A standard finite element analysis of individual components in aero engine and other systems shows a high accuracy compared to experimental measurements of the system response. However when it comes to assemblies, the conventional linear approaches fail to deliver good accuracy. This is due to the uncertain physical phenomena in the contact interface of the joints. A nonlinear contact problem is introduced by the joint and influences the overall dynamic behavior of the engine assembly. Therefore, the linear dynamic models must be coupled with nonlinear analysis of the assembly to investigate the accurate dynamics of the nonlinear system. Flanges are widely used joints that represent the main source of nonlinearities in assemblies. In this study, a finite element simulation of two bolted flanges is considered to identify the nonlinear behavior of the bolted flange joint caused by the presence of friction in contact interfaces. A detailed model of a bolted joint was built in ANSYS in order to evaluate the energy dissipated in a bolted joint and therefore provide accurate modeling of joint interfaces. Due to the high cost of the detailed model, an equivalent model was derived and predictions from this model are compared to the detailed model results in order to provide a robust model for designing bolted joints.