Robert Seifried

Propagation of guided waves in adhesive bonded components

The objective of this research is to use analytical and computational models to develop a quantitative understanding of the propagation of guided Lamb waves in multi-layered, adhesive bonded components. Key issues of this study include the effect of the adhesive bond layer, including its low stiffness (relative to the adherends) and viscoelastic behavior. The propagation of these guided waves are interpreted in terms of dispersion relationships, displacement profiles and attenuation curves (both as functions of frequency and wavenumber). The ultimate goal of this study is to determine the effectiveness and sensitivity of guided Lamb waves to determine the in situ properties of an adhesive bond. A combination of the analytical model, transient FEM simulation and experimental measurements provides a better understanding of the guided waves behavior in this layered waveguide.

Numerical and Experimental Investigation of Radial Impacts on a Half-Circular Plate

For investigations of impact problems, there are a lot of numericalsimulations available, however, how well these simulation resultscorrespond to the physical reality needs still further studies. The goalof this paper is to examine whether numerical results for impactresponses using the finite element method and experimental results usingadvanced measurement instruments agree well with each other and hownumerical results are influenced by some non-physical chosensimulation parameters in the numerical approach. For thesepurposes, the radial impact of a steel sphere colliding with ahalf-circular aluminum plate is used as an example and investigated bothnumerically and experimentally. In order to investigate the influence ofdifferent simulation parameters and the efficiency of different contactalgorithms, the impact of a steel sphere colliding with analuminium rod is first considered as a test case. Then, these resultsare used as guidelines for modeling the more complex radial impact.A comparison with experimental results shows for the centric andeccentric impact good agreement to the computed results if thesimulation parameters are chosen appropriately.

The role of the coefficient of restitution on impact problems in multi-body dynamics

Abstract This review article presents methods for treatment of impact problems in multi-body dynamics with a special focus on the coefficient of restitution. The impact modelling in multi-body systems is presented, including the impact kinematics, different definitions of the coefficient of restitution, the instantaneous impact modelling, and the continuous impact modelling. A main topic is the multi-scale simulation approach for the numerical evaluation of the coefficient of restitution using additional simulations on a fast time scale. Different models on the fast time scale are proposed and techniques for experimental validation of the models are given. Many numerical and experimental results for impacts of two and more bodies are presented. Thereby the efficiency and accuracy of the multi-scale simulation approach are verified; the influence of physical parameters on the impact process as well as the agreement of the instantaneous and continuous impact modelling are demonstrated. Finally, the extension of the presented methods to particle systems consisting of thousands of impacting bodies is briefly reviewed.

Dynamics of underactuated multibody systems : modeling, control and optimal design

1 Introduction 2 Multibody Systems 3 Feedback Linearization and Model Inversion of Nonlinear Systems 4 Trajectory Tracking of Multibody Systems 5 Model Inversion Using Servo-Constraints 6 Trajectory Tracking of Flexible Multibody Systems 7 Optimal System Design 8 Concluding Remarks Index

A Discrete Element Approach to Model Breakable Railway Ballast

A discrete element approach to assess degradation processes in ballast beds is presented. Firstly, a discrete element model describing strength and failure of strong rock is introduced. For this purpose a granular solid is created by bonding of adjacent particles. A method to define angular ballast stones made from the granular solid is proposed. The strength of these stones is evaluated by compression between parallel platens. Comparing these results to published experimental data yields very good qualitative and reasonable quantitative agreement. Finally, the failure of aggregates of breakable stones is investigated by simulation of oedometric compression tests and indentation of a sleeper into a ballast bed.

