Strain (materials science)

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**Continuum mechanics**
General
Classical mechanics Stress / Strain Tensor Conservation of mass Conservation of momentum
Solid mechanics
Solids Elasticity Plasticity Hooke's law Poisson's ratio Rheology
Fluid mechanics
Fluids Fluid statics Fluid dynamics Navier-Stokes equations Viscosity Newtonian fluids Non-Newtonian fluids

This article is about the deformation of materials. For other meanings, see strain.

In any branch of science dealing with materials and their behaviour, strain is the geometrical expression of deformation caused by the action of stress on a physical body. Strain is calculated by first assuming a change between two body states: the beginning state and the final state. Then the difference in placement of two points in this body in those two states expresses the numerical value of strain. Strain therefore expresses itself as a change in size and/or shape.

If strain is equal over all parts of a body, it is referred to as homogeneous strain; otherwise, it is inhomogeneous strain. In its most general form, the strain is a symmetric tensor.

In the case of geological action of the earth, if the release of stress through strain in rocks is sufficiently large, earthquakes may occur.

1 Quantifying strain
2 Linear axial strain at single point
3 The general case of linear strain
4 Shear strain
5 Volumetric strain
6 Strain in tensor notation
7 Principal strains in two dimensions
8 The case of big deformations
9 Engineering strain vs. true strain
10 See also
11 External links

[edit] Quantifying strain

Given that strain results in the deformation of a body, it can be measured by calculating the change in length of a line or by the change in angle between two lines (where these lines are theoretical constructs within the deformed body). The change in length of a line is termed the stretch, absolute strain, or extension, and may be written as $\delta \ell$ . Then the (relative) strain, $ε$ , is given by

$\varepsilon = \frac {\delta \ell}{\ell_0}$

where

$\varepsilon$ is strain in measured direction

$\ell_o$ is the original length of the material.

$\ell$ is the current length of the material.

The extension ( $\delta \ell$ ) is positive if the material has gained length (in tension) and negative if it has reduced length (in compression). Because $\ell_o$ is always positive, the sign of the strain is always the same as the sign of the extension.

Strain has no units of measure because in the formula the units of length are cancelled. Dimensions of metres/metre or inches/inch are sometimes used for convenience, but generally units are left off and the strain sometimes is given as a percentage.

[edit] Linear axial strain at single point

In the case of measuring strain in the selected point of the body, it is expressed as a strain where the distance $\ell$ between two points approaches zero:

$\varepsilon = \mathop {\lim_{\ell \to 0}} \frac {{\delta} {\ell} } {\ell}$

where

$\varepsilon$ is strain in measured direction

${{\delta} {\ell} }$ is the length difference for current length $\ell$ .

$\ell$ is the current length of the material, which approaches zero.

Therefore linear strain is defined as change of distance in the close proximity of selected point.

[edit] The general case of linear strain

For the body of any shape, subjected to any deformation the values of strain will be different depending on the spatial direction of measurement. Considering the linear deformation in the point A placed at the start of coordinate system and a second point B placed along the x axis, which due to deformation has moved to the point B' the linear strain will be expressed as:

$\varepsilon_x = \mathop {\lim_{B \to A}}{{|AB'|-|AB|} \over {|AB|}}$

Doing similar calculations for axes y and z respective values of ε_y and ε_z can be obtained. For any given displacement field $\overrightarrow u$ (the values of displacement vectors for all points in the body) the linear strain can be written as:

$\varepsilon_x = {{\partial u_x} \over {\partial x}}$ ; $\varepsilon_y = {{\partial u_y} \over {\partial y}}$ ; $\varepsilon_z = {{\partial u_z} \over {\partial z}}$

where

$\varepsilon_i$ is strain in direction along axis i

${{\partial u_i} \over {\partial i}}$ is a differential of $\overrightarrow u$ at any point in the direction along axis i

[edit] Shear strain

Similarly the angular change at any point between two lines crossing this point in a body can be measured as a shear (or shape) strain. Shear strain γ is the limit of ratio of angular difference between any two lines in a body before and after deformation, assuming that the lines lengths are approaching zero. Given a displacement field $\overrightarrow u$ like above, the shear strain can be written as follows:

$\gamma_{xy} = {{\partial u_x} \over {\partial y}} + {{\partial u_y} \over {\partial x}}$ ; $\gamma_{yz} = {{\partial u_y} \over {\partial z}} + {{\partial u_z} \over {\partial y}}$ ; $\gamma_{xz} = {{\partial u_x} \over {\partial z}} + {{\partial u_z} \over {\partial x}}$

[edit] Volumetric strain

Although linear strain ε and shear strain γ completely defines the state of deformation of a body, it is also possible to measure other characteristic strain values, like for example volumetric strain, which measures the ratio of change of body's volume. The definition of volumetric strain at selected point is:

