Size effects in epoxy resin by means of a Shear Transformation Zones model

During the last decades, the interest in composite materials never stopped growing in high-technological fields, due to their ability to obtain unique combination of properties. Along with the increase of computational power, a particular attention is paid to the modeling strategies of these materials since it allows a significant cost and time saving in the design process of composite-based components. Because of the structural nature of composite materials, their modeling follows a bottom-up approach in which the different constituents of the composite are simulated within their own scale and information are transferred from the bottom to the top to be used as building blocks for the higher layer models. At the bottom of this chain is situated the fiber-matrix system, which is characterized either by experimental testing or by a recent set of modeling tools known as computational micromechanisms. These models aim at describing the behavior of the matrix, the fibers and their interactions to give a proper description of the ply’s mechanical response. Even if a lot of work has been done during the past years in this domain, a lot of features of the fiber-matrix system are not modeled yet, such as the presence of size effects in the matrix when confined in small areas. In this master thesis, a Shear-Transformation-Zones approach is used to model the behavior of a commonly used matrix material, the epoxy resin RTM-6. The model is implemented in the finite element software Abaqus through the use of user-subroutines (UMAT). This STZ model is then used as a tool to capture size effects exhibited by epoxy resins under specific loadings. Several loading conditions are modeled such as biaxial compressions or simple shear, as well as the effect of strain gradient on the mechanical response of the RTM-6. The extension of the model to the 3D is implemented and nanoindentation tests are performed to capture the inverse dependence of the hardness of the material with regard to the indentation depth that is observed during experimental testing of epoxy resins. Thanks to these simulations, it is shown that STZ based models are effectively able to capture the presence of size effects due to the confinement of the matrix under simple shear loading. The management of the strain rate by the STZ model is highlighted and the inverse proportionality between the hardness and the indentation depth is captured as well. The model is then identified as a relevant tool for the characterization and modeling of epoxy resins. In fact, due to the physics-based nature of the model, only a limited number of parameters are needed to calibrate it and interesting information such as the description of the shear band pattern is available and can be used to get a better insight about the physical phenomena that control the deformation mechanisms of epoxy resins.