Designing a beam (repeated, one direction stress)

[mcfridge]

Hey everyone,

I have a steel W10x49 beam with a distributed pressure of 80 psi being applied to it (over a length of 90"). The beam is fixed at both ends. The pressure is applied and is held anywhere from 30 min to 12 hours.

I was looking through a machine design book (Mott) and it looked like this was repeated, one-direction fluctuating stress, since I will be applying and removing the load many times.

Since I already have this material, this is only really for my own interest. But what is the proper way to design for this type of stress? Is there a way to determine how many cycles it can go through before it might fail? The book mentions the Goodman method and such, but I wasn't really sure how to apply it. Thanks a lot.

 

[GBor]

Calculate stress in the beam due to the pressure loading. Look at the Goodman curve for the material...it will be stress vs. cycles. For the given max. stress, see how many cycles it will handle.

 

[mcfridge]

GBor,

Thanks for responding. I calculated (using the worst case of the beam being simply supported) that the maximum stress would be 15000 psi.

Are there Goodman curves for various materials available online? I searched but I couldn't find one, or do I have to develop the goodman curve myself?

 

[wes616]

Here is a pretty good and simple expl of fatigue analysis.

http://www.roymech.co.uk/Useful_Tables/Fatigue/Stress_levels.html

Wes C.
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When they broke open molecules, they found they were only stuffed with atoms. But when they broke open atoms, they found them stuffed with explosions...

 

[GBor]

mcfridge,

Wish I knew. The site that Wes C. has gives some good information, but search the Materials and Metallurgy forum...they've probably had this question more than once. What type of steel is it? Is this just an "over-the-counter" wide flange?

 

[rb1957]

15 ksi shouldn't be a problem to any type of steel (i'd imagine almost infinite life) ... a key consideration is the local geometry (holes and steps mostly) that create stress concentrations (but even with a Kt=3, open hole, 15 ksi should have a long life).

another thought, since the beam is cantilevered, then the support is reacting a couple, so there is load transfer out of the beam flanges into the support ... this is significant to the stress concentration.

good luck