# Large deformation of solids

Link to notebook with this question and code

I'd like to understand how large deformations of solid mechanics work and how they are implemented. For this am looking at the following reference problem: A long slender beam is fixated at the left. A larger force is pushing in the negative x direction and a smaller force in the negative y direction.

To keep things a simple as possible this example uses a linear plane stress model (a linear material model) - it's just about the large deformation.

A reference to this benchmark problem can be found here. There is also a video starting at time 28:00. Geometric nonlinearity is explained at 26:22.

The result for the plane stress case is a largely deformed beam. In the picture below (taken from the source given above) we are talking about the lower of the two beams.

Set up some parameters.

Needs["NDSolveFEM"]
length = 3.2;
hight = 1/10;
thickness = 1/10;
Ω = Rectangle[{0, 0}, {length, hight}];
mesh = ToElementMesh[Ω];
bmesh = ToBoundaryMesh[mesh];


The total force is given. I am not 100% sure what total force means in Comsol speak, but I assume it means force per area. The force in the negative x direction is 10^3 times larger then the one in the negative y direction. We use Poisson's ratio of 0 and Young's modulus of 210 GPa.

(*force per area*)
fx = -3.844*10^6/(hight*thickness);
fy = fx*10^-3;
bc2 = {NeumannValue[fx, x == length], NeumannValue[fy, x == length]};
bc1 = DirichletCondition[{u[x, y] == 0, v[x, y] == 0}, x == 0];
materialData = {nu -> 0, Y -> 210*10^9};


We start by using a linear - no large deformation - plane stress operator used on SE in several places and known to produce correct results. The only thing I changed is adding a 'thickness' factor to account for the thickness that is not 1.

PlaneStress[{Y_, nu_}, {u_, v_}, X : {x_, y_}] := Module[{pStress},
(* uses global thickness *)
pStress = -thickness *
Y/(1 - nu^2)*{{{{1, 0}, {0, (1 - nu)/2}}, {{0, nu}, {(1 - nu)/2,
0}}}, {{{0, (1 - nu)/2}, {nu, 0}}, {{(1 - nu)/2, 0}, {0, 1}}}};
{Inactive[Div][pStress[[1, 1]] . Inactive[Grad][u, X], X] +
Inactive[Div][pStress[[1, 2]] . Inactive[Grad][v, X], X],
Inactive[Div][pStress[[2, 1]] . Inactive[Grad][u, X], X] +
Inactive[Div][pStress[[2, 2]] . Inactive[Grad][v, X], X]}]


Solve the equation:

planeStress =
PlaneStress[{Y, nu} /. materialData, {u[x, y], v[x, y]}, {x, y}];
{uif2, vif2} =
NDSolveValue[{planeStress == bc2, bc1}, {u,
v}, {x, y} ∈ Ω];
Show[bmesh["Wireframe"],
deform2 =
ElementMeshDeformation[mesh, {uif2, vif2}, "ScalingFactor" -> 1][
"Wireframe"["MeshElementStyle" -> EdgeForm[Red]]]]


Next, I am solving the same problem with a different setup. This setup will allow me later to easily change to a large deformation setup. We use the following:

This is then converted to a 2 by 2 matrix:

Normal[
SymmetrizedArray[
Thread[Rule[{{1, 1}, {2, 2}, {1, 2}}, {Subscript[σ, xx],
Subscript[σ, yy], Subscript[τ, xy]}]], Automatic,
Symmetric]]

(* {{Subscript[σ, xx], Subscript[τ,
xy]}, {Subscript[τ, xy], Subscript[σ, yy]}} *)


And plugged into (with an additional thickness factor):

Table[
Inactive[Div][-{{Subscript[σ, xx], Subscript[τ,
xy]}, {Subscript[τ, xy], Subscript[σ,
yy]}}[[i, ;;]], X], {i, Length[U]}]

