Editing Elastic modulus (section)

== Types of elastic modulus ==
Specifying how stress and strain are to be measured, including directions, allows for many types of elastic moduli to be defined. The four primary ones are:
# ''[[Young's modulus]]'' (<var>E</var>) describes tensile and compressive [[Elasticity (physics)|elasticity]], or the tendency of an object to deform along an axis when opposing forces are applied along that axis; it is defined as the ratio of [[tensile stress]] to [[Deformation (mechanics)#Strain|tensile strain]]. It is often referred to simply as the ''elastic modulus''.
# The ''[[shear modulus]]'' or ''modulus of rigidity'' (<var>G</var> or <math>\mu \,</math>Lamé second parameter) describes an object's tendency to shear (the deformation of shape at constant volume) when acted upon by opposing forces; it is defined as [[shear stress]] over [[shear strain]].  The shear modulus is part of the derivation of [[viscosity]].
# The ''[[bulk modulus]]'' (<var>K</var>) describes volumetric elasticity, or the tendency of an object to deform in all directions when uniformly loaded in all directions; it is defined as [[Stress (physics)#Stress deviator tensor|volumetric stress]] over volumetric strain, and is the inverse of [[compressibility]]. The bulk modulus is an extension of Young's modulus to three dimensions.
# ''[[Flexural modulus]]'' (''E''<sub>flex</sub>) describes the object's tendency to flex when acted upon by a [[Moment (physics)|moment]].
Two other elastic moduli are [[Lamé's first parameter]], <var>λ,</var> and [[P-wave modulus]], ''M'', as used in table of modulus comparisons given below references. Homogeneous and [[isotropic]] (similar in all directions) materials (solids) have their (linear) elastic properties fully described by two elastic moduli, and one may choose any pair.  Given a pair of elastic moduli, all other elastic moduli can be calculated according to formulas in the table below at the end of page.

[[Fluids]] at rest are special in that they cannot support shear stress, meaning that the shear modulus is always zero. This also implies that Young's modulus for this group is always zero. When moving relative to a solid surface a fluid will experience shear stresses adjacent to the surface, giving rise to the phenomenon of [[viscosity]].

In some texts, the modulus of elasticity is referred to as the ''elastic constant'', while the inverse quantity is referred to as ''elastic modulus''.