Damping of structures with riveted joints

Abstract

The main purpose of the structural design is to restrict the vibration of structures at a desirable level as per requirements.Usually, structures inherently possess low structural damping necessitating the introduction of additional measures to improve their damping characteristics in order to control the harmful effects of vibration in normal operating conditions. In fact, most monolithic structures possess low inherent damping thereby causing serious problems which will impair the function and life of structures leading to their ultimate failure. It is always desirable to keep the vibration level as low as possible by introducing damping so that the performance and useful life of structures are enhanced largely. Since many decades, it has been a biggest challenge to the practicing engineers and designers to limit this unwanted vibration in structures. In view of this, structures must be properly designed to possess adequate damping so that the undesirable vibration levels will not build-up beyond a permitted limit. The sole contribution of the present investigation is intended in this direction only. The design concept evolved from this research work of using layered structures with riveted joints can be effectively utilized in trusses and frames, aircraft and aerospace structures, bridges, machine members, robots and many other applications where higher damping is required.The present investigation highlights the effect of interfacial slip on the damping of layered fixed beams jointed with rivets undergoing free vibration. The inclusion of mechanical joints bears a strong influence in the overall system performance and behavior, particularly the damping level of the structures. One of the techniques used in the present problem for improving damping is fabricating these structures in layers by means of riveted joints.The incorporation of such joints is the major source of energy dissipation through frictional effects associated with relative shear displacements at the interfaces of the various structural members. Most of the damping in built-up structures is thus attributed to micro-slip at the interfaces. The contribution of the micro-slip on the overall system damping is always significant in spite of its low magnitude. This thesis consists of two different parts: a theoretical analysis of the problem and an experimental work. The theoretical analysis proposes the classical method to calculate damping. The analyses are based on the assumptions of Euler-Bernoulli beam theory as the dimensions of test specimens satisfy the criterion of thin beam theory. A continuous model is characterized by a partial differential equation with respect to spatial and time coordinates. An analytical exact solution is obtained for the above differential equation from which the dynamic characteristics of the structure are represented accurately.The logarithmic decrement technique has been used for measuring the damping from the time history curve of the decaying signals recorded on the screen of digital storage oscilloscope.The experimental results are compared with the corresponding theoretical ones. The damping characteristics in jointed structure are influenced by the intensity of pressure distribution, micro-slip and kinematic coefficient of friction at the interfaces the above vital parameters are largely influenced by the thickness ratio of the beam and thereby affect the damping capacity of the structures. Number of layers, fixed beam length and diameter of connecting rivet also play a key role on the damping capacity of the jointed structures quantitatively.