quad - size and thickness
quad - size and thickness
(OP)
I was always taught that its good modelling practice when using shell type elements to have a reasonable relation between the element side lengths to the proposed material property thickness. For example if the material being modelled was 1mm thick then a typical minimum element size would be 1mm x 1mm square. Today i thought i test it, and set up a simple beam under pressure load fixed at both ends and compared FE results against hand calcs, and it didn't make a blind bit of difference to the deflection/stresses if the Quad4 element was 0.1mm x 0.1mm by 10mm deep (i used a 25mm x 10mm deep beam).
I suppose my question is two-fold, firstly why doesn't it make a difference and secondly, how does the old number cruncher calculate the extreme fibre stresses.
Oh, using MSC nastran & patran.
Cheers.
I suppose my question is two-fold, firstly why doesn't it make a difference and secondly, how does the old number cruncher calculate the extreme fibre stresses.
Oh, using MSC nastran & patran.
Cheers.





RE: quad - size and thickness
RE: quad - size and thickness
To answer your questions, the reason you do not see any sort of difference is probably due to the model you're considering. Typically, plate structures tend to act about 10% stiffer than beams with identical geometry.
If you want to understand how stresses are computed I'd recommend a good FE or Plate/Shell theory text book. In the end everything is calculated based on elasticity and the principle of virtual work. No short answer exists to your question.
RE: quad - size and thickness
RE: quad - size and thickness
While not true for all cases typically for shells (and plates) the stress computations simplify to (P/A +/- M y/I) i.e. a membrane and a bending stress component.....I'm not familiar with the specific element formulation of the Nastran elements but it's probably much as I have said.....If this is the case then the membrane stresses decrease with thickness (linearly) and the bending portion varies as (t/I). Thus to compute the extreme fiber stress I would take stresses at any 2 points through the thickness and linearly extrapolate them in the thickness direction...From your post I'm not sure I have said anything you didn't already know so maybe I don't fully understand the question......
Ed.R.
RE: quad - size and thickness
you had the same mesh and only varied the element thickness ?
the thickness direction is transverse (sort of like a web thickness, rather than a flange thickness) ?
my recollection is that CQUAD4s assume a constant stress state and that the output is truly correct at the element centroid. so if you've got several elements thru the depth of the web, plotting the element results at their centroid and extrapolating should yield the classic bending distribution.
lastly, i thought the rule of thumb was to make the element size > the thickness (2, 4, 10*)
RE: quad - size and thickness
I started off with a coarse mesh, and ran it for varying thickness, from 1mm to 25mm deep (i.e a square CSA)then subseuently finished at a mesh size of 0.1mm x 0.1mm by 10mm deep(250000 elems), and the results always stacked up to be the same (and within a gnats c*ck to results by hand).
Your right about the quad4 being a constant stress element.
I suppose my rule of thumb might have been hit with a hammer once or twice!! But from my very limited test, it seems to be a meaningless rule of thumb, as it doesn't matter if you greatly exceed it, it seems.
RE: quad - size and thickness
Hmmm.