An augmented free-interface-based modal substructuring for nonlinear structural dynamics including interface reduction

This work proposes two novel aspects for modal substructuring of geometrical nonlinear structural dynamics. Firstly, we present a non-intrusive model order reduction technique for substructures using an augmented free-interface method with residual flexibility attachment modes. Non-intrusive model reduction methods are beneficial, because they do not require to access substructures' closed form equations of motion while building nonlinear reduced models. Therefore, they can be easily combined with commercial finite element packages. Generally, the interface of substructures can contain many degrees-of-freedom leading to reduced order models, which are still computationally expensive. As a second novel point, the nonlinear reduced models of substructures are developed when the augmented free-interface-based method is combined with three interface reduction techniques (one system-level and two local-level), which so far have only been used with the fixed-interface method of Hurty/Craig-Bampton. The proposed methods are compared in terms of accuracy and computational efficiency with the two existing non-intrusive-based modal substructuring methods: nonlinear free-interface and nonlinear Hurty/Craig-Bampton methods. The performance of the improved free-interface method is examined on two different geometrically nonlinear structures by applying random pressure on them and comparing displacement spectral densities. The dynamic results show a remarkable improvement of accuracy compared to the nonlinear free-interface method and a slightly improved accuracy compared to the nonlinear Hurty/Craig-Bampton method. This improvement is achieved while the online computational costs are not increased significantly.