To meet the demand for high performance aircraft that can satisfy critical mission profiles, structural design limits need to be expanded. Linear models for thin structural panels undergoing bending tend to over predict the response at high amplitudes leading to thicker, heavier and less efficient designs. Larger operating ranges and thinner structural members are able to be utilized in aircraft design if the design space is expanded to incorporate the nonlinear response regime. However, when nonlinearity is allowed, the analysis and validation of the design becomes a challenging task. The response of such designs can be computed using the finite element (FE) method with nonlinear solution capabilities. To ensure confidence in the numerical model's results, model correlation and validation must be conducted. Unfortunately the linear model correlation and validation techniques that are commonly used in the aerospace industry are no longer valid in the nonlinear response regime so a new set of tools is required to validate nonlinear FE models. This work presents an algorithm to update nonlinear structural systems based upon their nonlinear normal modes (NNMs). The NNMs serve as a strong metric to validate the numerical models, because they represent the dynamics of the nonlinear system over a range of amplitudes and they arc independent of the loading applied to the system. NNMs are able to be extracted from experiments so numerical models can be correlated and validated with test data. This work presents a novel method of computing analytical gradients of NNM solutions with respect to system parameters. The procedure is applied to a single degree of freedom nonlinear oscillator to understand how each parameter affects the NNM and, when coupled with an optimization algorithm, this produces an efficient means of updating a nonlinear model to better correlate with measurements.
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