Dynamic behaviour of rubber during moderate extensions

The stresses and energy losses during simple extension cycles to about 130% maximum elongation have been determined for unfilled vulcanizates of natural rubber and acrylonitrile butadiene rubber at various rates of extension and temperatures. The results obey the Ferry transform, with the exception of natural rubber at the larger strains near the high temperature end of the experimental range. The stress-strain curves at increasing strain cannot be described as the product of one strain function and one time function, but the two-term Mooney equation with time dependent coefficients represents the results over a large strain range. Maxima in the strain-increasing stress curves have been observed which are not connected with necking. A suggestion is made as to why the negative slopes in these curves need not necessarily lead to instability.