Von mises stress above allowable stress in material? 1

[Yoman228]

I have modelled the material as following:
*Material, name=steel
*Conductivity
45.33,
*Elastic
207000., 0.3
*Plastic
423.,0.

Why do I have von mises stress of 470MPa using banded plot, and 433MPa for quilt plot?

Would't the stress should not pass the highest stress then the material?

Doing some more test, but wil be intrested if anyone have seen this before.

 

[dldhdz]

could that be a result of extrapolation?

 

[Yoman228]

results found so far with a simplified single element model with uniaxial or shear loading. Mises stress gives the same yield as material.

However, when under complex loading. The mises is above yield. but the stress at each integration point is same as yield.

Still studying.......

 

[Yoman228]

It seen that the element use have a big different to the von mises stress due to how they calculate results from intregation point.

Banded plot..

C3D8R - 421MPa
C3D8 - 516MPa
CPD8I - 476MPa
CPD8H - 516MPa

I was using CPD8I as my model have bening. It seen that it may be good idea to use the C3D8R as I used to.

Does anyone know where I can find a more explanation in thiese topic?

 

[dldhdz]

ABAQUS analysis user's manual, vol1, 4.1.2.-7

Extrapolation and interpolation of element output variables

The shape functions of the element are used for purposes of extrapolation and interpolation of output
variables. Extrapolated values are generally not as accurate as the values calculated at the integration
points in the areas of high stress gradients, particularly in the case of modified triangles and tetrahedra.
Therefore, adequately detailed meshing is necessary around nodes where accurate nodal values of such
element results are needed. If a cylindrical or spherical coordinate system is defined for the element
(see “Orientations,” Section 2.2.5), the orientation at each integration point may be different. When
the values at the integration points are extrapolated to the nodes, the difference in the orientation is not
taken into account; therefore, if the orientation varies significantly over the elements connected to a
node, the extrapolated values will not be very accurate. If the material orientation undergoes significant
spatial variation in a region of the model where the material behavior is truly anisotropic, a finer mesh
is required to obtain accurate results even at the integration points. In that situation once the overall
solution has converged with respect to the mesh density, the interpolation or extrapolation away from
the integration points can also be assumed to be reasonably accurate. Element output for second-order
elements with one collapsed side in two dimensions or one collapsed face in three dimensions should
not be extrapolated to the nodes.
In a coupled temperature-displacement analysis nodal temperatures (variable NT11) are more
accurate than temperatures at the integration point (variable TEMP) extrapolated to the nodes.
For derived variables, such as the Mises equivalent stress, the components are first extrapolated
or interpolated, then the derived value is calculated from the extrapolated or interpolated components.
However, in linear mode-based dynamic analysis procedures where values are obtained as nonlinear
combinations of modal response magnitudes (“Random response analysis,” Section 6.3.11, and
“Response spectrum analysis,” Section 6.3.10), the nonlinear combinations are first calculated at the
integration points. These derived values are extrapolated to the nodes or interpolated to the centroid.