Low-Cycle Fatigue 4

[jzhaan55]

If I have a rectangular sheet of gauge 12 steel (33 Fy, 45 Fu, 30% elongation), is there an easy way to estimate how many times that I can bend the sheet back and forth (holding the lower half in a vise and bending the upper half back and forth in 180 deg. cycles) before it starts to crack? Is there some sort of low-cycle fatigue equation that I can use to estimate this value.

Thanks,
J

 

[Tmoose]

I'd expect each time it is bent work hardening will take place, and unless a particular bend radius is enforced each of the next bend cycles will involve different material, until a tear finally opens up somewhere in the gnarly multi humped irregular shape.

 

[jzhaan55]

So you're saying since the material is work hardened, that after one bend is achieved (downward), then the upward bend will actually occur on a different part of the material adjacent to the first bend?

 

[metengr]

With this sheet thickness place it in a vise over a known bend radius and see how many cycles to failure - just like repeated bending of a paper clip to failure.

 

[jzhaan55]

are there any good equations to model this fatigue?

 

[metengr]

Yes, look up the strain-life method, calculate the strain induced in the sheet from reverse bending and plot the cycles to failure.

 

[davefitz]

There's an easier way-buy a dozen sheets of 12 gauge, and bend each one back and forth until it cracks. Find the mean number of cycles, and standard deviation, design for mean - 2 x standard deviation. Beats fatigue calculations every time.

 

[Guest102023]

I like davefitz' approach.

 

[NickE]

I have a question:
If the bending is exceeding the yield strength each time, is this still considered fatigue?

Nick
I love science!

 

[metengr]

Yes, it is considered low cycle fatigue crack propagation.

 

[redpicker]

The only problem I have with davfitz's solution is that you will probably need a larger sample size than 12 to get an accurate estimation of the standard deviation.

One might also argue that Mean - 2 Standard Deviations is not a very good design limit for low-cycle fatigue since over 2% could be expected to fall with fewer cycles, but that really was not part of the OP.

As a rule, though, I'd trust the answer using davefitz's method more than any calculated value.

rp

 

[LiquidSteel]

Once you bend it. Don't bend it back. Even if it doesn't break immediately, you've severly reduced its strength. I'm assuming you bent it 90 and you plan on bending it 180 opposite.

 

[Engdoitbetter]

As far as I remember from university courses, if you bend a beam beyond the elastic limit and then unload it, you leave a residual stress state in the material. This stress state increases material strength in case beam is bent in the same direction as before, while will decrease the strength in case of bending in the opposite direction.
So in low cycle fatigue you add the typical fatigue damage to work hardening beyond elastic limit and to residual stress effects.

Hope it helps.

Stefano