(* {Inactive[
Div][{-Subscript[σ, xx], -Subscript[τ, xy]}, {x, y}],
Inactive[Div][{-Subscript[τ, xy], -Subscript[σ, yy]}, {x,
y}]} *)

makeStressModel[e_] := Block[
{materialData, materialModel, exx, eyy, gxy, eVec, strain, nu, Y,
stress},
(* linear elastic plane stress model *)

materialModel =
Y/(1 - nu^2)*{{1, nu, 0}, {nu, 1, 0}, {0, 0, (1 - nu)/2}};
exx = e[[1, 1]];
eyy = e[[2, 2]];
gxy = (e[[1, 2]] + e[[2, 1]]);
(* for plane stress ezz =nu/(1-nu)*(exx+
eyy) does not play a role since nu = 0 *)
strain = {exx, eyy, gxy};
stress = (materialModel . strain);
Normal[SymmetrizedArray[
Thread[Rule[{{1, 1}, {2, 2}, {1, 2}}, stress]], Automatic,
Symmetric]]
]

(* uses global defined thickness *)
makePDEModel[stressMatrix_] :=
Table[Inactive[Div][-thickness*stressMatrix[[i, ;;]], X], {i,
Length[U]}]


We verify that this gives the same results as a the previous plane stress operator:

X = {x, y};
U = {u[x, y], v[x, y]};

(* geometric linear *)
gut = Transpose[gu];
e = 1/2 (gu + gut);
stress = makeStressModel[e];

pde = makePDEModel[stress /. materialData];
{uif, vif} =
NDSolveValue[{pde == bc2, bc1}, {u, v}, {x, y} ∈ mesh];

Show[bmesh["Wireframe"],
deform1 =
ElementMeshDeformation[mesh, {uif, vif}, "ScalingFactor" -> 1][
"Wireframe"["MeshElementStyle" -> EdgeForm[Brown]]]]


Norm[uif["ValuesOnGrid"] - uif2["ValuesOnGrid"]]
Norm[vif["ValuesOnGrid"] - vif2["ValuesOnGrid"]]

(* 1.43378*10^-10
6.49507*10^-9*)


With the framework in place and verified, I want to look at the large deformations. I set up the deformation gradient f and set up the non-linear strain.

(* geometric nonlinear *)
f = Grad[U, {x, y}] + IdentityMatrix[2];
e1 = 1/2 (Transpose[f] . f - IdentityMatrix[2]) // Expand // Simplify;


With the same approach as above

stress = makeStressModel[e1];
pde = makePDEModel[stress /. materialData];
{uif, vif} =
NDSolveValue[{pde == bc2, bc1}, {u,
v}, {x, y} ∈ Ω];
mesh = uif["ElementMesh"];
Show[ToBoundaryMesh[mesh]["Wireframe"],
deform2 =
ElementMeshDeformation[mesh, {uif, vif}, "ScalingFactor" -> 1][
"Wireframe"["MeshElementStyle" -> EdgeForm[Red]]]]


Show[deform1, deform2]


This is not correct. I would have expected a failure from the nonlinear solver for the large deformation or a much larger deformation. I am trying to understand what I am missing. Question: Why is this approach wrong or where did I make a mistake?

A second way to specify the large strains is by

gu = Grad[U, X];
gut = Transpose[gu];
e2 = 1/2 (gu + gut + gut . gu);
FullSimplify[e1 - e2]

(* {{0, 0}, {0, 0}} *)


But it gives the same result. I have verified that the PDE coefficients parse correctly

{state} =
NDSolveProcessEquations[{pde == bc2, bc1}, {u,
v}, {x, y} ∈ Ω];
GetInactivePDE[state] - pde

(*{0, 0}*)


The next idea was that using the Cauchy stress is not correct. So I changed to the second Piola-Kirchhoff stress. We compute the stress as before

stress = makeStressModel[e1];


Now, find the deformation gradient, it's inverse and transpose and J the determinant of f.

f = Grad[U, X] + IdentityMatrix[2];
invF = Inverse[f];
invFT = Transpose[invF];
piolaStress = Det[f]*invF . stress . invFT;
pde = makePDEModel[piolaStress /. materialData];
{uif, vif} =
NDSolveValue[{pde == bc2, bc1}, {u,
v}, {x, y} ∈ Ω];
deform3 =
ElementMeshDeformation[mesh, {uif, vif}, "ScalingFactor" -> 1][
"Wireframe"["MeshElementStyle" -> EdgeForm[Blue]]];
Show[deform2, deform3]


Still, no joy.

