Experimental Investigation on Characteristics of Combined Viscous and Coulomb Damped System

Most civil engineering structures exhibit a combination of viscous and Coulomb damping. It is known that the dominant mechanism of energy dissipation in the presence of structural defects such as cracks, defective connections etc. is due to dry Coulomb friction. Contrary to this, in the undamaged state of structures, the dissipation of energy is mostly due to material damping which is considered macroscopically viscous. Thus, identification of Coulomb friction in modal damping could reveal the presence of defects in a structure. Past research suggested that dry Coulomb friction damping is dependent on initial amplitude and normal force between contact surfaces. However, detailed investigations on this matter are scares. Thus, insight into the effect of these parameters on a combined dry Coulomb friction and viscous damped system could provide the basis for identifying and isolating the dominant energy dissipation mechanism. Therefore, this study investigates the influence of magnitude of Coulomb friction between contact surfaces and oscillation amplitude on the damping behavior of a system subjected to coexisting dry Coulomb friction and viscous damping. Experimental investigation was conducted using the free vibration decay of a cantilever beam under varying magnitudes of Coulomb friction and initial dis-placement amplitudes inside the laboratory. In the investigation, it was observed a strong correlation between increase in the Coulomb friction force and the nonlinear behavior of damping. Nonlinearity of damping behavior was seen increased when the Coulomb friction force was increasing. Nonlinearity of damping behavior is more pronounced at lower oscillation amplitude and magnitude of damping was observed become asymptotes to a constant damping value when oscillation amplitude is large irrespective of magnitude of applied Coulomb friction force. Increase in the magnitude of normal contact force always increased dry Coulomb damping. When both Coulomb friction and viscous damping were present, it was observed that dry Coulomb friction dominating the damping of oscillation at lower amplitudes and the viscous damping is dominating the damping behavior at higher oscillation amplitudes. Thus, it is evident that effect of dry Coulomb damping is most recognizable at lower oscillation amplitudes of free vibration.