Dictionary:Elastic constants, elastic moduli
Elasticity deals with deformations that vanish entirely upon removal of the stresses that cause them. For small deformations, Hooke's law holds and strain is proportional to stress. The passage of a low-amplitude seismic wave is an example. The general elasticity tensor relating stress and strain can be expressed as a matrix equation (Figure H-7). In anisotropic media this tensor possesses up to 18–21 independent constants. In transversely isotropic media (where properties are the same measured in two orthogonal directions but different in the third), these reduce to five independent constants (see also Thomsen parameters). Isotropic media (where properties are the same measured in any direction) have only two independent elastic constants. The stress-strain properties of isotropic materials that obey Hooke’s law are specified by elastic moduli (Figure E-6).