{state} =
NDSolveProcessEquations[{pde == bc2, bc1}, {u,
v}, {x, y} ∈ Ω];
GetInactivePDE[state] - pde
(* {0, 0} *)


Link to notebook with this question and code

Update:

From the video I understand that the following is use:

$$\nabla \cdot (F S)^T$$

$$F = \nabla u + I$$

The material law (Hooks law): $$S = ? F^{-T} (C:\epsilon) F^{-1}$$

Here, the $$?$$ is something I can not read - could be $$J = det(F)$$

$$\epsilon = \frac{1}{2} ((\nabla u)^T + (\nabla u) + (\nabla u)^T*(\nabla u))$$

• You seem to be concentrating on the stresses in this formulation. You also need to take account of the possible change in direction of the forces. So does the axial force Fx remain axial as the deflection occurs? Does the Fy force remain tangential to the end surface? As the deflection occurs should these forces rotate with the deflection? Such forces are called follower forces. Large deflections must take into account such changes in direction. Thus if Fy is caused by a weight hanging by a string vertically from the beam then as the beam deflects you will need to add cos and sin terms.
– Hugh
Jul 14, 2021 at 8:44
• @Hugh, Thanks for your comment. So you are saying that the boundary force should be multiplied with the boundary normal which is evaluated in the current frame and not the initial frame? My understanding was/is that the second PK stress allows for the force to remain in the initial frame - but maybe that's wrong. Jul 14, 2021 at 8:50
• Yes that is the issue. I think this is part of the problem definition. You have to decide if the direction of the external forces change with the deflection (the usual case of applied external loads) or follows the deflections.
– Hugh
Jul 14, 2021 at 9:00
• @user21 Very interesting problem! As far as I understand your question I think it's necessary to define first the deformation (usually one dimensional in beam theorie) and second calculate the strain tensor, which depends on the metrics of the undeformed and deformed "meshs". The stress tensor follows from the material model. Jul 15, 2021 at 6:43
• @UlrichNeumann, thank you for your comment. I am looking for a general solution, one that does not rely on analytical solutions or beam theory. I mean there are other FEM tools that compute this, and my question is how do those tools compute this. To the best of my knowledge the FEM derivation is based on variational principals but the question is then why does this not work in the coefficient form. Jul 15, 2021 at 7:20

I think you guys are using the wrong theory, as far as I understood the code above. Sadly I only have little time, and in this answer I can only give you the theory without code. If I read your code wrong and you are using the equivalent correct theory, then I apologize of course :D.

In continuum mechanics of solid bodies, you generally have material points $$X$$ and spatial positions $$x$$. The motion is usually defined as $$x = \chi(X)$$. The gradient of the motion $$\chi$$ w.r.t. (with respect to) matter is referred to as the deformation gradient $$F_{ij}(X) = \partial \chi_i(X) / \partial X_j$$.

Small deformations

When you assume small deformations, you implicitly assume $$F \approx I$$, which imply $$x \approx X$$. Because of this, you approximate the displacement field of a material point $$u(X)$$ by $$u(x)$$ (note the new dependency on spatial position $$x$$ instead of material point $$X$$). This yields that the equilibrium condition in terms of the Cauchy stress $$\sigma(x)$$ $$\sum_{j=1}^3 \frac{\partial \sigma_{ij}(x)}{\partial x_j} = 0 \ , \quad i=1,\dots,3$$ w.r.t. space can be solve for linear elasticity with the elastic law $$\sigma(x) = \mathbb{C}(x) \varepsilon(x)$$, some fourth-order tensor stiffness of space $$\mathbb{C}(x)$$ (e.g., with isotropic material behavior determined by Young's modulus and Poisson's ratio) and the infinitesimal strain tensor $$\varepsilon_{ij}(x) = \frac{1}{2}\left( \frac{\partial u_i(x)}{\partial x_j} + \frac{\partial u_j(x)}{\partial x_i} \right).$$ So, for small deformations you have everything parametrized w.r.t. space and you can go as you already now.

