Investigation of the nanoscale mechanical properties of carbon fiber/epoxy resin interphase. I. analysis of fiber-stiffening effect during the nanoindentation process based on numerical simulation

The interphase in fiber reinforced polymer composites is a narrow region around the fiber with the thickness in nanoscale, and its properties have a significant effect on the composite mechanical properties. Nanoindentation method was widely accepted for the measurement of the interphase properites. Different from the homogenous material, the modulus diffence between fiber and matrix is very obvious, it is difficult to get the intrinsic properties of the interphase just depending on the experimental data due to the fiber-stiffening effect. A numerical method is developed to simulate the nanoindentation process, to understand the fiber-stiffening effect during the nanoindentation process in the carbon fiber/epoxy composites. The predicted results reveal that the fiber impacts the indentation response in the vicinity region of the fiber. When the indent contacts the fiber, there will be a mutation in the loaddisplacement curve, and the fiber-stiffening effect is obvious, which is less affected by the thickness and modulus of the interphase and the resin matrix modulus. If the interphase thickness is smaller than the distance of indent contacting the fiber, it is nearly impossible to get the intrinsic modulus of the interphase due to the effect of fiber. On the other hand, if the interphase thickness is larger than that, in the near region about 100 nm away the indent contacting the fiber, the indentation response can reflect the difference of the interphase modulus. The gradient of the curve, that is, the indentation force varying with the distance from the fiber surface, decreases with the increase in the interphase modulus. POLYM. COMPOS., 2012. (c) 2012 Society of Plastics Engineers