The Tear Strength of Vulcanizates

Abstract

Abstract 1) The force acting upon the jaws of the tear testing machine is always measured in tear strength tests and its value serves as a criterion of the tear resistance. This force, measured at the instant of completion of the tear, is made up of the force necessary for tear propagation in the notch together with the force needed to produce the elongation of the material in the notched cross section. The proportion of these two components is quite different, depending on the form of the testpiece. The effort should be made to attain the highest possible stress concentration in the notch with the lowest possible elongation in the rest of the testpiece, so that the force that operates on the jaws will represent chiefly the force needed for tearing. 2) Two principle groups of testpieces are to be distinguished: those which show a tear propagation rate determined by the speed of the jaws and those with spontaneous tear propagation. 3) For the case of a thin cut with a razor blade in the testpiece, the tear which occurs upon stretching should not be thought of as a tear growth of the cut, but is to be regarded as a new tear initiation. 4) Several pieces of equipment with electronic force measuring instruments were devised to study the effect of jaw speed. The measurements were performed at jaw speeds up to 18 m/sec. 5) The tensile strengths of the vulcanizates which were studied fall off at first with increase in speed and then rise at high speeds. 6) The curve for tear strength as a function of speeds is similar to that for tensile strength. After passing through a minimum, the tear strengths of the vulcanizates generally rise for the higher speeds. 7) This increase in tear strength was especially large for the synthetic polymers studied. At a test speed of 18 m/sec, their tear strengths reached values several times larger than for lower speeds. 8) An explanation of the changes in tensile strength and tear strength with speed and temperature may be found in orientation and crystallization at high elongation on the one hand and in the flow processes on the other. 9) Changes in relative ratings appear when we compare the results of tear strength measurements on various vulcanizates because the testing speed affects them to different extents. 10) The material becomes anisotropic because of varying high local elongation and the accompanying orientation in the tear test specimen, and the resistance to tear propagation becomes dependent upon direction. The ruptured surfaces of the sample as well as the force/time diagram frequently show a saw-tooth character (“knotty tear”). Smooth or knotty tear occurs for every compound depending upon the testing conditions. 11) The ordinary tear strength test is, therefore, a test of structural strength, which is made up of the true tear propagation load, needed for the tearing, together with the load required for the deformation of the legs between jaws. Test speed and temperature exert different effects on each of these components. From what has been said, it follows that in the use of a given article, it is necessary that we give careful consideration to the sevice conditions, and hence we are not yet in a position to make a generally applicable recommendation for the form of the test specimen.