Large deformations

For large deformations the situation changes since the general equilibrium equation in terms of the Cauchy stress $$\sigma$$ $$\sum_{j=1}^3 \frac{\partial \sigma_{ij}(x)}{\partial x_j} = 0 \ , \quad i=1,\dots,3$$ is of course still valid, but now you have to take into account the difference between spatial positions $$x$$ and material points $$X$$. An easier way to do this is to solve the equivalent equilibrium condition in the reference placement formulated in terms of the first Piola-Kirchohoff stress tensor $$P(X)$$ [1PK] (note the dependency on matter $$X$$ and differentiation w.r.t. $$X$$ and not $$x$$) $$\sum_{j=1}^3 \frac{\partial P_{ij}(X)}{\partial X_j} = 0 \ , \quad i=1,\dots,3$$ see, e.g., equation (4.123) here. The 1PK is connected to the second Piola-Kirchoff stress tensor $$S(X)$$ [2PK] by $$P(X) = F(X)S(X)$$ In elasticity of large deformations, usually 2PK is modeled. The analogous elastic law is St.Venant's law $$S(X) = \mathbb{C}(X)E(X)$$ with Green's strain tensor $$$$E(X) = \frac{1}{2}(F^T(X)F(X) - I)$$$$ (Side note: St. Venant's law is not always a good idea, it yields unphysical behavior for very large deformations). Now, in order to have a PDE, we need to insert the displacement field $$u(X)$$ accordingly (note the dependency on matter). Use the relation $$\chi(X) = X + u(X)$$ in $$F(X) = I + \partial u(X)/\partial X$$, which then you can put into $$E(X)$$. This yields the in-$$u$$-highly-nonlinear PDE $$\sum_{j=1}^3 \frac{\partial [(I + \partial u(X)/\partial X)(\mathbb{C}(X)\{(I + \partial u(X)/\partial X)^T(I + \partial u(X)/\partial X) - I\}/2)]_{ij}}{\partial X_j} \ , \quad i=1,\dots,3$$ This is the 3D nonlinear PDE that you should solve in the reference (initial) configuration with BC's described in terms of $$X$$.

In your PDE, are you sure that the thickness thing really takes care of the rest of the nonlinearity? Seems odd to me, but I am only familiar with the general 3D theory, not with the beam theory for large deformations.

Again, if I read your code wrong and you are using the equivalent correct theory, then I apologize of course.

In terms of physical plausibility of the model the following points are important:

• which strain tensor do they use? Green's strain is fine only for moderate strains. There are alternatives, e.g., Hencky strain.
• which constitutive law (elastic law) is used? St. Venant's law is not applicable for very large strains. There are alternatives, e.g., based on hyperelastic models (stress $$P$$ or $$S$$ derived from scalar potential).
• Could you give an example with Mathematica code to illustrate your approach to the large deformation simulation? Jul 18, 2021 at 18:19
• As far as I understand the nonlinear FEM convention in Mathematica, you have to put your nonlinear PDE in coefficient form, see, e.g., this question. Sadly, I can only provide insight into the corresponding mechanical theory for the moment. I will try to work on this next week. Jul 19, 2021 at 12:35
• Thanks for your answer. I think in essence I have roughly done what you describe. I'll go through your post carefully. If you fell more comfortable in 3D that's fine too. I am not clinging to this specific example. Ideally an example with only displacement (no loads) would rule out that the boundary conditions are not handled properly. A code example would really go a long way. Jul 19, 2021 at 12:42
• I have added what I think they are doing, but again, another example is perfectly fine. Jul 19, 2021 at 12:56
• I mean like this: Table[Inactive[ Div][-{{Subscript[\[Sigma], xx], Subscript[\[Tau], xy], Subscript[\[Tau], xz]}, {Subscript[\[Tau], xy], Subscript[\[Sigma], yy], Subscript[\[Tau], yz]}, {Subscript[\[Tau], xz], Subscript[\[Tau], yz], Subscript[\[Sigma], zz]}}[[i, ;;]], {x, y}], {i, Length[{u[x, y, z], v[x, y, z], w[x, y, z]}]}] // MatrixForm Jul 19, 2021 at 13:02

