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problem C3D20R element

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sient

Structural
Oct 24, 2016
1
Hello,

I am working on a model of a masonry wall with a micromodelling approach,that includes also a steel plate on the top of the wall and a neoprene layer between the top part of the wall and this steel plate.
The input of the material parameters are the following:

Brick: Young Modulus= 7E9 Pa, density=14500 N/m3, Poisson's ratio= 0.18
Mortar: Young Modulus= 8E9 Pa, density=16650 N/m3, Poisson's ratio= 0.2
Steel: Young Modulus= 2.1e11 Pa, density=78450 N/m3, Poisson's ratio= 0.3
Neoprene: Young Modulus= 2e6 Pa, density=1.23e-5 N/m3, Poisson's ratio= 0.48

I made two models: one with C3D8R(Eight-node brick element with reduced integration) and the other with C3D20R element (Twenty-node brick element with reduced integration)

At the moment I am considering the elastic behavior of the wall, I carried out a modal analysis in both cases in order to compare the results with the experimental data. The wall is constrained at the base.

In the case of model with C3D20R, I noticed a ''distortion'' in the element of the neoprene layer from the 5th mode onwards, as showed in the attached figures, that there are not in the C3D8R model.
Is it an error due to the discretization or due to the elastic properties of the neoprene layer?

Thnaking you in advance
 
 http://files.engineering.com/getfile.aspx?folder=cd16d760-e157-41a5-affe-9932c1aa5552&file=MODES_WALL.tif
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