Shear modulus

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:
 * $$G \ \stackrel{\mathrm{def}}{=}\  \frac {\sigma} {\tau} =  \frac{F/A}{\Delta x/h} = \frac{F h}{\Delta x A}$$

where
 * $$\sigma = F/A \,$$ = shear stress;
 * $$F$$ is the force which acts
 * $$A$$ is the area on which the force acts
 * $$\tau = \Delta x/h \,$$ = shear strain;
 * $$\Delta x$$ is the transverse displacement
 * $$h$$ is the initial length

Shear modulus is usually measured in GPa (gigapascals) or ksi (thousands of pounds per square inch).

Explanation
The shear modulus is one of several quantities for measuring the strength of materials. All of them arise in the generalized Hooke's law:
 * Young's modulus describes the material's response to linear strain (like pulling on the ends of a wire),
 * the bulk modulus describes the material's response to uniform pressure, and
 * the shear modulus describes the material's response to shearing strains.

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 and paper exhibit differing material response to stress or strain when tested in different directions. In this case, when the deformation is small enough so that the deformation is linear, the elastic moduli, including the shear modulus, will then be a tensor, rather than a single scalar value.



Sound waves
In homogeneous and isotropic solids, there are two kinds of sound waves, pressure waves and shear waves. The speed of sound for shear waves, $$v_s$$ is controlled by the shear modulus,
 * $$v_s = \sqrt{\frac {G} {\rho} }$$

where
 * G is the shear modulus
 * $$\rho$$ is the solid's density.