It is not 2D problem, but 3D problem. So we actually need some 3D model to describe deformation. But before do this, we can describe the problem in the beam theory. Fortunately, this problem has exact solution - see, for instance, Landau and Lifshitz Theory of Elasticity. Parameters of the problem in the beam theory

inert = .1^4/12; L = 3.2; Y = 2.10 10^11; Fx = 3.84 10^6;


Maximum angle of deflection

l[t0_?NumericQ] :=
Sqrt[.5 inert Y/Fx] NIntegrate[1/Sqrt[Cos[t] - Cos[t0]], {t, 0, t0}];
x0 = x /. FindRoot[l[x] - L == 0, {x, 3.05}]

(*Out[]= 3.0716*)


Coordinates of beam axis

y[t_] := -Sqrt[2 inert Y/Fx] (Sqrt[1 - Cos[x0]] -
Sqrt[Cos[t] - Cos[x0]]);
x[t_?NumericQ] :=
Sqrt[.5 inert Y/Fx] NIntegrate[
Cos[tet]/Sqrt[Cos[tet] - Cos[x0]], {tet, 0, t}];

pic1=ParametricPlot[{x[t], y[t]}, {t, 0, x0}, PlotRange -> All,
FrameLabel -> {"x", "y"}, AspectRatio -> 1, Frame -> True,
ColorFunction -> Hue]


Final horizontal and vertical displacement at the tip

final = {x[x0]-L, y[x0]}

{-5.04634, -1.34933}


To compare with COMSOL we look at Table 1, and there are these values refer as follows {-5.05,-1.35}. Now we prepare two functions

v0 = Interpolation[Table[{l[t], y[t]}, {t, .01, x0, .001}]];

u0 = Interpolation[Table[{l[t], x[t] - l[t]}, {t, .01, x0, .001}]];


Let consider linear theory with Dirichletian conditions only

Needs["NDSolveFEM"]

x1 = 3.2;
y1 = 1/10;
z1 = 1/10;
reg = Cuboid[{0, 0, 0}, {x1, y1, z1}]; mesh =
ToElementMesh[reg, MaxCellMeasure -> 0.00001]
mesh["Wireframe"["MeshElementStyle" -> EdgeForm[Blue]]]

ClearAll[stressOperator];
stressOperator[
Y_, ν_] := {Inactive[
Div][{{0, 0, -((Y*ν)/((1 - 2*ν)*(1 + ν)))}, {0, 0,
0}, {-Y/(2*(1 + ν)), 0, 0}} .
Inactive[Grad][w[x, y, z], {x, y, z}], {x, y, z}] +
Inactive[
Div][{{0, -((Y*ν)/((1 - 2*ν)*(1 + ν))),
0}, {-Y/(2*(1 + ν)), 0, 0}, {0, 0, 0}} .
Inactive[Grad][v[x, y, z], {x, y, z}], {x, y, z}] +
Inactive[
Div][{{-((Y*(1 - ν))/((1 - 2*ν)*(1 + ν))), 0,
0}, {0, -Y/(2*(1 + ν)), 0}, {0, 0, -Y/(2*(1 + ν))}} .
Inactive[Grad][u[x, y, z], {x, y, z}], {x, y, z}],
Inactive[Div][{{0, 0, 0}, {0,
0, -((Y*ν)/((1 -
2*ν)*(1 + ν)))}, {0, -Y/(2*(1 + ν)), 0}} .
Inactive[Grad][w[x, y, z], {x, y, z}], {x, y, z}] +
Inactive[
Div][{{0, -Y/(2*(1 + ν)),
0}, {-((Y*ν)/((1 - 2*ν)*(1 + ν))), 0, 0}, {0, 0,
0}} . Inactive[Grad][u[x, y, z], {x, y, z}], {x, y, z}] +
Inactive[
Div][{{-Y/(2*(1 + ν)), 0,
0}, {0, -((Y*(1 - ν))/((1 - 2*ν)*(1 + ν))), 0}, {0,
0, -Y/(2*(1 + ν))}} .
Inactive[Grad][v[x, y, z], {x, y, z}], {x, y, z}],
Inactive[Div][{{0, 0, 0}, {0,
0, -Y/(2*(1 + ν))}, {0, -((Y*ν)/((1 -
2*ν)*(1 + ν))), 0}} .
Inactive[Grad][v[x, y, z], {x, y, z}], {x, y, z}] +
Inactive[
Div][{{0, 0, -Y/(2*(1 + ν))}, {0, 0,
0}, {-((Y*ν)/((1 - 2*ν)*(1 + ν))), 0, 0}} .
Inactive[Grad][u[x, y, z], {x, y, z}], {x, y, z}] +
Inactive[
Div][{{-Y/(2*(1 + ν)), 0, 0}, {0, -Y/(2*(1 + ν)), 0}, {0,
0, -((Y*(1 - ν))/((1 - 2*ν)*(1 + ν)))}} .
Inactive[Grad][w[x, y, z], {x, y, z}], {x, y, z}]}
peram = {Y -> 210 10^1, ν -> 0.0};(*We don't use load, therefore scale of Y taken as arbitrary*)


