Crack velocities in natural rubber

Measurements are reported of the limiting velocity of a running crack in biaxially stretched sheets of unfilled and carbon-black-filled natural rubber. The crack velocity was found to increase with the cleavage strain,e_y, and also with the strain,e_x, parallel to the direction of tearing, reaching values of over 100 m sec^–1 at the highest strain levels employed. These crack velocities are shown to be close to those predicted by Mott's theory, i.e., about one-third of the velocity of sound, when a strong strain-dependence is recognized for the velocity of sound in rubber.