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Inhaltsangaben
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Buch, Hörbücher:

Maschinendynamik

Seite 6

On the Dynamics of Axially Moving Strings

Ulrike Zwiers

Taschenbuch

The primary objective of this thesis is to provide a coherent and concise elaboration of the modeling and analysis of axially moving strings. Both analytical and numerical solution techniques are used to study the free vibrations of strings in stationary and accelerated motion. Axially moving continua are prevalent in many engineering applications such as power transmission belts, magnetic tapes, elevator and crane hoist cables, band saw blades, and textile-fiber manufacture. During the past several decades, those systems have been extensively studied by many researchers, frequently referring to the translating uniform string as the simplest representative of distributed gyroscopic systems. This thesis addresses the modeling and analysis of strings moving axially between fixed supports. First, the problem of stationary motion is defined, and it is demonstrated that a perfectly flexible string can undergo a snake-like motion, that is, a motion along an arbitrary fixed path in space, only at constant speed. Along a straight line, however, motion at time-varying speed is feasible. For this special case, the linearized equation governing the transverse vibrations of an axially accelerated string is derived in both Lagrangian and Eulerian description by superposing small perturbations on the stationary solution. As the resulting equation of motion reveals a conflict with the one commonly referred to in literature, a comparison of both models is given including a physical interpretation of the well-known, but apparently incorrect equation. Also a numerical stability analysis based on Floquet's theory is presented.

Dynamic Torsion Test for the Mechanical Characterization of Soft Biological Tissues

Davide Valtorta

Taschenbuch

In this thesis, a novel measurement method for the characterization of the mechanical properties of soft biological tissues is presented. The linear viscoelastic properties are determined through dynamic torsion tests by applying forced torsional oscillations to soft tissue samples. This work presents the definition of the measurement principle, with the design of torsional resonating sensors and the development of analytical and finite elements methods used for the inverse material characterization. The reliability and limitations of the proposed measurement technique have been assessed with experiments on soft biological materials as well as with synthetic materials. The viscoelastic response of soft materials is characterized for harmonic shear deformations at high frequencies (1-12 k Hz) and small strains (up to 0. 2% nominal strain for the soft biological tissues considered). Experiments are performed using a torsional resonating sensor, hereafter referred to as the torsional resonator device ( T R D), which consists of a rod excited to vibrate at resonance, with one end in contact with a material sample. The resonating sensor induces shear waves in the material analyzed. Adherence between vibrating sensor and material sample is ensured by vacuum clamping in the contact area. The response of the material results in changes in the dynamic behavior of the vibrating system sensor + material sample. The damping characteristics and resonance frequency of the vibrating system are inferred from the control variables of a phase stabilization loop. These quantities are then related to the mechanical properties of the material using analytical and finite element models that describe the interaction between sensor and material sample.