Solution and visualization

{U, V, W} =
NDSolveValue[{stressOperator[Y, ν] == {0, 0, 0},
DirichletCondition[{u[x, y, z] == 0, v[x, y, z] == 0,
w[x, y, z] == 0}, x == 0],
DirichletCondition[{u[x, y, z] == u0[x], v[x, y, z] == v0[x]},
1.06 <= x <= x0],
DirichletCondition[{u[x, y, z] == -5.05, v[x, y, z] == -1.35},
x == x1]} /. peram, {u, v, w}, {x, y, z} ∈ mesh];

Show[{mesh["Wireframe"["MeshElementStyle" -> EdgeForm[Gray]]],
ElementMeshDeformation[mesh, {U, V, W}][
"Wireframe"[
"ElementMeshDirective" ->
Directive[EdgeForm[Blue], FaceForm[]]]]}, Axes -> True,
AxesLabel -> {x, y, z}]


It looks nice, but there is a message

ElementMesh::femimq: The element mesh has insufficient quality of -0.967913. A quality estimate below 0. may be caused by a wrong ordering of element incidents or self-intersecting elements.


Now we can turn to the nonlinear case. First we verifier 2D linear model

Needs["NDSolveFEM"]
length = 3.2;
hight = 1/10;
thickness = 1/10;
\[CapitalOmega] = Rectangle[{0, 0}, {length, hight}];
mesh = ToElementMesh[\[CapitalOmega]]
bmesh = ToBoundaryMesh[mesh];
(*force per area*)
fx = -3.844*10^6/(hight*thickness);
fy = fx*10^-3;
bc2 = {NeumannValue[fx, x == length], NeumannValue[fy, x == length]};
bc1 = {DirichletCondition[{u[x, y] == 0, v[x, y] == 0}, x == 0],
DirichletCondition[{u[x, y] == u0[x], v[x, y] == v0[x]},
1.06 <= x <= x0],
DirichletCondition[{u[x, y] == -5.05, v[x, y] == -1.35},
x == length]};
materialData = {nu -> 0., Y -> 210*10^9};

makeStressModel[e_] := Block[
{materialData, materialModel, exx, eyy, gxy, eVec, strain, nu, Y,
stress},
(* linear elastic plane stress model *)
materialModel =
Y/(1 - nu^2)*{{1, nu, 0}, {nu, 1, 0}, {0, 0, (1 - nu)/2}};
exx = e[[1, 1]];
eyy = e[[2, 2]];
gxy = (e[[1, 2]] + e[[2, 1]]);
(* for plane stress ezz =nu/(1-nu)*(exx+
eyy) does not play a role since nu = 0 *)
strain = {exx, eyy, gxy};
stress = (materialModel . strain);
Normal[SymmetrizedArray[
Thread[Rule[{{1, 1}, {2, 2}, {1, 2}}, stress]], Automatic,
Symmetric]]
]

