Modal Analysis Of Nonlinear Mechanical Systems PDF Download

"In structural dynamics, autonomous conservative systems commonly exhibit continuous families of periodic orbits in the phase space, usually known as modes of vibration. The main task of modal analysis is to accurately compute natural frequencies and corresponding mode shapes of vibratory mechanical systems as they are known, at least in a linear context, to properly predict the conditions under which the associated periodically forced and slightly damped systems will resonate.Characterizing the modes of vibration of nonlinear yet smooth mechanical systems (systems governed by ordinary or partial differential equations that are smooth with respect to the unknown displacement and velocity) is a current topic of interest in the industrial and academic spheres. Many useful tools, such as the Finite Element Method (FEM), the Harmonic Balance Method (HBM), the continuation techniques and the Frequency--Energy Plots (FEP) provide great assistance in understanding the modal dynamics. Theoretical as well as numerical issues arise when extending these tools to nonsmooth problems such as unilateral contact formulations. The dynamics of two impacting bodies is characterized by travelling waves emanating from the contact interface. In the one-dimensional setting, chosen in this work, these waves couple time and space, in the sense that they are functions of the form f(x+ct) or f(x-ct) where c is the wave velocity. Uncoupling time t and space x leads to numerical and theoretical issues. In FEM, the displacement commonly takes the form u(x,t)= \sum_i \phi_i(x) u_i(t), where u_i(t) is the i-th displacement participation and \phi_i(x), the corresponding shape function. This leads to spurious oscillations, dispersion, and energy dissipation, for most numerical schemes dealing with unilateral contact conditions. Additionally, an impact law is required to uniquely describe the time-evolution of a space semi-discretized formulation. The impact law should be purely elastic to preserve energy, making it difficult to describe lasting contact phases which are expected in the continuous framework. Time-Domain Boundary Element Medthod (TD-BEM) which appropriately combines space and time seems promising as it uses Green's functions that are travelling waves.In this work, unilateral contact conditions are considered for a one-dimensional bar system clamped on one end and undergoing a complementarity condition on the other end. The complementarity form is dealt with as a switch between Dirichlet and Neumann boundary conditions. In dynamics, the solution can thus be retrieved through time marching using TD-BEM with a switch between fixed state when it is in contact and free when it is released. In vibration analysis of autonomous systems, periodic solutions are sought to obtain the mode shapes of the system. In this thesis, TD-BEM presumes the existence of periodic solutions and shooting is employed to find the initial conditions that lead to the assumed periodic solutions. Backbone curves in frequency-energy are constructed via continuation. Existing analytical solutions serve as references for validating the suggested scheme. TD-BEM does not numerically dissipate energy unlike FEM and properly captures wave fronts as expected. The proposed formulation is capable of capturing main, subharmonic as well as internal resonance backbone curves known to emerge in nonlinear dynamics. For the system of interest, the main and subharmonic mode shapes are piecewise-linear function in space and time, as opposed to the linear mode shapes that are half sine waves in space and full sine waves in time." --