Experimental Identification of a Structure with Internal Resonance

Engineered structures are becoming increasingly lightweight and flexible, and as such more likely to achieve large amplitude and nonlinear vibratory responses. This leads to a demand for new methods and experimental test structures to see how in practice nonlinearity can be handled. In previous work, the authors studied a continuous modal structure with a local nonlinearity. The structure has been designed to have transparent underlying physics, and easily adjustable natural frequencies, and this leads to the ability to investigate an approximately 3:1 internal resonance between the 1st and 2nd modal frequencies. Therefore the structure exhibits complex responses to harmonic excitation, including isolated regions of the frequency response and quasiperiodic behaviour. In the present work we discuss a rapid means of identifying the structure with the minimum requirements of test data and time. A particular aim is to characterise the underlying linear system using data that is strongly influenced by nonlinearity. A harmonic balance procedure is used to identify a nonlinear discrete spring-mass system, that is modally equivalent to the structure under test. It is found that the inclusion of harmonic components in the test data and the presence of internal resonance leads to surprising amounts of information about modes that are not directly excited by the fundamental stepped-sine excitation.