Stress at fixed boundary condition
Stress at fixed boundary condition
(OP)
Hi, I recently started working on Finite element analysis. Is it correct that the stress values at the fixed boundary condition are not realistic? I created a simple cantilevered beam and therotically calculated the bending stress values at the fixed Boundary Condition (B.C). Even with mesh refinement, the FEA stress values at the fixed boundary were slightly different from therotical value. I think that this difference increased when the mesh was corser. So I was thinking that I should not believe the stress values at the fixed BC and at tie regions. I was thinking that the stress may be realistic at one or two elements after the fixed B.C or tie regions. But I do not know the reason why the stresses are not real at boundaries. Can anyone please explain me this?
I was going through the ABAQUS user manual, where they have a fixed B.C to a cantilever (a lug). The example determines the stress at integration point and reaction force at nodes on the elements at fixed B.C, when a bearing pressure load is applied on a hole at the free end. The example uses 20 node brick elements with 8 integration points for meshing.
Since the manual listed the stress value at the fixed B.C, I am not clear whether the stress value for the element invlolved in the fixed boundary condition are realistic? The manual listed the stress at integration point. When I created a simple cantilevered beam for comparison with theorotical bending stress, I was looking at the stress contour plot. I remember reading some where that the contour plot stress, plots nodal stress and that the nodal stress is the average of stress from the surrounding integration points.
So is it correct that I can believe in the stress values at the integration points for the elements at fixed boundary condition but I should not believe in the stress values listed for the nodes at the fixed boundary condition? But again, I read in the manual that for a good refined mesh, the stress at integration point should be close to the nodal stresses. The basic question still remains for me. Are the stresses at the boundary condition realistic and if not why? Please help me understand the concept clearly. Thank you.
I was going through the ABAQUS user manual, where they have a fixed B.C to a cantilever (a lug). The example determines the stress at integration point and reaction force at nodes on the elements at fixed B.C, when a bearing pressure load is applied on a hole at the free end. The example uses 20 node brick elements with 8 integration points for meshing.
Since the manual listed the stress value at the fixed B.C, I am not clear whether the stress value for the element invlolved in the fixed boundary condition are realistic? The manual listed the stress at integration point. When I created a simple cantilevered beam for comparison with theorotical bending stress, I was looking at the stress contour plot. I remember reading some where that the contour plot stress, plots nodal stress and that the nodal stress is the average of stress from the surrounding integration points.
So is it correct that I can believe in the stress values at the integration points for the elements at fixed boundary condition but I should not believe in the stress values listed for the nodes at the fixed boundary condition? But again, I read in the manual that for a good refined mesh, the stress at integration point should be close to the nodal stresses. The basic question still remains for me. Are the stresses at the boundary condition realistic and if not why? Please help me understand the concept clearly. Thank you.





RE: Stress at fixed boundary condition
There are some specifics related to the integration points and their accuracy, but I'm not the person to answer that...not ABAQUS saavy.
Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group
RE: Stress at fixed boundary condition
i'm also not an ABAQUS user, possibly the elements don't particularly like moments (like NASTRAN's CQUAD4).
RE: Stress at fixed boundary condition
Every engineering model of reality requires you to make certain key assumptions to reduce the problem of reality to something more manageable. With the wonderful tool of FEA, the best you can do is compute the exact solution for the model of reality you have made; systematic approaches for computing how far you are from the exact solution is different thread. Nevertheless, I think that it is possible to get good reliable FEA results if you always keep in mind what the goals of the analysis are--if you are interested in stress 'far away' from the support, you can use a different strategy than if you need stresses at the support.
RE: Stress at fixed boundary condition