Stress‐strain testing of rubbers at high rates of elongation

A high speed stress‐strain apparatus has been constructed to measure the strength and elongation of rubbery materials at extension rates roughly 250 and 550 in./sec. The test sample is stretched by falling weight which catches a protruding lug on the lower jaw of the sample. The weight falls from a magnetic grip at one of two selected stations. The potential energy of the falling weight is large compared to the energy under the stress‐strain curve in order to avoid appreciable slowing of the weight during the period of stretch. The excess energy is dissipated in several kinds of damping mechanisms without damage to other than expendable crash pads. The tensile load on the rubber sample is carried by a stiff‐strain‐gage beam, the output of which is displayed on an oscilloscope for photographic recording. Stress‐strain curves of rubber recorded for stretch rates about 550 in./sec. show evidence of sonic wave pulses in the early portions of the extension cycle. Ultimate tensile and elongation values differ somewhat from values obtained using conventional test machines.