Issue with stress calculation using quadratic tetra elements

[janek]

Hi Forum!
I have problem with calculation of stress on tetrahedral mesh. This is probably something with solver configuration but I already spend hours trying to find an issue hence this post. I'd like to demonstrate my problem on simple case. I created simple beam in gmsh:

Code: Select all

Point(1) = {0,0,0,0.1};

Extrude {100,0,0} {
    Point{1}; Layers{20};
}

Extrude {0,10,0} {
    Line{1}; Layers{2};
}

Extrude {0,0,10} {
    Surface{5}; Layers{2};
}

then I transformed it from d4 to d10 tetras using:

Code: Select all

ElmerGrid.exe 14 4 beam.msh -out beam-d10.msh -increase

I imported this beam into elmer GUI and created default scenario. All I did was selecting default and adding support and some force on both ends of beam. Solver file is:

Code: Select all

Header
  CHECK KEYWORDS Warn
  Mesh DB "." "."
  Include Path ""
  Results Directory ""
End

Simulation
  Max Output Level = 5
  Coordinate System = Cartesian
  Coordinate Mapping(3) = 1 2 3
  Simulation Type = Steady state
  Steady State Max Iterations = 1
  Output Intervals = 1
  Timestepping Method = BDF
  BDF Order = 1
  Solver Input File = case.sif
  Post File = case.ep
End

Constants
  Gravity(4) = 0 -1 0 9.82
  Stefan Boltzmann = 5.67e-08
  Permittivity of Vacuum = 8.8542e-12
  Boltzmann Constant = 1.3807e-23
  Unit Charge = 1.602e-19
End

Body 1
  Target Bodies(1) = 1
  Name = "Body 1"
  Equation = 1
  Material = 1
End

Solver 1
  Equation = Linear elasticity
  Procedure = "StressSolve" "StressSolver"
  Variable = -dofs 3 Displacement
  Exec Solver = Always
  Stabilize = True
  Bubbles = False
  Lumped Mass Matrix = False
  Optimize Bandwidth = True
  Steady State Convergence Tolerance = 1.0e-5
  Nonlinear System Convergence Tolerance = 1.0e-7
  Nonlinear System Max Iterations = 20
  Nonlinear System Newton After Iterations = 3
  Nonlinear System Newton After Tolerance = 1.0e-3
  Nonlinear System Relaxation Factor = 1
  Linear System Solver = Iterative
  Linear System Iterative Method = BiCGStab
  Linear System Max Iterations = 500
  Linear System Convergence Tolerance = 1.0e-10
  BiCGstabl polynomial degree = 2
  Linear System Preconditioning = Diagonal
  Linear System ILUT Tolerance = 1.0e-3
  Linear System Abort Not Converged = False
  Linear System Residual Output = 1
  Linear System Precondition Recompute = 1
End

Equation 1
  Name = "Equation 1"
  Calculate Stresses = True
  Active Solvers(1) = 1
End

Material 1
  Name = "Iron (generic)"
  Heat expansion Coefficient = 11.8e-6
  Electric Conductivity = 10.30e6
  Heat Conductivity = 80.2
  Sound speed = 5000.0
  Heat Capacity = 449.0
  Mesh Poisson ratio = 0.29
  Electric Conductivity = 10.30e6
  Density = 7870.0
  Poisson ratio = 0.29
  Youngs modulus = 193.053e9
End

Boundary Condition 1
  Target Boundaries(1) = 3 
  Name = "support"
  Displacement 3 = 0
  Displacement 2 = 0
  Displacement 1 = 0
End

Boundary Condition 2
  Target Boundaries(1) = 5 
  Name = "force"
  Force 2 = 10000
End

Result in postprocessor is here:

and:


What I'm trying to understand is why stress not uniform but instead follows element boundaries. If I understand correctly this should not happen for d10 elements. Can you please advice what I did wrong?
Regards
Janek

[raback]

Hi Janek

Looks ok to me. Found this random picture from google that looks quite similar:

https://homesecurity.press/quotes/stres ... lever.html

-Peter