Accelerated flex fatigue testing was carried out on nylon 6.6 tire cords, under various applied tensions (stress), at standard atmospheric conditions of 65% relative humidity and 21 degrees C. Fatigue lifetime was measured and scanning electron microscope used to investigate the nature of failure. The study concluded that there was an exponential dependence of fatigue life on stress. The mode of failure depended on the tension applied, the underlying modes being those of kink band at lower stress levels and tensile failures as the stress level increased. (C) 2000 John Wiley & Sons, Inc.