Pressure and resulting stress on surface
Pressure and resulting stress on surface
(OP)
Hi,
In essence, I'm simulating a plate which is supported by bars that run along the plate's edges. A pressure is applied on the plate's surface and the system is exposed to a gravitational field.
I run the simulation and then look at the stresses in the plate's normal direction on the plate's surface. Shouldn't these stresses be equal to the applied pressure?
Pingen
In essence, I'm simulating a plate which is supported by bars that run along the plate's edges. A pressure is applied on the plate's surface and the system is exposed to a gravitational field.
I run the simulation and then look at the stresses in the plate's normal direction on the plate's surface. Shouldn't these stresses be equal to the applied pressure?
Pingen





RE: Pressure and resulting stress on surface
It sounds like you are using an isotropic plate, but have you done a mesh sensitivity analysis? What are your boundary conditions? How is the load applied? And, what software are you using? How thick is your plate?
There are many things that go in to producing an FEA result. Without a few of these questions being answered, it is difficult to determine whether your results are correct.
Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group
Magnitude The Finite Element Analysis Magazine for the Engineering Community
RE: Pressure and resulting stress on surface
I don't have the book, but I just ordered it.
-The plate is a curved rectangle, shaped like a half-pipe
-The pressure is unevenly distributed on the concave surface
-Two surfaces are fixed. If the plate was extended to a full cylinder, then the fixed surfaces would be located by the cylinder's openings. (I hope that explanation can be understood)
-It's a relatively thin plate, yet I'm using solid elements
-It's a fine mesh and the stresses appear in smooth transitions. I have not done a mesh sensitivity analysis though
-I'm using ANSYS
-The model is currently linear
Regards,
Pingen
RE: Pressure and resulting stress on surface
RE: Pressure and resulting stress on surface
Sorry I missed your last post, Pingen, but rb1957 is more than qualified to solve your problem.
Sounds like you have something similar to half a pressure vessel, which is where the pR/t and pR/2t equations would come from.
RE: Pressure and resulting stress on surface
GBor's right about the shear stress (funnily enough it equals the longitudinal load in the pressure vessel), but the hoop stress is pR/t
but the structure is more like a doubly cantilevered beam.
i guess we could turn the question around and ask why would you think the stresses in a beam should be equal to the applied pressure ?
RE: Pressure and resulting stress on surface
RE: Pressure and resulting stress on surface
i'd say that's right if you're close to an axis of symmetry ... the further away for this the plate sigma zz has to react the sum of the applied pressure (effectively the integral of the applied pressure). in any case, i'd expect that this stress would be much less than sigma xx due to bending.
RE: Pressure and resulting stress on surface
Z=sig_zz * n + sig_xz * l + sig_yz * m
where Z is the z traction and n,l and m are the direction cosines which for the upper and bottom surfaces of a flat plate have values of +-1, 0 and 0.
Whether or not this guy has a flat plate I have no idea..he didn't say one way or the other.
RE: Pressure and resulting stress on surface
RE: Pressure and resulting stress on surface
If the plates are curved, if they are flat, doesn't matter--if the load is only a pressure (indicating a stress normal to the surface everywhere) and no shear tractions are applied, then the normal stress has to be equal to the pressure; in this case the direction cosine of the surface is the same whether you are computing pressure or normal traction. Now the reasons for the discrepancy in your FE model can only be speculated. Bad mesh comes to mind first, that is not enough elements or poorly placed elements. Are you doing this with plates or shell elements, or are you doing this with 3D brick elements? It will help if you identify the element as precisely as possible--not just "brick" but how many nodes are in the brick.
RE: Pressure and resulting stress on surface
To see why it makes a difference, it might help to think of the shell as the two dimensional version of a beam; certainly the equations that describe equilibrium in shells and beams are similar, with similar approximations in the models. Now think of a beam, simply supported, loaded by a constant pressure 'q' along the beam (say in the 'y' direction, the x direction is aligned with the axis of the straight beam). Certain approximations are made to make this an easy problem to compute. Now try to calculate the complete stress state, sxx, syy, szz, sxy, sxz, syz in this beam. szz is zero of course, so are sxz and syz. sxx is from the classical My/I, shear from VQ/It. What is syy? Zero! Completely counterintuitive, nevertheless the right answer for the model, that is, a product of the formulation (beam equations).