American Institute of Aeronautics and Astronautics 1 Nonlinear Model Updating of a Cantilevered Plate and a Stiffened Skin Panel from a Lynx Helicopter Matthew S. Allen 1 Engineering Physics Department, University of Wisconsin-Madison Ben Weekes 2 Mechanical Engineering Department, University of Bristol 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. I. Introduction EOMETRIC nonlinearities are often important in thin walled structures whenever the shell displacement becomes an appreciable fraction of the thickness. While it has been possible to model these types of structures in nonlinear finite element codes for a few decades [1], these models are exceedingly expensive to integrate and so they have not been used widely for dynamic analysis or design. Fortunately, considerable progress has been made in recent years to develop methods that extract a reduced order model (ROM) for a geometrically nonlinear structure from a series of static loads or displacements applied to the finite element model [2, 3]. Kuether and Allen recently suggested that the nonlinear normal modes (NNMs) of a structure can be compared between candidate ROMs to evaluate the convergence/accuracy of the ROM, as they provide an amplitude dependent view of the dynamics of the structure (or ROM) that is independent of the loading [4, 5]. They have also presented a method whereby the true NNMs of the finite element model can be computed [6]. Peeters, Kerschen and Golinval recently laid the foundation for measuring the nonlinear normal modes of a lightly damped structure [7], and hence it begins to be appealing to use NNMs as a metric to validate and update a nonlinear structural model. This work seeks to combine these tools to model and then use tests to update/validate geometrically nonlinear finite element models. The methods are evaluated on two structures. The first is a cantilevered plate that is clamped at its base and exhibits a lightly damped two-stripe mode; this type of mode can be problematic in the compressor blades of some turbine engines. The plate of interest is constructed of titanium and was designed to be used to evaluate life prediction and crack propagation methods [8], and so it is important to have an accurate understanding of the stress field in the part as it undergoes geometrically nonlinear motions. This simple structure was found to exhibit several subtleties which make model updating challenging and shed light on best practices for future studies. 1 Associate Professor, 535 Engineering Research Building, 1500 Engineering Drive, Madison, WI 53706-1609, USA, msallen@engr.wisc.edu , AIAA Lifetime Member. 2 Postdoctoral Researcher, Queen’s Building, University Walk, BS8 1TR, UK, B.Weekes@Bristol.ac.uk G