Academic literature on the topic 'Joint damping'

Bolted joints are integral parts of mechanical systems, and bolt preload loss is one of the major failure modes for bolted joint structures. Understanding the damping and frequency response to a varying preload in a single-bolted lap-joint structure can be very helpful in predicting and analyzing more complicated structures connected by these joints. In this thesis, the relationship between the bolt preload and the natural frequency, and the relationship between the bolt preload and the structural damping, have both been investigated through impact hammer testing on a single-bolted lap-joint structure. The test data revealed that the bolt preload has nonlinear effects on the structural damping and on the natural frequency of the structure. The damping ratios of the test structure were determined to increase with decreasing preload. An increase in structural damping is beneficial in most engineering circumstances, for it will reduce the vibrational response and noise subjected to external excitations. It was also observed that the modal frequency increased with increasing preload, but remained approximately constant for preload larger than 30% in the bolt yield strength. One application for studying the preload effect is the detection for loose bolts in structures. The possibility of using impact testing for estimating preload loss has been confirmed, and the modal damping was determined to be a more sensitive indicator than the natural frequency in a single-bolted lap-joint structure.

ABSTRACT This thesis is concerned with the investigation of the influence of damping on the frequencies of plane frames in the presence of rotational joint damping. Usually damping is ignored in dynamic analysis of structures, or is taken into account using the simple concept of Rayleigh damping. However, damping is always present, and must be considered for a realistic analysis of structures, especially if a matching is desired with frequencies obtained from vibration testing of the structure. In the thesis, the view point is adopted that the damping is caused mainly by the relative slip of the joined parts at various joints of the structure. The slip may either be translational or rotational. However, it appears that purely translational slip is not as significant as that caused by relative rotation of members at a joint. The thesis considers frames in which joints are not perfectly rigid, and hence allow relative rotation of members meeting at the joint. The resistance to the relative rotation is modeled by (1) assigning rotational stiffness to the joints, and (2) by assigning rotational damping coefficients to the joints. The damping is considered to be of the viscous variety, so that the damping moments are proportional to the rotational velocities at the joint. The resulting problem for free vibrations is a complex-valued eigenvalue problem. The thesis investigates this problem analytically, experimentally and numerically. Exact free vibration analyses were performed for straight beams with translational and rotational dampers at the ends. Experiments were conducted on a simple frame to determine the effect of joint damping on its in-plane frequencies. A finite element program was constructed to determine frequencies of general plane frames in the presence of rotational joint damping.<br>RésuméCette thèse présente une recherche portant sur l'influence de l'amortissement sur les fréquences des cadres en présence d'amortissement de rotation aux joints. Normalement on ne tient pas compte des amortissements dans une analyse dynamique des structures, si ce n'est par le concept d'amortissement de Rayleigh. Toutefois, l'amortissement est toujours présent et doit être pris en considération pour une analyse réaliste des structures surtout si on souhaite obtenir un résultat correspondant aux fréquences obtenues lors des tests de vibration de la structure. Dans cette thèse on considère que l'amortissement provient surtout du glissement se produisant aux parties jointes à plusieurs joints de la structure. Il peut s'agir d'un glissement de translation ou encore de rotation. Il semble toutefois qu'un simple glissement de translation ne soit pas aussi important qu'un glissement de rotation des membres à un joint. Cette thèse porte sur les cadres dans lesquels les joints ne sont pas parfaitement rigides et permettent donc une certaine rotation des membres aux joints. On crée un modèle de résistance relative à la rotation en donnant (1) des rigidités de rotation et (2) des coefficients d'amortissement de rotation aux joints. L'amortissement est considéré de type visqueux afin que le moment de glissement soit proportionnel à la vélocité de rotation aux joints.Le problème résultant des vibrations libres est un problème de valeurs propres (eigen-value) Dans cette thèse on étudie le problème de façon analytique, expérimentale et numérique. Des analyses précises de vibration libre sont faites pour des poutres droites avec des amortisseurs de translation et de rotation aux extrémités. Des expériences ont été faites sur un cadre simple afin de déterminer l'effet de l'amortissement aux joints sur la fréquence dans le plan du cadre. Un programme d'éléments finis a ét

