Experimental Modal Analysis of Geometrically Nonlinear Structures by Using Response-Controlled Stepped-Sine Testing

Abstract

The everlasting competition in the industry to achieve higher performance in aircraft, satellites, and wind turbines encourages lightweight design more than ever, which eventually gives birth to more flexible engineering structures exhibiting large deformations in operational conditions. Accordingly, continuously distributed geometrical nonlinearity resulting from large deformations is currently an important design consideration. Being guided with this motivation, this paper investigates the performance of a recently developed promising nonlinear experimental modal analysis method on a clamped-clamped beam structure which exhibits geometrical nonlinearity continuously distributed throughout the entire structure. The method is based on response-controlled stepped-sine testing (RCT) where the displacement amplitude of the excitation point is kept constant during the frequency sweep. In this study, the nonlinear beam structure is instrumented with multiple accelerometers at several different locations along its length and is excited at a single point. Tests are conducted at energy levels where no internal resonance occurs, yet the beam structure exhibits strong stiffening nonlinearity which results in jump phenomenon in the case of classical constant-force sine testing. Nonlinear modal parameters are experimentally identified as functions of modal amplitude by applying standard linear modal identification methods to quasi-linear frequency response functions (FRFs) measured with RCT. Validation of the identified modal parameters is accomplished by comparing the constant-force FRFs synthesized using the identified modal parameters with the ones obtained from constant force testing and also with the ones extracted from the harmonic force surface (HFS).