calculate stress, interpolate stress to nodes using lsq fitting

test/test_extrapolate_to_nodes.jl

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+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+using JuliaFEM
+using JuliaFEM.Postprocess
+using JuliaFEM.Test
+
+@testset "extrapolate stress from gauss points to nodes" begin
+    X = Dict{Int, Vector{Float64}}(
+        1 => [0.0, 0.0],
+        2 => [6.0, 0.0],
+        3 => [6.0, 6.0],
+        4 => [0.0, 6.0],
+        5 => [12.0, 0.0],
+        6 => [12.0, 6.0])
+    el1 = Element(Quad4, [1, 2, 3, 4])
+    el2 = Element(Quad4, [2, 5, 6, 3])
+    el1.id = 1
+    el2.id = 2
+    elements = [el1, el2]
+    time = 0.0
+    update!(elements, "geometry", X)
+    update!(get_integration_points(el1), "stress", time => [1.0, 2.0, 3.0])
+    update!(get_integration_points(el2), "stress", time => [2.0, 3.0, 4.0])
+    field_name = "stress"
+    field_dim = 3
+    calc_nodal_values!(elements, field_name, field_dim, time)
+    s1 = el1("stress", [0.0, 0.0], time)
+    s2 = el2("stress", [0.0, 0.0], time)
+    # visually checked, see blog post "Postprocessing stress"
+    @test isapprox(s1, [1.125, 2.125, 3.125])
+    @test isapprox(s2, [1.875, 2.875, 3.875])
+end
+