Nonlinear Model Updating of a Cantilevered Plate and a Stiffened Skin Panel from a Lynx Helicopter

Abstract

This work explores technologies for nonlinear modeling, testing and nonlinear model updating of geometrically nonlinear structures. The methodology centers around a finite element model (FEM) of the structure, created and solved using commercial software. A nonlinear reduced order model (ROM) is extracted from the FEM using the implicit condensation and expansion method and its nonlinear normal modes are computed using a pseudo-arclength continuation algorithm. The nonlinear modes provide insight into the dynamics of the FEM/ROM and also provide a comparison that guides model updating and validation. Measurements from the structure using swept or stepped sine excitation are compared with the nonlinear mode backbones to assess their accuracy. Both structures show deformation shapes that change in specific ways with increasing response amplitude, so full-field measurements would be helpful in understanding how the underlying linear modes of the structure interact as the response amplitude increases. These concepts are illustrated on two structures, a cantilevered flat plate which was used in other works to study crack growth in titanium, and a curved exterior panel from a Lynx helicopter. These structures reveal the potential, as well as the limitations, of the current state of the art in modeling and testing these types of structures.