Simplified phenomenological model of the nonlinear behavior of FRPs under combined stress states

Nonlinear material behavior of FRPs under shear loading is widely observed and investigated. In case of combined stress states under tension and shear, an interaction between the macroscopic shear stress–strain curve evolution and the applied tension has been observed and described by several publications in the past. In the present work, the available experimental data with combined stress states are evaluated and a specific threshold shear stress is found, above which nonlinear material behavior occurs for all stress states. Further, a new simplified phenomenological model is derived to model the nonlinear behavior of FRPs when the threshold shear stress is exceeded. This simplified model only needs the threshold shear stress and one evolution parameter, both derived from a pure shear test, to model nonlinear behavior for all combined stress states. A comparison with the available experimental results and with the predictions of the WWFE-III participants for WWFE-III test case 1 shows a very good agreement.