AbstractTensile measurements on fibers from nylon 66 and polyethylene terephthalate have shown that the fiber tenacities at relatively high temperatures (about 250 to −100°C.) are controlled by the internal molecular structure of the fiber and not by any local defect. While it is true that fibers always break at some local defect, the actual strength was determined by a yielding of the molecular structure which occurred before fiber breakage. In contrast, the tenacity at low temperatures (generally −100 to −196°C.) was controlled by local defects which propagated under stress, causing fiber failure before the internal molecular structure exhibited any yield. The temperature at which the mechanism changed was dependent on the severity of local defects and on the internal molecular structure as well. The low‐temperature tenacity was found to be independent of the actual temperature of measurement, and this constant level of tenacity, at low temperatures, was shown to be a measure of the severity of the most critical local defect in the fib
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