Tensile Tests of Vulcanized Rubber at High Speed

Abstract The types of apparatus most frequently used for elongation tests of rubber have only a slow speed. Thus the maximum rate of elongation of the Schopper dynamometer is 120 cm. per minute and in ordinary practice 60 cm. per minute is the speed used. The normal rate ordinarily used in America with the Scott apparatus is 20 in. per minute. On the other hand, it has been found by experience that slightly vulcanized rubber which, on the Schopper machine, gives very good values of tensile strength and resilient energy at rupture, even excellent values, is on the contrary brittle when stretched very rapidly by hand and breaks with a very low elongation. Consequently it is of interest, chiefly from the point of view of a study of rubber, to carry out elongation tests at high speed. As is easily seen from the technical literature, this problem has been neglected. At the Bureau of Standards the influence of the rate of elongation on the results of tensile tests has already been studied, but the rate varied only between five and forty-five inches per minute. Briefly, the results of this study seem to indicate a general tendency for the elongation and resistance to extension to increase with increase in speed of elongation. Hauser and Rosbaud have also made tests on the influence of the speed of elongation, but their tests were on raw rubber and at extremely slow speeds. As for the brittleness which appears at high rates of elongation, only very casual references are to be found in the literature. By a study of gas black Park has proved that the maximum physical properties found by hand-testing do not agree with the maximum obtained with the Scott dynamometer. Recently Wright5 has also pointed out the “shortness” of vulcanized rubber and has emphasized the importance of tensile tests at high speed. Unfortunately, we did not have at our disposal any apparatus which permitted recording the stress-strain curve at high speed and reading the ultimate elongation and tensile strength. Accordingly, we used in our tests a Charpy ram pendulum, belonging to the section of metallography of the Polytechnical School of Delft. It is known that this apparatus as well as others similar to it are used in determining the energy necessary to rupture various materials. This apparatus was changed and on the two supporting surfaces were attached two pins in such a way that a rubber ring could be placed there without elongation as seen in Fig. 1. This ring was then broken with the ram pendulum. The greater the energy expended in breaking the ring, the less the pendulum rises. As is known, the energy expended in breaking the ring is calculated by the aid of a numerical table by comparison with an oscillation without rupture of the ring. For each type of vulcanizate, at least six rings were tested by this pendulum method. In addition some rings from the same vulcanizates were given the usual tensile test on the Schopper dynamometer. From the resulting curve the energy of rupture can be determined in kilograms in the usual manner by the aid of a planimeter.