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Preliminary data on the speed of testing of plastics have been obtained in the Ordnance Corps' Plastics Laboratory using a constant ϵ rate tension test instead of the usual constant rate of crosshead motion test. The data were obtained by the use of a three-coil transformer attached to the specimen. The output voltage of this coil was determined by the length of iron core inserted within the coils. A similar transformer was set up so that the core was inserted at a constant predetermined rate. The output voltages of these two transformers were compared, and any error present was corrected by the automatic speeding up or slowing down of the crosshead motion. In this fashion, the stressing was performed so as to result in a constant ϵ rate. The maximum deviation as shown by ϵ—time curves was within 1 per cent. Details of the construction and operation of this equipment are contained in a report by S. Strella. Five materials were tested at both constant rate of ϵ increase and at constant crosshead speed. Three specimens of each material were run by each method. Although most of the tests were conducted at a speed one tenth of that recommended by the ASTM the results obtained at the ASTM speed confirmed the findings. A straight polystyrene showed that ϵ values at various stresses, the breaking ϵ, and the breaking stress were all in better agreement when tested at a constant ϵ rate increase. A standard phenolformaldehyde material, which is similar to the polystyrene in being a rather brittle material but which differs in that it is crosslinked, showed similar results.
Ruggers, George R.
Ordnance Corps, Dover, N. J.