# Introduction to Elasticity/Kinematics example 2

## Example 2

Given: A body occupies the unit cube ${\displaystyle X_{i}\in [0,1]}$ in the reference configuration. The mapping between the current and the reference configuration given by ${\displaystyle x_{1}=X_{1}+\kappa ~X_{2}}$, ${\displaystyle x_{2}=X_{2}\,}$, ${\displaystyle x_{3}=X_{3}\,}$.

Find:

1. Sketch current configuration.
2. Show that motion is isochoric.
3. Find stretches in the directions ${\displaystyle {\widehat {\mathbf {e} }}_{1}}$, ${\displaystyle {\widehat {\mathbf {e} }}_{2}}$, ${\displaystyle (1/{\sqrt {2}})({\widehat {\mathbf {e} }}_{1}+{\widehat {\mathbf {e} }}_{2})}$, and ${\displaystyle (1/{\sqrt {2}})({\widehat {\mathbf {e} }}_{1}-{\widehat {\mathbf {e} }}_{2})}$.
4. Find the orthogonal shear strain between the reference material directions ${\displaystyle {\widehat {\mathbf {e} }}_{1}}$ and ${\displaystyle {\widehat {\mathbf {e} }}_{2}}$. Also between directions ${\displaystyle (1/{\sqrt {2}})({\widehat {\mathbf {e} }}_{1}+{\widehat {\mathbf {e} }}_{2})}$ and ${\displaystyle (1/{\sqrt {2}})({\widehat {\mathbf {e} }}_{1}-{\widehat {\mathbf {e} }}_{2})}$.
5. Find principal stretches and principal directions of stretch (${\displaystyle \kappa =0.4\,}$).

### Solution:

 Deformed shape.
• All parallelograms that have the equal heights and the same base have equal areas. Hence, there is no volume change in this deformation. Hence isochoric.
• Stretches in the material direction ${\displaystyle {\widehat {\mathbf {G} }}}$ are given by
${\displaystyle \lambda ({\widehat {\mathbf {G} }})={\sqrt {{\widehat {\mathbf {G} }}:{\boldsymbol {C}}:{\widehat {\mathbf {G} }}}}}$

where ${\displaystyle {\boldsymbol {C}}\,}$ is the Cauchy-Green deformation tensor

${\displaystyle {\boldsymbol {C}}={\boldsymbol {F}}^{T}\bullet {\boldsymbol {F}}}$

We will use Maple to calculate the stretches in the four directions.

      with(linalg):
x := array(1..3): X := array(1..3):
x[1] := X[1] + k*X[2]: x[2] := X[2]: x[3] := X[3]:
F := linalg[matrix](3,3):
for i from 1 to 3 do
for j from 1 to 3 do
F[i,j] := diff(x[i],X[j]);
end do;
end do;
evalm(F);
C := evalm(transpose(F)&*F);
e1 := linalg[matrix](1,3,[1,0,0]):
e2 := linalg[matrix](1,3,[0,1,0]):
e1pe2 := evalm((e1 + e2)/sqrt(2)):
e1me2 := evalm((e1-e2)/sqrt(2)):''
lambda[1] := sqrt(evalm(evalm(e1&*C)&*transpose(e1))[1,1]);
lambda[2] := sqrt(evalm(evalm(e2&*C)&*transpose(e2))[1,1]);
lambda[3] := simplify(sqrt(evalm(evalm(e1pe2&*C)&*transpose(e1pe2))[1,1]));
lambda[4] := simplify(sqrt(evalm(evalm(e1me2&*C)&*transpose(e1me2))[1,1]));

${\displaystyle F:={\begin{bmatrix}1&k&0\\0&1&0\\0&0&1\end{bmatrix}}}$
${\displaystyle C:={\begin{bmatrix}1&k&0\\k&k^{2}+1&0\\0&0&1\end{bmatrix}}}$
${\displaystyle \lambda _{1}:=1\,}$
${\displaystyle \lambda _{2}:={\sqrt {k^{2}+1}}}$
${\displaystyle \lambda _{3}:={\frac {\sqrt {4+4k+2k^{2}}}{2}}}$
${\displaystyle \lambda _{4}:={\frac {\sqrt {4-4k+2k^{2}}}{2}}}$

The following figure shows that the calculated stretches are correct.

