Cette thèse porte sur l’étude du comportement mécanique et vibratoire d’un composite biosourcé incorporant un matériau viscoélastique. Les matériaux étudiés sont des stratifiés en composite lin/greenpoxy et des stratifiés viscoélastiques composés d'un noyau viscoélastique en caoutchouc naturel confiné entre deux composites. La première partie du travail a été consacrée à l’étude de l’influence de l’intégration de la couche viscoélastique sur le comportement mécanique des composites. L’analyse des résultats expérimentaux et l’observation des signaux d’émission acoustique obtenus dans ces composites soumis à différentes sollicitations mécaniques en statique et en fatigue ont permis d’identifier les signatures acoustiques des mécanismes d’endommagement prépondérants dans les deux matériaux. Dans un deuxième temps, les propriétés dynamiques de ces composites ont été déterminées à partir des essais de vibration. Les résultats obtenus ont montré que la couche viscoélastique a joué un rôle majeur dans l'amortissement et la dissipation d'énergie des composites. Suite à cette analyse, nous avons mis en place une procédure, utilisant la méthode des éléments finis, pour calculer l’amortissement de ces matériaux. Dans le but de mettre en évidence l’influence des caractéristiques de la couche viscoélastique, une étude paramétrique a été menée sur le composite viscoélastique, permettant d’optimiser l’amortissement de ce matériau en faisant varier divers paramètres. Enfin, le comportement visqueux des composites a été caractérisé par la méthode de résonance non linéaire en faisant varier l’amplitude d’excitation. / This thesis focuses on the study of the mechanical and vibration behaviour of a flax fibre reinforced composites with and without an interleaved natural viscoelastic layer. The composite materials have been characterized experimentally using different mechanical and vibrational tests. First, both types of composites were studied using uni-axial tensile and three-points bending tests. Acoustic emission (AE) has been often used for the identification and characterization of micro failure mechanisms in composites. The results showed that these composites have very high specific characteristics. It can be used for applications currently using composites reinforced with synthetic fibres such glass, carbon…. Next, experimental and finite element vibration analyses were carried out on the composites with and without an interleaved natural viscoelastic layer. A good agreement between the two methods was obtained. It has been shown that the viscoelastic layer plays a major role in damping because it has a high level of energy dissipation. Therefore, it improves with a significant way the modal properties of the composite. Finally, nonlinear resonance tests were performed on the composites. It has been shown that the viscoelastic layer generates a nonlinear behaviour in the material. The linear and nonlinear, elastic and dissipative parameters have been calculated to deduce finally that nonlinear parameters are more sensitive to heterogeneities than those derived from linear vibration tests.

In this master thesis the performance of external viscous dampers attached to cables in cable-stayed bridges have been studied. A comparison has been performed between a linear and a non-linear cable model. The comparison was carried out for two bridge cables, one from the Dubrovnik Bridge and the other from the Normandie Bridge. The performance of the dampers have been measured in terms of maximum achieved damping ratio and minimum amplitude of vibration. The analysis was performed using the finite element method. The damping ratio was measured using both the half-power bandwidth method and by calculating the loss factor. The half-power bandwidth method can only be applied to a linear system. Therefore, the loss factor was evaluated for the linear model and compared to the results obtained using the half-power bandwidth method. From the comparison, it was concluded that the damping ratio evaluated using the loss factor was similar to the results obtained when using the half-power bandwidth method. However, when calculating the loss factor, it was of great importance that the resonance frequency of the system was accurately determined. The loss factor was then calculated for the non-linear model and compared to the results obtained for the linear model. Since the loss factor measures the energy dissipated in a system, it could be utilised for the non-linear model. When computing the strain energy for the non-linear model an approximate method was used to take into consideration the strain energy caused by the static deformation of the cable. From the comparison between the linear and non-linear cable models, it was concluded that the optimal damper coefficients obtained by both models are not significantly different. However, there is an uncertainty in the results due to the fact that an approximate method was used when calculating the strain energy for the nonlinear model. It was also observed that a very accurate evaluation of the system’s resonance frequency was needed to calculate the loss factor. It was also observed that the variation in amplitude of vibration for varying damper coefficient was small for all modes of vibration for the Dubrovnik Bridge Cable as well as for the first mode of vibration for the Normandie Bridge Cable. The difference in the results between the two bridge cables needs to be investigated further in order to get a better understanding of the results.

年々増大するエンジントルク変動を低減するために広角トーショナルダンパの適用例が増してきているが，それらに起因する非線形振動のメカニズムや振動低減のための研究例は少ない．本論文の目的は，これらに関する非線形振動の解明および改善，および精度と計算コストを両立するモデルの開発である．そのため，実験検証に基づくモデルを開発し，非線形振動を大幅に削減する手法を検討した．さらに，計算コストを増加させることなく解析精度を改善する不等ピッチ離散化モデル化手法を検討した． / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

Aufgrund der stetig wachsenden Anforderungen hinsichtlich Gewichtsreduzierung und Funktionsintegration, besonders im Bereich des Automobilbaus, werden traditionell aus metallischen Werkstoffen gefertigte Komponenten immer häufiger durch Kunststoffbauteile substituiert. Dem entgegen steht derzeit die Tatsache, dass, trotz hohen Prozessverständnisses und des Wissens um die Prozess-Struktur-Eigenschafts-Beziehungen beim Vibrationsschweißen, die theoretisch erzielbaren Schweißnahtfestigkeiten, von 90 % bis 100 % des unverstärkten Grundmaterials, in der industriellen Serienfertigung bei weitem nicht erreicht werden. Die Komplexität eines industriell gefertigten Bauteils wird an Plattenprüfkörpern simuliert. Die Ergebnisse der Schweißversuche zeigen, dass unterschiedliche Wandstärken im Schweißnahtbereich, Bauteilverzug und unterschiedliche Schwingrichungen innerhalb einer Schweißnaht zu ungleichmäßigen lokalen Prozessbedingungen währenden des linearen Vibrationsschweißprozesses führen. Diese hinterlassen lokale Schwachstellen, welche das Gesamtbauteilversagen bestimmen. Durch alternative Prozessführungsstrategien, wie das Hochdruckanfahren und die IR-Vorwärmung, können diese Schwachstellen reduziert und die Gesamtbauteilfestigkeit angehoben werden. Dies wird am Beispiel des Bauteilverzugs veranschaulicht. / Due to the increasing demands for weight reduction and integration of function, especially in the field of automotive, components made of metallic materials are increasingly being substituted by components made of thermoplastic materials. In contrast to this there is currently the fact that, despite the high process understanding of the vibration welding and the knowledge of the process-structure-property relationships, the theoretically achievable weld strengths of 90 % to 100 % of the unreinforced base material strength are far to be achieved in industrial series production. The complexity of an industrially manufactured component is simulated by using plate test specimens. The results of the welding experiments show that different wall thicknesses in the weld area, component warpage and different friction angle within the weld leads to nonuniform local process conditions during linear vibration welding process. This results in local weak spots, which reduce the total component strength. These local weak spots can be reduced by using alternative process strategies, such as in-process pressure variation and IR preheating. So not only the local strengths but also the total component strength will be increased. This is shown on the example of component warpage.