Creep Behavior Study for Carbon Fiber Nano-Composites

The surface modification of carbon nanotubes (CNTs) has been recently observed to influence the distribution of CNTs in epoxy resin and the mechanical properties and electrical conductivities of these CNTs. Accordingly, the treatment of CNTs to with organic acids to oxidize them generates functional groups on the surface of CNTs. This investigation studies the consequent enhancement of the mechanical properties and electrical conductivities of CNTs. The influence of adding various proportions of CNTs to the epoxy resin on the mechanical properties and electrical conductivities of the composites thus formed is investigated, and the strength of the material is tested at different temperatures.The test results also indicate that mechanical strength and electrical conductivity increase with the amount of CNTs added to the composites. Different coefficients of expansion of the matrix, fiber and CNTs, are such that overexpansion of the matrix at high temperature results in cracking in it.Moreover, the creep behaviors of carbon fiber (CF) /epoxy resin thermosetting composites and CNTs/CF/ epoxy resin composites were tested and analyzed at different stresses, orientations of fiber, temperatures and humidities. The creep exhibits only two stages-primary creep and steady-state creep. The effects of creep stress, creep time, and humidity on the creep of composites that contain various proportion of CNTs were investigated at various temperatures.Additionally, increasing the number of cycles in cyclic creep tests at room temperature resulted in a decrease in creep strain even at a high temperature of 55°C. Possible room temperature creep mechanisms have been proposed and discussed. With increasing number of creep tests, the creep strain decreased due to strain hardening which occurred during creep. Creep strain is believed to increase with applied stress, creep time, humidity, temperature and degree of the angle θ between the orientation of fiber and the direction of the applied stress.Finally, the test results of creep strain of CF/epoxy resin composites and CNTs/CF/epoxy resin composites tested under various conditions can be smoothly fitted by the fitting curves of Findley power law.