diff --git a/test/test_elasticity_hollow_sphere_with_surface_pressure.jl b/test/test_elasticity_hollow_sphere_with_surface_pressure.jl
index 061215a..c3b548d 100644
--- a/test/test_elasticity_hollow_sphere_with_surface_pressure.jl
+++ b/test/test_elasticity_hollow_sphere_with_surface_pressure.jl
@@ -5,6 +5,67 @@ using JuliaFEM
 using JuliaFEM.Preprocess
 using JuliaFEM.Testing
 
+function get_stress_tensor(element, ip, time)
+    haskey(element, "displacement") || return nothing
+    gradu = element("displacement", ip, time, Val{:Grad})
+    eps = 0.5*(gradu' + gradu)
+    E = element("youngs modulus", ip, time)
+    nu = element("poissons ratio", ip, time)
+    mu = E/(2.0*(1.0+nu))
+    la = E*nu/((1.0+nu)*(1.0-2.0*nu))
+    S = la*trace(eps)*I + 2.0*mu*eps
+    return S
+end
+
+""" Return stress vector in "ABAQUS" order 11, 22, 33, 12, 23, 13. """
+function get_stress(element, ip, time)
+    S = get_stress_tensor(element, ip, time)
+    return [S[1,1], S[2,2], S[3,3], S[1,2], S[2,3], S[1,3]]
+end
+
+""" Return principal stresses. """
+function get_stress_principal(element, ip, time)
+    S = get_stress_tensor(element, ip, time)
+    return sort(eigvals(S))
+end
+
+""" Make least squares fit for some field to nodes. """
+function lsq_fit(elements, field, time)
+    A = SparseMatrixCOO()
+    b = SparseMatrixCOO()
+    volume = 0.0
+    for element in elements
+        gdofs = get_connectivity(element)
+        # increase integration order by 1 from default
+        for ip in get_integration_points(element, 1)
+	        detJ = element(ip, time, Val{:detJ})
+	        w = ip.weight*detJ
+	        N = element(ip, time)
+	        f = field(element, ip, time)
+	        add!(A, gdofs, gdofs, w*kron(N', N))
+	        for i=1:length(f)
+                add!(b, gdofs, w*f[i]*N, i)
+            end
+            volume += w
+	    end
+    end
+    info("Mass matrix for least-squares fit is assembled. Total volume to fit: $volume")
+    A = sparse(A)
+    b = sparse(b)
+    A = 1/2*(A + A')
+    
+    SparseArrays.droptol!(A, 1.0e-6)
+    SparseArrays.dropzeros!(A)
+    nz = get_nonzero_rows(A)
+    F = ldltfact(A[nz,nz])
+
+    x = zeros(size(b)...)
+    x[nz, :] = F \ b[nz, :]
+
+    nodal_values = Dict(i => vec(x[i,:]) for i in nz)
+    return nodal_values
+end
+
 #=
 test subjects:
 - surface pressure load in curved surface
@@ -20,11 +81,12 @@ a = inner surface radial distance, b = outer surface ...
 
 if a=0.9, b=1.0, ν=1/3, E = 24580 and p = 7317 equation yields
 -9/10 for radial displacement
-=#
 
-@testset """1/8 hollow sphere with surface load""" begin
+http://mms2.ensmp.fr/emms_paris/plasticite3D/exercices/eSpherePress.pdf
+=#
+function test_wedge_sphere(model)
     mesh_file = Pkg.dir("JuliaFEM") * "/test/testdata/primitives.med"
-    mesh = aster_read_mesh(mesh_file, "HOLLOWSPHERE8_WEDGE6")
+    mesh = aster_read_mesh(mesh_file, model)
     body = Problem(Elasticity, "hollow sphere 1/8 model", 3)
     body.elements = create_elements(mesh, "HOLLOWSPHERE8")
     update!(body, "youngs modulus", 24580.0)
@@ -48,13 +110,52 @@ if a=0.9, b=1.0, ν=1/3, E = 24580 and p = 7317 equation yields
     nids = sort(collect(keys(X)))
     umag = Float64[norm(u[id]) for id in nids]
     um = mean(umag)
-    info("mean umag = $um")
-    info("std umag = ", std(umag))
+    us = std(umag)
     rtol = norm(um - 0.9) / max(norm(um), 0.9) * 100.0
-    info("rtol = $rtol")
+    info("mean umag = $um, std umag = $us, rtol = $rtol")
     @test rtol < 1.5 # percents
+    
+    # for linear model
+    #=
     u_CA = [-8.85861895037377E-01, -3.46944695195361E-18, -3.46944695195361E-18]
     rtol = norm(u[38] - u_CA) / max(norm(u[38]), norm(u_CA)) * 100.0
     info("rel diff to CA = $rtol %")
     @test isapprox(u[38], u_CA)
+    =#
+
+    S = lsq_fit(body.elements, get_stress, 0.0)
+    
+    # Calculate principal stresses in nodes
+    Sp = Dict()
+    for (nid, s) in S
+        # order is: 11, 22, 33, 12, 23, 13
+        stress_tensor = [
+            s[1] s[4] s[6]
+            s[4] s[2] s[5]
+            s[6] s[5] s[3]]
+        Sp[nid] = sort(eigvals(stress_tensor))
+    end
+    
+    node_ids = sort(collect(keys(Sp)))
+    for (i, nid) in enumerate(node_ids)
+        println("$nid -> $(Sp[nid])")
+        if i > 9
+            println("...")
+            break
+        end
+    end
+    
+    for element in body.elements
+        ip = [1/3, 1/3, 1/3]
+        time = 0.0
+	    stress_tensor = get_stress_tensor(element, ip, time)
+        principal_stresses = sort(eigvals(stress_tensor))
+        println("princial stresses = $principal_stresses")
+    end
 end
+
+@testset """1/8 hollow sphere with surface load""" begin
+    test_wedge_sphere("HOLLOWSPHERE8_WEDGE6")
+    #test_wedge_sphere("HOLLOWSPHERE8_WEDGE15")
+end
+