways...……….

Lever attached to an axle. All of the flex is mid span on the lever. The moment connections are highly reinforced. But ya gotta be an absolute moron to figure there ain't a ton of stress on the connection to the axle.

Combined flex is probably additive. Flex on the lever, and torque flex on the axle.

Total flex is probably about 3/16". Annoying, but maybe liveable. That's the killer. It's hard to see inside the steel.

You can make everything rigid, but if it ain't necessary...…….why bother.

You can hang enough steel on anything, but in the end...is it warranted. I can make anything into a boat anchor, but that ain't fabrication.
Yield and fatigue ARE two different concepts. You can resist deformation in the elastic range a finite number of times without inducing stress fracture.
Once you go beyond that limit, it's only a matter of time before the fracture occurs.

Different materials react to fatigue differently. Aluminum WILL crack eventually. The inspection and repair of airframes bears this out, with an airframe having a finite lifespan.

The page you referenced says to hold the stress level down to 50% of ultimate strength in MOST steels.

More on this later.
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Old 08-06-2018, 10:34 PM
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Quote:
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How so? When do you approach the point where too many cycles are too much. I know civil engineers take this into account Generally deeper beams are the answer......which has to take yield into account
Yield stress is the stress at which plastic deformation (aka permanent bend) happens FROM A SINGLE LOAD. Below yield stress, the material returns to its original shape. This is in the elastic range. FYI, permanent change in shape is known as plastic deformation.

Fatigue is a factor of multiple or cyclic loading. Fatigue resistance is only loosely related to yields, but is more a factor of the separation of yield stress and ultimate failure stress.

Fatigue can happen at stresses above OR above the yield point. Depending on the material and load situation.
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