(* uses global defined thickness *)
makePDEModel[stressMatrix_] :=
Table[Inactive[Div][-thickness*stressMatrix[[i, ;;]], X], {i,
Length[U]}]

X = {x, y};
U = {u[x, y], v[x, y]};
(* geometric linear *)
gut = Transpose[gu];
e = 1/2 (gu + gut);
stress = makeStressModel[e];
pde = makePDEModel[stress /. materialData];
{uif, vif} =
NDSolveValue[{pde == {0, 0}, bc1}, {u, v}, {x, y} \[Element] mesh];
pic2=Show[bmesh["Wireframe"],
deform1 =
ElementMeshDeformation[mesh, {uif, vif}, "ScalingFactor" -> 1][
"Wireframe"["MeshElementStyle" -> EdgeForm[Brown]]]]


As result we have nice picture and same message as above

ElementMesh::femimq: The element mesh has insufficient quality of -0.966859. A quality estimate below 0. may be caused by a wrong ordering of element incidents or self-intersecting elements.


Nevertheless we can compare the beam theory and linear theory combining Figure 1 and Figure 3 in one plot as Show[pic2, pic1]

It looks like a perfect agreement. No wonder since we use the beam theory solution as input in a form of bc1. Finally we try to solve nonlinear model. Note, that in this case we use the first and last points of the beam theory in bc1

Needs["NDSolveFEM"]
length = 3.2;
hight = 1/10;
thickness = 1/10;
\[CapitalOmega] = Rectangle[{0, 0}, {length, hight}];
mesh = ToElementMesh[\[CapitalOmega]]
bmesh = ToBoundaryMesh[mesh];

(*force per area*)
fx = -3.844*10^6/(hight*thickness);
fy = fx*10^-3;
bc2 = {NeumannValue[fx, x == length], NeumannValue[fy, x == length]};
bc1 = {DirichletCondition[{u[x, y] == 0, v[x, y] == 0}, x == 0],
DirichletCondition[{u[x, y] == -5.05, v[x, y] == -1.35},
x == length]};
materialData = {nu -> 0., Y -> 210*10^9};

makeStressModel[e_] := Block[
{materialData, materialModel, exx, eyy, gxy, eVec, strain, nu, Y,
stress},
(* linear elastic plane stress model *)
materialModel =
Y/(1 - nu^2)*{{1, nu, 0}, {nu, 1, 0}, {0, 0, (1 - nu)/2}};
exx = e[[1, 1]];
eyy = e[[2, 2]];
gxy = (e[[1, 2]] + e[[2, 1]]);
(* for plane stress ezz =nu/(1-nu)*(exx+
eyy) does not play a role since nu = 0 *)
strain = {exx, eyy, gxy};
stress = (materialModel . strain);
Normal[SymmetrizedArray[
Thread[Rule[{{1, 1}, {2, 2}, {1, 2}}, stress]], Automatic,
Symmetric]]
]
X = {x, y};
U = {u[x, y], v[x, y]};

(* uses global defined thickness *)
makePDEModel[stressMatrix_] :=
Table[Inactive[Div][-thickness*stressMatrix[[i, ;;]], X], {i,
Length[U]}]

(* geometric nonlinear *)
f = Grad[U, {x, y}] + IdentityMatrix[2];
e1 = 1/2 (Transpose[f] . f - IdentityMatrix[2]) // Expand // Simplify;

stress = makeStressModel[e1];
pde = makePDEModel[stress /. materialData];
{uif, vif} =
NDSolveValue[{pde == {0, 0}, bc1}, {u, v}, {x, y} \[Element] mesh];
pic3 = Show[ToBoundaryMesh[mesh]["Wireframe"],
deform2 =
ElementMeshDeformation[mesh, {uif, vif}, "ScalingFactor" -> 1][
"Wireframe"["MeshElementStyle" -> EdgeForm[Red]]]]


The picture below looks different from the linear and beam theory as well

Show[pic3, pic1]


Also we have a message

ElementMesh::femimq: The element mesh has insufficient quality of -0.00267866. A quality estimate below 0. may be caused by a wrong ordering of element incidents or self-intersecting elements.


