Under harmonic excitation, soil exhibits softening behaviour that can be captured through the so-called hyperbolic soil model. The response of systems with such a material model can elegantly be obtained using the classical Harmonic Balance Method (HBM). Soil also exhibits nonlinear hysteretic damping under harmonic excitation, feature which is not incorporated in the hyperbolic soil model. The response of a system that includes also the nonlinear hysteretic damping cannot be obtained using the classical HBM. This work demonstrates the application of an advanced HBM (more specifically, alternating frequency-time HBM) for finite and infinite systems that exhibit softening behaviour and nonlinear hysteretic damping. The purpose of this model is to, in the future, investigate the influence of the nonlinear hysteretic damping on the response of such systems, as opposed to linear viscous or hysteretic damping that is usually adopted. To conclude, we show that the advanced HBM is an effective tool for revealing fundamental characteristics of continuous systems with softening behaviour and nonlinear hysteretic damping whose stationary responses consist of either standing or propagating waves.

We report experiments on high-amplitude sound wave propagation in an acoustic metamaterial composed of an air-filled waveguide periodically side-loaded by holes. In addition to the linear viscothermal and radiation losses, high amplitude sound waves at the locations of the sideholes introduce nonlinear losses. The latter result in an amplitude-dependent reflection, transmission, and absorption, which we experimentally characterize. First, we evidence that nonlinear losses change the nature of the device from a reflective to an absorbing one, showing thepossibility to use the system as a nonlinear absorber. Second, we study the second-harmonic generation and its beating phenomenon bothexperimentally and analytically. We find that when considering the propagation of both the fundamental and the second harmonic, nonlinear losses cannot be neglected. Our results reveal the role of nonlinear losses in the proposed device and also provide a quite accurate analytical model to capture the effect of such losses.

The Harmonic Balance Method (HBM) is often used to determine the stationary response of nonlinear discrete systems to harmonic loading. The HBM has also been applied to nonlinear continuous systems, but in many cases the nonlinearity consists of discrete nonlinear elements. This chapter demonstrates the application of the HBM to dissipative continua with distributed nonlinearity by analysing three canonical problems: (a) 1-D layer with a free surface and rigid base (interfering upward and downward propagating shear waves), (b) 1-D half-space with a rigid base (vertically propagating shear waves), and (c) 2-D axially symmetric semiinfinite medium with a circular cavity (radially propagating compressional waves), all of them subject to harmonic excitation at a boundary. Results show that systems (a) and (c) exhibit softening behaviour and super-harmonic resonances, while only the former displays multiple response amplitudes for certain excitation frequencies; the unique frequency-amplitude relationship of system (c) is due to the strong damping (i.e., radiation damping and internal dissipation). Furthermore, although system (b) essentially does not resonate, the third-harmonic component exhibits a maximum caused by the interplay between the dissipative and nonlinear effects, a phenomenon that also occurs in system (c). Finally, the considered systems have applications in earthquake and geotechnical engineering, among others, but the presented methodology is generic.