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Shear strain

In materials science, shear modulus or modulus of rigidity, denoted by G, or sometimes S or μ, is defined as the ratio of shear stress to the shear strain:

$G\ {\stackrel {\mathrm {def} }{=}}\ {\frac {\tau _{xy}}{\gamma _{xy}}}={\frac {F/A}{\Delta x/l}}={\frac {Fl}{A\Delta x}}$

where

$\tau _{xy}=F/A\,$ = shear stress;

$F$ is the force which acts

$A$ is the area on which the force acts

in engineering, $\gamma _{xy}=\Delta x/l=\tan \theta \,$ = shear strain. Elsewhere, $\gamma _{xy}=\theta$

$\Delta x$ is the transverse displacement

$l$ is the initial length

Shear modulus' derived SI unit is the pascal (Pa), although it is usually expressed in gigapascals (GPa) or in thousands of pounds per square inch (ksi). Its dimensional form is M¹L^-1T⁻².

The shear modulus is always positive.

Explanation

Material	Typical values for shear modulus (GPa) (at room temperature)
Diamond	478.0
Steel	79.3
Copper	44.7
Titanium	41.4
Glass	26.2
Aluminium	25.5
Polyethylene	0.117
Rubber	0.0006

The shear modulus is one of several quantities for measuring the stiffness of materials. All of them arise in the generalized Hooke's law:

• Young's modulus describes the material's response to linear stress (like pulling on the ends of a wire or putting a weight on top of a column),
• the bulk modulus describes the material's response to uniform pressure (like the pressure at the bottom of the ocean or a deep swimming pool)
• the shear modulus describes the material's response to shear stress (like cutting it with dull scissors).

The shear modulus is concerned with the deformation of a solid when it experiences a force parallel to one of its surfaces while its opposite face experiences an opposing force (such as friction). In the case of an object that's shaped like a rectangular prism, it will deform into a parallelepiped. Anisotropic materials such as wood, paper and also essentially all single crystals exhibit differing material response to stress or strain when tested in different directions. In this case one may need to use the full tensor-expression of the elastic constants, rather than a single scalar value.

One possible definition of a fluid is material with zero shear modulus.

Waves

File:SpiderGraph ShearModulus.GIF

Influences of selected glass component additions on the shear modulus of a specific base glass.

In homogeneous and isotropic solids, there are two kinds of waves, pressure waves and shear waves. The velocity of a shear wave, $(v_{s})$ is controlled by the shear modulus,

$v_{s}={\sqrt {\frac {G}{\rho }}}$

where

G is the shear modulus

$\rho$ is the solid's density.

Shear modulus of metals

Shear modulus of copper as a function of temperature. The experimental data are shown with colored symbols.

The shear modulus of metals is usually observed to decrease with increasing temperature. At high pressures, the shear modulus also appears to increase with the applied pressure. Correlations between the melting temperature, vacancy formation energy, and the shear modulus have been observed in many metals.

Several models exist that attempt to predict the shear modulus of metals (and possibly that of alloys). Shear modulus models that have been used in plastic flow computations include:

1. the MTS shear modulus model developed by and used in conjunction with the Mechanical Threshold Stress (MTS) plastic flow stress model.
2. the Steinberg-Cochran-Guinan (SCG) shear modulus model developed by and used in conjunction with the Steinberg-Cochran-Guinan-Lund (SCGL) flow stress model.
3. the Nadal and LePoac (NP) shear modulus model that uses Lindemann theory to determine the temperature dependence and the SCG model for pressure dependence of the shear modulus.

MTS shear modulus model

The MTS shear modulus model has the form:

$\mu (T)=\mu _{0}-{\frac {D}{\exp(T_{0}/T)-1}}$

where µ₀ is the shear modulus at 0 K, and D and T₀ are material constants.

SCG shear modulus model

The Steinberg-Cochran-Guinan (SCG) shear modulus model is pressure dependent and has the form

$\mu (p,T)=\mu _{0}+{\frac {\partial \mu }{\partial p}}{\frac {p}{\eta ^{1/3}}}+{\frac {\partial \mu }{\partial T}}(T-300);\quad \eta :=\rho /\rho _{0}$

where, µ₀ is the shear modulus at the reference state (T = 300 K, p = 0, η = 1), p is the pressure, and T is the temperature.

NP shear modulus model

The Nadal-Le Poac (NP) shear modulus model is a modified version of the SCG model. The empirical temperature dependence of the shear modulus in the SCG model is replaced with an equation based on Lindemann melting theory. The NP shear modulus model has the form:

$\mu (p,T)={\frac {1}{{\mathcal {J}}({\hat {T}})}}\left[\left(\mu _{0}+{\frac {\partial \mu }{\partial p}}{\cfrac {p}{\eta ^{1/3}}}\right)(1-{\hat {T}})+{\frac {\rho }{Cm}}~k_{b}~T\right];\quad C:={\cfrac {(6\pi ^{2})^{2/3}}{3}}f^{2}$

where

${\mathcal {J}}({\hat {T}}):=1+\exp \left[-{\cfrac {1+1/\zeta }{1+\zeta /(1-{\hat {T}})}}\right]\quad {\text{for}}\quad {\hat {T}}:={\frac {T}{T_{m}}}\in [0,1+\zeta ],$

and µ₀ is the shear modulus at 0 K and ambient pressure, ζ is a material parameter, k_b is the Boltzmann constant, m is the atomic mass, and f is the Lindemann constant.

See also

• Shear strength
• Dynamic modulus
• Impulse excitation technique

References

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