Effect of linear viscoelasticity on stress transfer in a numerical model of a single fiber fragmentation test

The interfacial region is important for the performance of a fiber-reinforced composite. However, in the case of a carbon fiber (CF) and epoxy resin composite, the chemical interactions in the vicinity of the CF may cause the properties of the resin to be different from the rest of the matrix obtaining shorter relaxation times and a greater sensitivity to temperature. This viscoelastic behavior becomes relevant when using the single-fiber fragmentation test to characterize the interfacial shear strength. Such behavior was characterized, by means of the Prony Series of the epoxy resin with two different proportions of curing agent each representing the interfacial region and the matrix. Next, it was modeled considering the elastic properties of the CF, the viscoelastic properties of the interfacial region and the matrix with a three-phase model using the finite element method (FEM). The model represented a fiber fragment from the single fiber fragmentation test (SFFT). A parametric analysis was performed considering different important components in the stress transfer from the matrix to the fiber, especially the elongation rate and the temperature. It was observed that the viscoelastic behavior of the interphase has a larger effect on the interfacial shear strength (IFSS), and notorious differences were observed on the stress transfer efficiency and hereby on the importance of selecting an appropriate testing speed for the SFFT. This analysis is also important in the design or inspection of composite structures/components used in various engineering applications and subjected to large temperature gradients during their operation.