A study of two-stage cumulative fatigue behavior for CNT/epoxy composites

In this study, the two-stage cumulative fatigue behavior for the carbon nanotube (CNT)/epoxy composites was experimentally investigated. The content of the chemically modified multi-walled carbon nanotubes used for the employed specimen is 0.5 wt. %. The loading sequence effect and the influence of the cycle ratio of the first stage on the cumulative fatigue life were studied herein. Two loading sequences, i.e., high-to-low and low-to-high cases, were considered in the experimental program to study the interaction of applied loads at the two stages. The high and low loading levels were selected as the applied loads corresponding to the basic fatigue lives of 20,000 and 500,000 cycles, respectively. Furthermore, five cycle ratios of the first stage, i.e., 0.1, 0.2, 0.5, 0.7 and 0.9, were chosen as the experimental variables in the fatigue tests. All the tests were performed using an Instron 8872 servo-hydraulic testing system and the employed loading frequency is 7 Hz. Moreover, all the fatigue tests were stress-controlled at room temperature with the stress ratio of 0.1. To evaluate the cumulative fatigue life, the Miner's rule was employed to predict the remaining fatigue life of the second stage. Besides, the specimen stiffness and the electrical resistance during the preliminary constant-amplitude fatigue tests were utilized to describe the fatigue damage and the corresponding damage curves were developed to predict the cumulative fatigue life. By comparing the prediction results with the experimental data, it shows that the Miner's rule fails to evaluate the cumulative fatigue life, while the non-linear damage theories based on the variation of the stiffness and the electrical resistance provide good estimated results of the cycle ratio of the second stage. (C) 2010 Published by Elsevier Ltd.