Study of damping in layered and welded beams

Vibration and noise reduction are crucial in maintaining high performance level and prolonging the useful life of machinery, automobiles, aerodynamic and spacecraft structures. Notwithstanding the variety and immensity of work done within this domain of study, and despite all possibly most accurate solutions and arduous experiments, many aspects related to damping remain poorly examined. In fact, the damping and its improvement in machines or structures are one of the biggest challenges to the practicing engineers. Following the requirements of modern technology, there is an increasing demand for machine tools and fabricated structures with high stiffness, high damping capacity and light weight. Such requirements necessitated the use of layered and welded cantilever beams as structural members. Alternatively, cast cantilever beams can be used, but unfortunately, these are more expensive to manufacture. As a result, the deployment of welded layered beams is becoming increasingly common in the machine tool industry and fabricated construction. Many structures are made by connecting structural members through joints. Due to very low material damping of built-up structures, sufficient damping has to come from the joints. Damping in built-up structures is often caused by energy dissipation due to micro-slip along frictional interfaces (e.g., at welded joints), which provides a beneficial damping mechanism and plays an important role in the vibration behavior of such structures. The research presented in this thesis is devoted to the problem of damping estimation in engineering structures, typically welded and layered cantilever beams, through analytical and experimental work. The ultimate goal of this project is to develop a damping model that is capable of describing the effects of welded joints on a vibrating structure. In order to do this, it is not necessary to model the actual physics at the microscopic level, instead, the macroscopic effects of the joint on the gross vibration characteristics of the structure are considered and a way for modeling these effects is sought. A careful theoretical and experimental study to quantify the effects of the joints on the structural damping is an integral part of this effort. This thesis consists of two different parts: a theoretical analysis of the problem and an experimental work. The theoretical analysis proposes three different methods to evaluate damping: ii classical, finite element and response surface method. It is a general fact that the theoretically computed results will differ from the actual values due to the assumptions made in the theoretical analyses. In view of this discrepancy in results, experiments are conducted for different set of mild steel and aluminium specimens under different vibrating conditions. Time and frequency domain approaches have been adopted to experimentally evaluate the damping capacity. Both the numerical and experimental results are compared for authentication. Finally, useful conclusions have been drawn from both the numerical and experimental results