In-situ characterization of microstructural changes in a carbon nanotube sheet under sustained load

Abstract

The present study characterizes the creep behavior of carbon nanotube (CNT) sheets under externally applied sustained loads. Creep loads at levels ranging from 85% to 98% of the ultimate tensile strength of the CNT sheet were applied using a tensile tester inside a scanning electron microscope chamber. The microscope enabled in-situ characterization of the microstructural changes in the CNT sheet under the influence of the applied load. The loads were sustained for 1,000,000s or failure, whichever occurred first. A computational pattern recognition technique was also developed that enabled quantitative approximation of the time dependent changes in distribution of individual CNT orientation with reference to the loading direction. It was observed that the CNTs increasingly aligned along the loading direction during the initial loading phase when the CNT sheet was ramped up to the desired load level. Further microstructural changes by way of individual CNTs continuing to gradually align along the loading direction was observed after the loads were sustained at the desired levels. The pattern recognition computational results validated the experimental findings. Slight relaxation of the CNTs was also observed upon the removal of load once 1,000,000s was reached.