2 POISSON'S RATIO

When a material is stretched there is also contraction in the direction perpendicular to the direction of stretching. The ratio of the lateral contraction to the longitudinal extension is Poisson's ratio. For incompressible materials, Poisson's ratio is 0.5 and as rubbers are very nearly incompressible they have values close to this.

Methods have not been standardised for measuring Poisson's ratio for rubber but the most obvious approach is to use a second "extensometer" to measure the change in the width of the test piece during a normal tensile test. Since the lateral contraction is half the tensile strain and the width of dumbbells is much smaller than the gauge length, a very high performance is needed from the "extensometer" to achieve sensible accuracy. Not surprisingly, accurate measurements have proved very difficult to obtain.

Alternative procedures are to use a dilatometer to measure volume change or to calculate Poisson's ratio from measurement of two moduli. Laufer et al ^[5] concluded that for soft rubbers the dilatometer is the best approach and describe a suitable apparatus. Kugler et al ^[6] have given a review of attempts at measurement and describe an optical system which they employed on a series of filled rubbers. It would seem reasonable that modern instrumentation, such as a video extensometer, could be used but this does not appear to have been reported.

Perhaps the most reliable approach is to obtaining Poisson's ratio is to calculate it from bulk and Young's or shear moduli...