Finally we can test the second Piola-Kirchhoff stress model on the moving grid as follows

Needs["NDSolveFEM"]
length = 3.2;
hight = 1/10;
thickness = 1/10;
\[CapitalOmega] = Rectangle[{0, 0}, {length, hight}];
mesh[0] = ToElementMesh[\[CapitalOmega]];
bmesh[0] = ToBoundaryMesh[mesh[0]];
LL[0] = Line[Take[bmesh[0][[1]], -5]];

(*force per area*)
fx = -3.844*10^6/(hight*thickness);
fy = fx*10^-3;
bc1[0] = {DirichletCondition[{u[x, y] == 0, v[x, y] == 0}, x == 0]};
materialData = {nu -> 0., Y -> 210*10^9};

makeStressModel[e_] := Block[
{materialData, materialModel, exx, eyy, gxy, eVec, strain, nu, Y,
stress},
(* linear elastic plane stress model *)
materialModel =
Y/(1 - nu^2)*{{1, nu, 0}, {nu, 1, 0}, {0, 0, (1 - nu)/2}};
exx = e[[1, 1]];
eyy = e[[2, 2]];
gxy = (e[[1, 2]] + e[[2, 1]]);
(* for plane stress ezz =nu/(1-nu)*(exx+
eyy) does not play a role since nu = 0 *)
strain = {exx, eyy, gxy};
stress = (materialModel . strain);
Normal[SymmetrizedArray[
Thread[Rule[{{1, 1}, {2, 2}, {1, 2}}, stress]], Automatic,
Symmetric]]
]

X = {x, y};
U = {u[x, y], v[x, y]};

(* uses global defined thickness *)
makePDEModel[stressMatrix_] :=
Table[Inactive[Div][-thickness*stressMatrix[[i, ;;]], X], {i,
Length[U]}]

(* geometric nonlinear *)
f = Grad[U, {x, y}] + IdentityMatrix[2];
e1 = 1/2 (Transpose[f] . f - IdentityMatrix[2]) // Expand // Simplify;
stress = makeStressModel[e1];
pde = makePDEModel[stress /. materialData]; invF = Inverse[f];
invFT = Transpose[invF];
piolaStress = 1/Det[f]*invF . stress . invFT;
pde1 = makePDEModel[piolaStress /. materialData];

n = 100; bc2[0] = {NeumannValue[fx , Element[{x, y}, LL[0]]],
NeumannValue[fy , Element[{x, y}, LL[0]]]}; Do[{uif[i], vif[i]} =
NDSolveValue[{pde1 == bc2[i - 1]/n, bc1[0]}, {u,
v}, {x, y} \[Element] mesh[i - 1]];
mesh[i] = ElementMeshDeformation[mesh[i - 1], {uif[i], vif[i]}];
bmesh[i] = ToBoundaryMesh[mesh[i]];
LL[i] = Line[Take[bmesh[i][[1]], -5]];
bc2[i] = {NeumannValue[fx , Element[{x, y}, LL[i]]],
NeumannValue[fy , Element[{x, y}, LL[i]]]};, {i, 20}]

Table[Show[ToBoundaryMesh[mesh[0]]["Wireframe"],
mesh[i]["Wireframe"["MeshElementStyle" -> EdgeForm[Red]]],
pic1], {i, 20}]


Note, this model is not correlated well with the beam theory. But it gives large deformation in iteration process with our arbitrary choice. Therefore if we add time then this model probably can be useful.

• Thank you for your answer so far, I am looking forward to the nonlinear case. Jul 15, 2021 at 6:27
• We can't solve this problem with the linear model, therefore we can't solve it with nonlinear model as well. The message we got for 3D and 2D linear model means, that there is mesh large deformation. Probably we need implement some relaxation method to solve linear problem. Jul 15, 2021 at 10:48
• This last idea of yours is an interesting approach - with this I could try to rule out that this is a boundary condition issue. Thanks. Jul 20, 2021 at 7:01