Changes in the mechanical properties of cords subjected to dynamic fatigue were measured for cotton, mercerized cotton, and rayon tire cords. These data were obtained by the use of an instrument which subjected the cords under tension to localized dynamic forces under controlled atmospheric conditions. The forces, when observed on an oscilloscope screen, were found to vary during the flex cycle. Flex life and ultimate elongation were found to be cumulative, regardless of whether the cords were fatigued continuously or were allowed to relax with or without static tension between periods of flexing. Calculated activation energies for fatigue failure were less than those for heat degradation. From the differences in properties a qualitative evaluation of tire cord behavior could be made. THE FATIGUE-RESISTANCE of an automobile tire cord plays an important part in the performance of the tire. A tire in service must endure periodic and unequal stresses, relaxations, and compressions under varying conditions of moisture and temperature. The ability of the cord, the unit supporting structure of the tire wall, to withstand these varying forces depends upon its physical properties -its strength, elongation, and elastic behavior. While much has been reported in the literature about the effects of the forces on these properties, the interpretation of this type of information in terms of tire performance is still in the experimental stage.The previous interpretations have been made on the basis of data on the dynamic fatigue obtained on testing machines of radically different designs [1,2,7,12,14,19,20,25,27] . But since none of the available machines was considered sufficiently reliable to be accepted as the standard by the American Society for Testing Materials, a special instrument was constructed for the present work which bends cords while subjecting them to localized dynamicfatigue factors. This machine provided a means of comparing the changes undergone in strength, elongation, and energy characteristics by cotton, mercerized cotton, and rayon tire cords after fatiguing under different conditions. The results, after the necessary limitations of the test are taken into account, indicate a characteristic pattern of performance for each of the materials examined.The Instrument