 Stretches
• The orthogonal shear strain between two orthogonal units vectors ${\displaystyle {\widehat {\mathbf {G} }}_{1}}$ and ${\displaystyle {\widehat {\mathbf {G} }}_{2}}$ in the reference material co-ordinate system is given by
${\displaystyle \gamma ({\widehat {\mathbf {G} }}_{1},{\widehat {\mathbf {G} }}_{2})=\sin ^{-1}\left({\frac {{\widehat {\mathbf {G} }}_{1}:\mathbf {C} :{\widehat {\mathbf {G} }}_{2}}{\lambda ({\widehat {\mathbf {G} }}_{1})\lambda ({\widehat {\mathbf {G} }}_{2})}}\right)}$

Once again, we will use Maple to calculate these strains. The steps are the same upto the calculation of the stretches.

      numer1 := evalm(evalm(e1&*C)&*transpose(e2))[1,1]:
denom1 := lambda[1]*lambda[2]:
ratio1 := numer1/denom1:
gam1 := arcsin(ratio1);
numer2 := simplify(evalm(evalm(e1pe2&*C)&*transpose(e1me2))[1,1]):
denom2 := lambda[3]*lambda[4]:
ratio2 := numer2/denom2:
gam2 := arcsin(ratio2);

${\displaystyle \gamma _{1}:=\sin ^{-1}({\frac {k}{\sqrt {k^{2}+1}}})}$
${\displaystyle \gamma _{2}:=-\sin ^{-1}({\frac {2k^{2}}{{\sqrt {4+4k+2k^{2}}}{\sqrt {4-4k+2k^{2}}}}})}$

The following figure shows that the calculated orthogonal shear strains are correct.

 Orthogonal shear strains

The value of ${\displaystyle \gamma _{1}\,}$ can easily be verified using the definition of ${\displaystyle \sin {\gamma }\,}$. For the verification of the value of ${\displaystyle \gamma _{2}\,}$, the cosine law

${\displaystyle \cos(A)={\frac {b^{2}+c^{2}-a^{2}}{2bc}}\,}$

has to be used. Note that the actual length of the sides ${\displaystyle b\,}$ and ${\displaystyle c\,}$ of the triangle is obtained after multiplying the values shown in the figure by the length of the diagonal (${\displaystyle {\sqrt {2}}}$). Hence, the calculated values are correct.

      b := sqrt(2)/2*lambda[4]:
c := sqrt(2)/2*lambda[3]:
a := 1:
A := (b^2 + c^2 - a^2)/(2*b*c);

${\displaystyle A:={\frac {2k^{2}}{{\sqrt {4+4k+2k^{2}}}{\sqrt {4-4k+2k^{2}}}}}}$
• The principal stretches are given by the square roots of the eigenvalues of ${\displaystyle {\boldsymbol {C}}}$. The principal directions are the eigenvectors of ${\displaystyle {\boldsymbol {C}}}$.

Once again, we use Maple for our calculations.

      CC := linalg[matrix](3,3):
for i from 1 to 3 do
for j from 1 to 3 do
CC[i,j] := eval(C[i,j], k=0.4);
end do;
end do;
evalm(CC);
eigvals := eigenvals(CC);
PrinStretch[1] := sqrt(eigvals[1]);
PrinStretch[2] := sqrt(eigvals[2]);
PrinStretch[3] := sqrt(eigvals[3]);
PrinDir := eigenvects(CC);

${\displaystyle CC:={\begin{bmatrix}1&0.4&0\\0.4&1.16&0\\0&0&1\end{bmatrix}}}$
${\displaystyle {\text{eigvals}}:=0.67,1.0,1.49}$
${\displaystyle {\text{PrinStretch}}[1]:=0.82~,~~{\text{PrinStretch}}[2]:=1.0~,~~{\text{PrinStretch}}[3]:=1.22}$
${\displaystyle {\text{PrinDir}}[1]:=[0.77,-0.63,0.]~;~~{\text{PrinDir}}[2]:=[0,0,1]~;~~{\text{PrinDir}}[3]:=[0.63,0.77,0.]}$