The strain tensor [ε] is a symmetric tensor used to quantify the strain of an object undergoing a 3-dimensional deformation:

• the diagonal coefficients ε_ii are the relative change in length in the direction of the i direction (along the x_i-axis) ;
• the other terms ε_ij (i ≠ j) are the γ, half the variation of the right angle (assuming a small cube of mattter before deformation).

The deformation of an object is defined by a tensor field, i.e. this strain tensor is defined for every point of the object. This field is linked to the field of stress tensor by the generalized Hooke's law.

In case of small deformations, the strain tensor is the Green tensor, defined by the equation:

Where u represents the displacement field of the object's configuration (i.e. the difference between the object's configuration and its natural state). This is the 'symmetric part' of the Jacobian matrix.

Demonstration in simple cases

One-dimensional elongation

When the [AB] segment, parallel to the x₁-axis, is deformed to become the [A'B' ] segment, the deformation being also parallel to x₁

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Deformation_lineaire_et_deplacement.png
Image:Deformation lineaire et deplacement.png

the ε₁₁ strain is (expressed in algebraic length):

<math>\varepsilon_{11} = \frac{\Delta l}{l_0} = \frac{\overline{A'B'}-\overline{AB}}{\overline{AB}}<math>

Considering that

<math>\overline{AA'} = u_1(A)<math> and <math>\overline{BB'} = u_1(B)<math>

the strain is

<math>\varepsilon_{11} = \frac{\overline{A'A} + \overline{AB} + \overline{BB'}}{\overline{AB}} - 1<math>

<math>\varepsilon_{11} = \frac{u_1(B)-u_1(A) + \overline{AB}}{\overline{AB}} - 1<math>

The series expansion of u₁ is

<math>u_1(B) \simeq u_1(A) + \frac{\partial u_1}{\partial x_1} \cdot \overline{AB}<math>

and thus

<math>\varepsilon_{11} = \frac{\partial u_1}{\partial x_1}<math>

And in general

<math>\varepsilon_{ii} = \frac{\partial u_i}{\partial x_i} = \frac{1}{2} \left ( \frac{\partial u_i}{\partial x_i} + \frac{\partial u_i}{\partial x_i} \right )<math>

Pure shear strain

Let us now consider a pure shear strain. An ABCD square, where [AB] is parallel to x₁ and [AD] is parallel to x₂, is transformed into a AB'C'D' rhombus, symmetric to the first bisecting line.

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Cisaillement_et_deplacement.png
Image:Cisaillement et deplacement.png

The tangent of the γ angle is:

<math>\tan(\gamma) = \frac{\overline{BB'}}{\overline{AB}}<math>

for small deformations,

<math>\tan(\gamma) \simeq \gamma<math>

and

<math>\overline{BB'} = u_2(B) \simeq u_2(A) + \frac{\partial u_2}{\partial x_1} \cdot \overline{AB}<math>

and u₂(A) = 0. Thus,

<math>\gamma \simeq \frac{\partial u_2}{\partial x_1}<math>

Considering now the [AD] segment:

<math>\gamma \simeq \frac{\partial u_1}{\partial x_2}<math>

and thus

<math>\gamma = \varepsilon_{12} = \frac{1}{2} \left ( \frac{\partial u_1}{\partial x_2} + \frac{\partial u_2}{\partial x_1} \right )<math>

It is interesting to use the average because the formula is still valid when the rhombus rotates; in such a case, there are two different angles <math>\gamma_B = \widehat{B'AB}<math> et <math>\gamma_D = \widehat{D'AD}<math>.

Relative variation of the volume

The relative variation of the volume ΔV/V₀ is the trace of the tensor :

<math>\frac{\Delta V}{V_0} = \varepsilon_{11} + \varepsilon_{22} + \varepsilon_{33}<math>

Actually, if we considere a cube with an edge length a, it is a quasi-cube after the deformation (the variations of the angles do not change the volume) with the dimensions <math>a \cdot (1 + \varepsilon_{11}) \times a \cdot (1 + \varepsilon_{22}) \times a \cdot (1 + \varepsilon_{33})<math> and V₀ = a³, thus

<math>\frac{\Delta V}{V_0} = \frac{\left ( 1 + \varepsilon_{11} + \varepsilon_{22} + \varepsilon_{33} + \varepsilon_{11} \cdot \varepsilon_{22} + \varepsilon_{11} \cdot \varepsilon_{33}+ \varepsilon_{22} \cdot \varepsilon_{33} + \varepsilon_{11} \cdot \varepsilon_{22} \cdot \varepsilon_{33} \right ) \cdot a^3 - a^3}{a^3}<math>

as we considere small deformations,

<math>1 >> \varepsilon_{ii} >> \varepsilon_{ii} \cdot \varepsilon_{jj} >> \varepsilon_{11} \cdot \varepsilon_{22} \cdot \varepsilon_{33} <math>

therefore the formula.

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Approximation_volume_deformation.png



Real variation of volume (top) and the approximated one (bottom): the green drawing shows the estimated volume and the orange drawing the neglected volume

In case of pure shear, we can see that there is no change of the volume.

fr:Tenseur des déformations