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Methods for evaluation of the low-temperature resistance of non-rigid insulating materials have progressed from haphazard arbitrary methods to one widely used standard test. Not too long ago, this property was determined by pinching the material with pliers or bending it in half while it was in a can of dry ice. This basic idea then was refined somewhat by standardizing the exposure time in a cold box and bending a specimen around definite diameter mandrels. This was commonly known as the mandrel-bend test, but was found to be non-reproducible due to the effects of variations in bending force and speed, and mandrel temperature. Another step in the evolution of a low-temperature test for insulating materials was development of a jig to hold and bend the specimen a controlled distance. This test has been adopted as ASTM Method D 736 and has been used quite extensively in the rubber industry. However, results from this method have been found to be affected by the rate of flexing, which cannot be closely controlled, thus reducing reproducibility. A little over a decade ago, an impact failure test was introduced and adopted as ASTM Method D 746. This test introduced a new criterion for low-temperature failure—cracking of the specimen by high-speed impact while in a cold liquid bath. Results from this method were found to be fairly reproducible, and, with slight modifications, it rapidly has become standard for evaluation of most non-rigid insulation materials. Comparatively little consideration has been given to the significance of the results.
Craig, C. L.
Senior Materials Engineer, Sperry Gyroscope Co., Great Neck, N. Y.