Evaluation of the creep behaviour of the carbon fibre in an unidirectional pultruded reinforced composite using nano-indentation technique

The interfacial strength (IFS) between carbon fibres and polymeric matrices has important implications for the mechanical properties of composite materials, which can be evaluated by a push-out method using an instrumented nano-indentation machine to assess the fibre's movement under increasing load and constant load (creep). In this paper, the nano-creep behaviour and time-dependent properties of carbon fibres in a reinforced composite were investigated in the first time. The indentation displacement and indentation creep rate of the fibre in the composite were measured at different testing conditions in terms of loading rate, peak load, holding time, elevated temperature (room temperature, 50 and 100 degrees C). Berkovich and cone-shaped diamond indenters were used in the creep tests. The cone-shaped indenter had some advantages over the Berkovich indenter in terms of an extended range of displacement before interfering with the surrounding resin. A single fibre in a thin slice required a critical vertical load to be pushed out under the continuous loading mode, which was strongly linked to the thickness of the composites sample, the location of the fibre in the resin and other environmental factors such as temperature. Both the displacement and creep rate reduced with the increase of temperature due to the resistance caused by the expansion of the fibre and the resin. The nanoindentation creep test results were analysed by an instrumental logarithmic software, and it was found that the creep strain rate sensitivity parameter increased with the load and the holding time but decreased with the increment of the test temperature.