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ptc-53663871-Visitor
1-Visitor
February 9, 2015
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Creo Simulate 3.0: Cyclical Stress Analysis?

  • February 9, 2015
  • 1 reply
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A project I'm working on will have a 1/16" thick, 25"x25" stainless steel sheet that will be mounted to hinges along two parallel edges. The middle will be supported by a bar attached to a linear actuator. The sheet will go from being flat to convex and concave depending on the positioning of the linear actuator.

Is there a way to determine the lifespan of this sheet before metal fatigue wears in? I envision it going from flat -> 6cm convex -> 2cm concave -> flat once every 10 hours. For 3 years. How can I determine if it will last?

I'm fairly proficient in Creo, but have never used Simulate before in my life.

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More Info: This project is sort of like an automatic cookie sheet (minus the heat)- material gets deposited on top of the sheet, cures/hardens, sheet bends (causing material to "pop" off), "wiper" pushes material off the sheet, process repeats. One of the hinges is free to slide back and forth so this system won't be overconstrained


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Best answer by unickque

Normally I would say:

  • Calculate stress in the plate in convex state and in concave state. (both using Creo and manually to see if the results are in the right ball-park, so that you have confidence in the FEA).
  • Find a few locations that look critical (because they have high stress in one of the states, or because the stress difference between the two states is large.)
  • Perform a manual fatigue calculation on those locations.

Make sure you use a large safety factor, because fatigue has many uncertain influences.

But reading your message again, you have 3*365*24/10 = 2628 cycles over the 3 years. For fatigue that is almost nothing. Meaning you will be good to go even when you get close to yield stress of the material. But again: take plenty of margin just to be safe. So go up to half the yield stress and you'll be okay.

If at all possible, test your assembly after it is made. (Or test a simplified version before finalising the design). A test for 10000 cycles doesn't take that long, and then you can say with near certainty that your design is okay.

1 reply

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unickque1-VisitorAnswer
1-Visitor
February 9, 2015

Normally I would say:

  • Calculate stress in the plate in convex state and in concave state. (both using Creo and manually to see if the results are in the right ball-park, so that you have confidence in the FEA).
  • Find a few locations that look critical (because they have high stress in one of the states, or because the stress difference between the two states is large.)
  • Perform a manual fatigue calculation on those locations.

Make sure you use a large safety factor, because fatigue has many uncertain influences.

But reading your message again, you have 3*365*24/10 = 2628 cycles over the 3 years. For fatigue that is almost nothing. Meaning you will be good to go even when you get close to yield stress of the material. But again: take plenty of margin just to be safe. So go up to half the yield stress and you'll be okay.

If at all possible, test your assembly after it is made. (Or test a simplified version before finalising the design). A test for 10000 cycles doesn't take that long, and then you can say with near certainty that your design is okay.

P
ptc-53663871-VisitorAuthor
1-Visitor
February 9, 2015

Thanks for the tip about half yeild stress; a few engineer friends of mine confirmed that this is likely pretty good. I'm also glad to hear that I shouldn't be too worried about fatigue. I'll double the expected cycles as well, just to be super safe in case some (future) customers run the rig for smaller but more frequent uses.

Because I'll be steering clear of yeild stress and fatigue, do you think that if my design were simplified to only push the sheet to convex and then the actuator would retract back to its original state, the sheet would snap back to being perfectly flat? Or would it gradually become curved? Let's assume the hinge mechanisms on the side remain lubricated throughout the lifetime.

(Since push-only design would not require the pushbar under the plate to be welded or otherwise attached to the plate, reducing stress at that point)

D
DenisJaunin15-Moonstone
15-Moonstone
February 9, 2015

Hello, Paul,

Could you attach your file so that we can better realize your problem.

Best Regards.

Denis.

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