bending or shear?

[jsboy]

When we look at FEA results, is there any way to cleary determine what is dominating the results e.g. it is bending or shear? I am performing FEA and wants to know if it is bending dominating or shear by looking at output, stress.

 

[nagi]

Hi Jsboy,

Are you talking about a shell or plate problem? In that case, there are two types of loading: bending-dominated and membrane-dominated. Is that what you mean?

Nagi

 

[jsboy]

Thanks Nagi for the comment. The model is shell and as you said that there are both bending and membrane loading. What I want to know is that is there any way e.g. by looking at some stress value, to clealy determine which load is dominating in particular case.

 

[corus]

As FEA in stress analysis mostly looks at statically indeterminate problems it generally tends to be the relative stiffnesses of the structure that dominates the solution. I tend to look at the overall displaced shape of the structure in making a judgement of what dominates the results. You could look at the membrane stress in comparison to the bending stress at any one position but that would most likely just tell you what is happening locally.

corus

 

[jsboy]

Thanks corus!!

 

[nagi]

I think the best measure is to check the ratio of bending or membrane energy to total internal energy in the model, but I don't believe most FE codes give such output.

One qualitative estimate would be to plot the stresses at the shell midsurface and the stresses at the top (or bottom) surface and to visually compare the results. Regions where there is not much difference are membrane-dominated while regions which have a big difference are bending-domninated.

Nagi

 

[glenn11897]

just adding my 2 cents,

as you already know, stress depends on the area and force. if you are familiar with the geomtry of your fem, then check the output force. this way you actually see the diffenece between the two loads (bending or shear).

when comparing shear and bending its probably almost like comparing apples to oranges. but the loads should tell you which is worse... good luck...

 

[magicme]

hi there

i agree with nagi's message of march 16....look at the stress components (sx, sy, sz) on top and bottom surfaces of the shell elements.....if they are very similar in sign and magnitude, then membrane stress > bending stress. don't make the mistake of looking at the combined (or von mises) stress as that won't tell the story.

daveleo

 

[joekm]

You could also look at your shell elements, the plate-derived portion of that formulation will be based either on Mindlin or Kirchoff plate theory. Perhaps you could introduce discreet Kirchoff elements and see if there is a significant change?

I would probably just check a reference like Roarke and Young and see if there was a way I could empirically or semi-empirically make the determination.

--
Joseph K. Mooney
Director, Airframe Structures - FAA DER
Delta Engineering Corporation