<div>EACS 2016 Paper No. 105</div><div><br></div>This paper investigates the impact of inerter nonlinearities on the performance of tunedinerter- dampers (TID). The inerter completes the force-current analogy between mechanical and electrical networks, and it represents the mechanical element equivalent to a capacitor. The TID is a vibration suppression system designed to limit unwanted vibrations of civil engineering structures and has a layout similar to that of a tuned-mass-damper (TMD), where the mass element was replaced by an inerter, making it a two-terminal device. The TID offers significant advantages over TMDs, in terms of performance over a wider frequency band, installation and dimensions. Nevertheless, the study carried out to date has been mainly numerical, considering a linear, purely inertial model of the inerter, where the generated force was proportional to the relative acceleration between the devices terminals. In practice, the dynamics of an inerter include nonlinearities, especially at the low frequencies associated with civil engineering applications. Here, the identification of these nonlinearities is done experimentally, based on an off-the-shelf inerter. The structural system, a single-degree-of-freedom host structure with an attached TID, is tested using Real Time Dynamic Substructuring (RTDS). The inerter represents the physical substructure, while the remaining elements of the TID and the host structure form the numerical substructure. This split allows the optimisation of the TID parameters, since the values of the spring and the damper can be changed without altering the experimental setup. In addition, this configuration takes into account the inerters potentially complex dynamics by testing it experimentally together with the characteristics of the host structure. The experimental and numerical results show that with appropriate retuning of the stiffness and damping components of the nonlinear TID system, the performance of the linear TID can be regained.