$\vartheta = \lim_{V^{(0)} \to 0}{V - V^{(0)} \over {V^{(0)}}}$

where

$\vartheta$ is volumetric strain

V (0)

is initial volume

V

is final volume

For cartesian coordinate system following expression is always true:

$\vartheta = \varepsilon_x + \varepsilon_y + \varepsilon_z$

where

$\vartheta$ is volumetric strain

$\varepsilon_x , \varepsilon_y , \varepsilon_z$ are strains along x, y and z axis

[edit] Strain in tensor notation

Using above notation for linear and shear strain it is possible to express strain as a strain tensor

$\varepsilon_{ij} = {1 \over 2} \left({\nabla_i u_j + \nabla_j u_i}\right)$

using indicial notation or using vector notation:

$\varepsilon = {1 \over 2} ( \vec{\nabla}\vec{u} + (\vec{\nabla}\vec{u})^T)$

Comparing traditional notation with tensor notation following is obtained for cartesian coordinate system:

$\varepsilon_{ij}= \left[{\begin{matrix} {\varepsilon _x } & \frac {\gamma _{xy} } {2} & \frac {\gamma _{xz} } {2} \\ \frac {\gamma _{xy} } {2} & {\varepsilon _y } & \frac {\gamma _{yz} } {2} \\ \frac {\gamma _{xz} } {2} & \frac {\gamma _{yz} } {2} & {\varepsilon _z } \end{matrix}}\right]$

Then volumetric strain equals:

$\vartheta = \varepsilon_{ij}g^{ij}$

where g^ij is a contravariant metric tensor (using tensor notation: $\vartheta = tr(\varepsilon)$ )

[edit] Principal strains in two dimensions

Because the strain tensor is a real symmetric matrix, by singular value decomposition it can be represented as a set of orthogonal eigenvectors, directions along which there is no shear, only stretching or compression.

Assuming the two dimensional strain tensor given as:

$\varepsilon_{ij}= \left[{\begin{matrix} {\varepsilon _x } & {\frac {\gamma _{xy}} {2}} \\ {\frac {\gamma _{xy}} {2}} & {\varepsilon _y } \\ \end{matrix}}\right]$

Then principal strains $\varepsilon _1, \varepsilon _2$ are equal to:

$\varepsilon _1 = \frac {\varepsilon _x + \varepsilon _ y}{2} + \sqrt{ \left( \frac {\varepsilon _x - \varepsilon _y}{2} \right)^2 + \left( \frac{\gamma _{xy}} {2}\right)^2 }$

$\varepsilon _2 = \frac {\varepsilon _x + \varepsilon _ y}{2} - \sqrt{ \left( \frac {\varepsilon _x - \varepsilon _y}{2} \right)^2 + \left( \frac{\gamma _{xy}} {2}\right)^2 }$

[edit] The case of big deformations

Above reasoning assumes that body is subject to small deformations. It must be rememberred that with increasing deformation the linear strain error increases. For big deformations the strain tensor can be written as:

$\varepsilon_{ij} = {1 \over 2}({g_{ij}-g_{ij}^{(0)}})$

where

g_ij is the metric tensor of body after deformation

g_ij⁽⁰⁾ is metric tensor of the undeformed body

[edit] Engineering strain vs. true strain

In the definition of linear strain (known technically as engineering strain), strains cannot be totaled. Imagine that a body is deformed twice, first by $\delta \ell_1$ and then by $\delta \ell_2$ (cumulative deformation). The final strain

$\epsilon = \frac{\delta \ell_1 + \delta \ell_2}{\ell_0}$

is slightly different from the sum of the strains:

$\epsilon_1 = \frac{\delta \ell_1}{\ell_0}$

and

$\epsilon_2 = \frac{\delta \ell_2}{\ell_0 + \delta \ell_1}$

As long as $\delta \ell_1 \ll \ell_0$ , it is possible to write:

$\epsilon_2 \simeq \frac{\delta \ell_2}{\ell_0}$

and thus

$\epsilon \simeq \epsilon_1 \; + \epsilon_2 \;$

True strain (aka natural strain and logarithmic strain), however, can be totaled. This is defined by:

$\exp(\epsilon) = \frac{\ell_f}{\ell_0}$

and thus

$\epsilon = \ln \left (\frac{\ell_f}{\ell_0} \right )$

The engineering strain formula is the series expansion of the true strain formula.

[edit] See also

[edit] External links

Strain types listed: engineering, true, logarithmic and lagrange

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Strain (materials science)

From Wikipedia, the free encyclopedia

Contents

[edit] Quantifying strain

[edit] Linear axial strain at single point

[edit] The general case of linear strain

[edit] Shear strain

[edit] Volumetric strain

[edit] Strain in tensor notation

[edit] Principal strains in two dimensions

[edit] The case of big deformations

[edit] Engineering strain vs. true strain

[edit] See also

[edit] External links

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