Bending stress through perforated plate

[tr6]

I've performed an FEA on a 1" thick plate, that is welded to the interior of the vessel. The plate has 2" dia holes on a 2-5/16" pitch. The edges of the holes are chamfered, to reduce stress risers.

I am now at the point of performing the stress linearizations. From Div 2, part 5, table 5.6 - "Perforated Head or shell", the membrane stesses can be obtained by running the SCL through the thickness of the plate. No problem. I've selected a mid-point in the ligament between the holes.

The description of the bending stress is confusing, to me anyway. "Averaged through width of ligament, but gradient through plate". My first thought was to run the SCL at an angle from the top edge of the hole, through the plate, across the ligament to the bottom edge of the adjacent hole. But then Section 5.A.3.c provides guidelines that the SCL should be normal to the mid-surface.

Or, do you average the bending stress through the thickness of the plate by running SCLs across the ligament between the corresponding nodes of the two adjacent holes? I hope I described that clearly.

Anyway, Thanks for your replies.

 

[RXer]

If I understand the geometry correctly, take the net ligament normal force divided by ligament area for the membrane stress. For bending, take the net bending moment in the ligament and divide by the section modulus of the ligament for bending stress. Combine to get memb +/- bending.

How you determine net normal and bending forces will depend on your software.

 

[BPVFEA]

Can anybody please give satisfactory reply to this post. I also would like to know how this bending stress is found escpecially through ANSYS.
Or does this requirement applicable only when the plate is modeled with Shell elements?
Thanks in advance

 

[prex]

You could use a number of control lines through plate thickness, one at mid ligament, one at hole border, and some others, possibly just one, midway of these (of course repeating towards the other hole, if there is no symmetry). Then you calculate the bending stress for each SCL and make an average based onto their distances.
Another possibility, that the code doesn't resolve, is to average the stresses (at the 6 component level) across the ligament at different depths through plate, then use those averages to get the bending stress.
The two procedures should anyway give close results.
As to ANSYS and linearization, I've found incorrect results (in a far past, don't know if anything has been changed more recently) from the built-in feature. I'm afraid that you should do that manually or writing a specific routine. Of course it helps a lot if you decide in advance where to place your SCL and you force nodes to be along those.

prex

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[tr6]

prex,
Would you mind clarifying the last part of your second sentence, "...make an average based onto their distances."?

Thanks,

 

[prex]

Take x as the abscissa along the ligament, 0 is at one hole border, 1 at the other one. Suppose that you have just 3 SCL, one at x1=0, one at x3=1 and one at x2=0.5. Calling b1,b2,b3 the 3 bending stresses for each SCL, the average would be:
b=0.25*b1+0.5*b2+0.25*b3
If x2 was =0.25 (unrealistic, but just as an example), then
b=0.125*b1+0.5*b2+0.375*b3.
Note that b1,b2,b3 as referred above should not be the bending stress intensities, but, more correctly IMO, the averaging should be done separately for each bending principal stress, then the stress intensity obtained from the averaged principal stresses.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes and launchers for fun rides

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