How to write the stress field into vtk file？

[zyhsjtu]

How to write the stress field into vtk file.
I add the following codes and calculate the strain tensor and stress at quadrature point，
`

struct StateVariable{T}
   ε::SymmetricTensor{2,3,T} # strain in each quadrature point
  σ::SymmetricTensor{2,3,T} # stress in each quadrature point
  end
 function StateVariable()
      return StateVariable(zero(SymmetricTensor{2, 3}),zero(SymmetricTensor{2, 3}))
end

`

Then I write the follow codes to extract the stress field and write it into vtk but failed.
`

function compute_stress(states, cellvalues::CellVectorValues{dim, T}, dh::DofHandler) where {dim, T}
      σ = [SymmetricTensor{2,3, Float64}[] for _ in 1:getncells(dh.grid)]
  for (i, cell) in enumerate(CellIterator(dh))
	  state = @view states[:, i]
	  σ_cell = σ[i]
	  for q_point in 1:getnquadpoints(cellvalues)
		  push!(σ_cell, state[q_point].σ)
	  end
  end
  return σ
end
 vtk_grid(filename, grid) do vtk
       vtk_point_data(vtk,  σ, "σ(MPa)")
    end

`

and the error information is:

ERROR: LoadError: DimensionMismatch: incorrect dimensions of input array.

So How should I do to write the stress field and how to project them into nodes?

[termi-official]

Quadrature data is usually projected to nodes with an L2 projection. Here you solve the problem $\int uv = \int fv$ where the expression on the right hand side is your quadrature data. For this we provide a small helper function project and the associated L2Projector struct, for which you can find an example in the second tutorial, which is about postprocessing.

[zyhsjtu]

Thanks for your quick response!! I have done it as follow, but still there some errors, could you give me some advices?
`

 projector = L2Projector(ip, grid);
 σ = compute_stress(states, cellvalues, dh);
 σ_projected = project(projector, σ, qr; project_to_nodes = false);
 vtk_grid(filename, grid) do vtk
   vtk_point_data(vtk, projector, σ_projected, "σ(MPa)")
end

`

[KristofferC]

https://ferrite-fem.github.io/Ferrite.jl/stable/examples/plasticity/ should probably be updated to show how to do this properly instead of the slightly inelegant way that is currently used there.

[zyhsjtu]

Thanks for your suggestion!

[fredrikekre]

In https://ferrite-fem.github.io/Ferrite.jl/dev/tutorials/incompressible_elasticity/ there is volume averaging within the element.

[termi-official]

https://ferrite-fem.github.io/Ferrite.jl/stable/examples/plasticity/ should probably be updated to show how to do this properly instead of the slightly inelegant way that is currently used there.

Yea, I will try to update the example with FerriteViz after release, so you can inspect the true stress fields.

[zyhsjtu]

In https://ferrite-fem.github.io/Ferrite.jl/dev/tutorials/incompressible_elasticity/ there is volume averaging within the element.
Thanks for your reply, I was using the unupdated example earlier and didn't see the code that output the stress field.