Tan delta

The tangent of the phase angle (= delta) of the sample.  

Similar to phase angle this it is a relative measure of the viscous and elastic properties of a material.

It ranges from 0 for a fully elastic material to infinity for a fully viscous material, with a value of 1 where the storage modulus = Viscous modulusThe complex modulus (viscous component), loss modulus, or G’’, is the “imaginary” part of the samples the overall complex modulus. This viscous component indicates the liquid like, or out of phase, response of the sample being measurement. loss modulus (see Cross over pointIn rheological test such as a frequency sweep or a time/temperature sweep, the cross over point is a convenient point of reference to indicate a “transition” point of the sample.Cross over).

Technically, it is the ratio of the loss modulus over the storage modulus, and hence indicates the tendency of a material to dissipate energy (i.e., more viscous-like) or to store energy (i.e., more elastic-like).

Tan δ = G''/G'