Viscoplastic Effects Occurring in Impacts of Aluminum and Steel Bodies and Their Influence on the Coefficient of Restitution

Generally speaking, impacts are events of very short duration and a common problem in machine dynamics. During impact, kinetic energy is lost due to plastic deformation near the contact area and excitation of waves. Macromechanically, these kinetic energy losses are often summarized and expressed by a coefficient of restitution, which is then used for impact treatment in the analysis of the overall motion of machines. Traditionally, the coefficient of restitution has to be roughly estimated or measured by experiments. However, more recently finite element (FE) simulations have been used for its evaluation. Thereby, the micromechanical plastic effects and wave propagation effects must be understood in detail and included in the simulations. The plastic flow, and thus the yield stress of a material, might be independent or dependent of the strain-rate. The first material type is called elastic-plastic and the second type is called elastic-viscoplastic. In this paper, the influence of viscoplasticity of aluminum and steel on the impact process and the consequences for the coefficient of restitution is analyzed. Therefore, longitudinal impacts of an elastic, hardened steel sphere on aluminum AL6060 rods and steel S235 rods are investigated numerically and experimentally. The dynamic material behavior of the specimens is evaluated by split Hopkinson pressure bar tests and a Perzyna-like material model is identified. Then, FE impact simulations and impact experiments with laser-doppler-vibrometers are performed. From these investigations it is shown that strain-rate effects of the yield stress are extremely small for impacts on aluminum but are significant in impacts on steel. In addition, it is demonstrated that it is possible to evaluate for both impact systems the coefficient of restitution numerically, whereas for the aluminum body a simple elastic-plastic material model is sufficient. However, for the steel body an elastic-viscoplastic material model must be included.

A Stable Inversion Method for Feedforward Control of Constrained Flexible Multibody Systems

The inverse dynamics of flexible multibody systems is formulated as a two-point boundary value problem for an index-3 differential-algebraic equation (DAE). This DAE represents the equation of motion with kinematic and trajectory constraints. For so-called nonminimum phase systems, the remaining dynamics of the inverse model is unstable. Therefore, boundary conditions are imposed not only at the initial time but also at the final time in order to obtain a bounded solution of the inverse model. The numerical solution strategy is based on a reformulation of the DAE in index-2 form and a multiple shooting algorithm, which is known for its robustness and its ability to solve unstable problems. The paper also describes the time integration and sensitivity analysis methods that are used in each shooting phase. The proposed approach does not require a reformulation of the problem in input-output normal form, which is known from nonlinear control theory. It can deal with serial and parallel kinematic topology, minimum phase and nonminimum phase systems, and rigid and flexible mechanisms.

Design of Feed-Forward Control for Underactuated Multibody Systems with Kinematic Redundancy

For underactuated multibody systems with kinematic redundancy the design of a feed-forward control for end-effector trajectory tracking is presented. The feed-forward control design is based on an inverse model of the multibody system which is derived from the nonlinear input-output normal-form. The computation of the inverse model requires a bounded solution of the internal dynamics. This yields a two-sided boundary value problem which in general has a non-causal solution, yielding a pre- and post-actuation phase. In this paper it is shown that in the case of output trajectory tracking the additional degrees of freedom, resulting from the kinematic redundancy, can be used to introduce free design parameters with which a bounded and causal solution for the internal dynamics can be determined.

Extended Luenberger Observer for a MIMO Nonlinear Nonholonomic System

Abstract State of the art high speed color printers require sheets being accurately positioned as they arrive to the image transfer station (ITS). This goal has been achieved by constructing a steerable nips mechanism, which is located upstream from the ITS. This mechanism consists of two rollers which not only rotate to advance the paper along the track, but also steer the paper in the yaw direction. A recently developed nonlinear control strategy for the position of the sheet is briefly reviewed. The core of this paper focuses on the addition of a nonlinear observer used to estimate the longitudinal, lateral, and angular positions of a sheet, by detecting its motion along two of its perpendicular sides. The success of the approach presented is corroborated through simulations, in which the estimates from the extended Luenberger observer designed are used on a nonlinear feedback control strategy.

Characterization of adhesive bond properties with Lamb waves

The objective of this research is to use analytical and computational models to develop a quantitative understanding of the propagation of guided Lamb waves in multi-layered, adhesive bonded components. A specific goal of this study is to develop a theoretical understanding of the experimental behavior observed in [1]. Key issues of this study include the effect of the adhesive bond layer, including its thickness, low stiffness (relative to the aluminum adherends) and viscoelastic behavior. The propagation of these guided waves are interpreted in terms of dispersion relationships, displacement profiles and attenuation curves (both as functions of frequency and wavenumber). The ultimate goal of this study is to determine the effectiveness and sensitivity of guided Lamb waves to determine the in situ properties of an adhesive bond.