Vibration problems are still the major constraint in modern machining processes that seek higher material removal rate, shorter process time, longer tool life and better product quality. Depending on the process, the weaker structure element can be the tool/tool holder, workpiece/fixture or both. When the tool/tool holder is the main source of vibration, the stability limit is determined in most cases by the ratio of length-to-diameter. Regenerative chatter is the most significant dynamic phenomenon generated through the interaction between machine tool and machining process. As a rule of thumb, the ratio between the tool’s overhang length and the tool’s diameter shouldn’t exceed 4 to maintain a stable machining process while using a conventional machining tool. While a longer tool overhang is needed for specific machining operations, vibration damping solutions are required to ensure a stable machining process. Vibration damping solutions include both active and passive damping solutions. In the passive damping solutions, damping medium such as viscoelastic material is used to transform the vibration strain energy into heat and thereby reduce vibration amplitude. For a typical cantilever tool, the highest oscillation displacement is near the anti-node regions of a vibration mode and the highest oscillation strain energy is concentrated at the node of a vibration mode. Viscoelastic materials have been successfully applied in these regions to exhibit their damping property. The node region of the 1st bending mode is at the joint interfaces where the cantilever tools are clamped. In this thesis, the general method that can be used to measure and characterize the joint interface stiffness and damping properties is developed and improved, joint interfaces’ importance at optimizing the dynamic stiffness of the joint interface is studied, and a novel advancing material that is designed to possess both high young’s modulus and high damping property is introduced. In the joint interface characterization model, a method that can measure the joint interface’s stiffness and damping over the full frequency range with only the assembled structure is presented. With the influence of a joint interface’s normal pressure on its stiffness and damping, an optimized joint interface normal pressure is selected for delivering a stable machining process against chatter with a boring bar setting at 6.5 times overhang length to diameter ratio in an internal turning process. The novel advancing material utilizes the carbon nano particles mixed in a metal matrix, and it can deliver both high damping property and high elastic stiffness to the mechanical structure.<br><p>QC 20130521</p><br>PoPJIM, HydroMod, XPRES, NanoComfort

Rigidizable/Inflatable space structures have been the focus of renewed interest in recent years due to efficient packaging for transport. In this work, we examine new mathematical systems used to model small-scale joint dynamics for inflatable space truss structures. We investigate the regularity and asymptotic behavior of systems resulting from various damping models, including Kelvin-Voigt, Boltzmann, and thermoelastic damping. Approximation schemes will also be introduced. Finally, we look at optimal control for the Kelvin-Voigt model using a linear feedback regulator.<br>Ph. D.

The precision of machining systems is ever increasing in order to keep up with components’ accuracy requirements. At the same time product variants areincreasing and order quantities are decreasing, which introduces high demands on the capability of machining systems. The machining system is an interaction between the machine tool structure, the process and the control system and is defined in terms of capability by the positional, static, dynamic and thermal accuracy. So far, the control of the machining system, in terms of static and dynamic stability is process based which is often translated into sub-optimum process parameters and therefore low productivity.This thesis proposes a new approach for control of the machining systemwhich is based on the capability to control the structural properties of themachine tool and as a result, controlling the outcome of the machining process.The control of the structural properties is realized by carefully designed Joint Interface Modules (JIMS). These modules allow for control of the stiffness and damping of the structure, as a result of tuning the contact conditions on the interface of the JIM; this is performed by control of the pre-load on the interface,by treatment of the interface with damping enhancing materials, or both. The thesis consists of a presentation of the motivation behind this work, the theoretical basis on which the proposed concept is based and a part describing the experimental investigations carried out. Two prototype JIMs, one for a milling process and one for a turning process were used in the experimental investigations that constitute the case studies for examining the validity of the proposed concept and demonstrating its applicability in a real production environment.<br><p>QC 20140611</p><br>EU FP7 POPJIM

Les structures aérospatiales sont de plus en plus légères, ce qui les rend sensibles aux vibrations. Pour ces structures le frottement pourrait fournir plus d’amortissement que les polymères d’une part à cause des températures extrêmes de fonctionnement (&lt;-20°C et &gt;100°C) et d’autre part parce qu’il est plus efficace que les absorbeurs actifs ou passifs à masse ajoutée qui ne sont pas adaptés aux problèmes sur larges bandes de fréquence, qui comprennent plusieurs fréquences propres. Pour que l’amortissement induit par frottement soit efficace, il faut asservir la charge normale aux amplitudes vibratoires. Le premier objectif de la thèse est de définir un coefficient indicateur de la capacité du changement des forces normales de serrage de vis à contrôler les niveaux vibratoires. Ce coefficient sera un outil de dimensionnement des structures réelles à contrôler. Le deuxième objectif est de concevoir des lois de contrôle semi-actif de la force de serrage pour des régimes libre et forcé d’un problème modal à deux degrés de liberté. L’objectif final est de concevoir un dispositif pouvant s’intégrer à une structure assemblée pour contrôler la charge normale dans les liaisons et de tester expérimentalement l’efficacité des lois de contrôle conçues pour une structure académique ou industrielle. Le contrôle des forces de serrage s’effectue avec l’utilisation des patchs piézoélectriques<br>Because of their lightness the aerospace structures are vulnerable to vibrations whose the amplitudes need to be mitigated through the active and the semi active methods. For extreme temperatures (&lt;20oC and &gt; 100oC), Coulomb friction might provide more structural damping than polymers. Nevertheless, there is a consensus to say that these friction-damping values are often lower than 5%. Moreover friction damping is amplitudedependent. Therefore, engineers have to tune the normal load in order to get the maximum damping for the average vibration level. The first objective of the thesis is to define a coefficient indicator of the ability of the tightening force to control the vibration levels. This coefficient is a design tool of real structures witch need controlling. The second objective is to design semi-active control laws of the clamping force for free and forced modal problem of two degrees of freedom. The ultimate goal is to design a device that can be integrated to an assembly structure for controlling the normal load in the joints and then experimentally test the effectiveness of the laws control for an academic or an industrial structure. The tightening control forces is effected with the use of piezoelectric patches