To find a relationship between moment versus curvature in a traction-free ovine artery, a pure moment was applied to a radially cut ovine artery (length 50.23 mm). The curvature of the segment opposite the cut was calculated and used to calculate the pre-stresses using a Fung type model. The pre-stresses were then used to calculate the moment. The moment applied during the experiment was calculated by recording the twist applied and the stiffness of the wire applying the moment. The artery was sutured symmetrically with a custom jig, and then sutured to two blocks, one fixed and one subject to the pure moment. The axial strain was assumed unity. The Fung model yielded a linear moment versus curvature relationship, as well as the moment versus curvature relationship for the experiment. Despite both small and large stretches, the strains felt by the artery were not influential enough to display a non-linear correlation for moment vs curvature.

Abstract In this thesis pre-stressed piezoelectric actuators for micro and fine mechanical applications have been developed. First, RAINBOW (Reduced And INternally Biased Oxide Wafer) and thick film actuators were manufactured and their electromechanical properties were characterised. In the second part, the novel pre-stressed piezoelectric actuator PRESTO (PRE-STressed electrOactive component by using a post-fired biasing layer) was developed and its electrical and electromechanical properties were measured. Commercial piezoelectric PZT 5A and PZT 5H discs were used in the RAINBOW and PRESTO actuators and PLZT paste for thick film actuators. The pre-stressing of the PRESTO actuators was based on the sintering shrinkage and different thermal expansion coefficient of the piezoelectric disc and passive material. Dielectric LTCC tape and AgPd paste were utilized as pre-stressing media and passive layer by using lamination and screen-printing, respectively. Different active and passive layer thicknesses and electrode materials were realized in order to obtain high displacements and good load bearing capability for actuators. The PRESTO actuators showed a significantly higher coercive electric field than their bulk counterparts, but a decreased remanent polarisation. The displacement as a function of load was measured under 0.3–3 N loads and electric fields up to ±0.75 MV/m. The highest displacement of 118 μm was obtained with a 250 μm thick PZT 5H actuator (Ø 25 mm) with LTCC tape (thickness ~96 μm) as the pre-stressing material. The corresponding actuator with AgPd pre-stressing material (thickness ~33 μm) produced 63 μm displacement. Additionally, PRESTO actuators were tested with a glued steel layer in a mechanical amplifier which obtained displacements up to 1.2 mm. Effective d31 coefficients of the PRESTO actuators were derived using an analysis based on unimorph model and measured displacement data. The actuators exhibited significantly enhanced effective d31 coefficients (d31eff = -690 pm/V and d31eff = -994 pm/V for PZT 5A and 5H, respectively) comparable to the RAINBOW actuators. Mass-producible PRESTO actuators with high displacement, moderate load bearing capabilities and integration possibilities can be utilised in various micro and fine mechanical devices e.g. dosing devices, electromechanical locks, regulators, positioners vibrators, speakers, adjusters, pumps, valves, relays, dispensers, micromanipulators, etc.

In this thesis, the effect of inhomogeneous pre-stress on elastic wave propagation and scattering in nonlinear elastic materials is investigated. Four main problems are considered: 1. torsional wave propagation in a pre-stressed annular cylinder, 2. the scattering of horizontally polarised shear waves from a cylindrical cavity in a pre-stressed, infinite, nonlinear elastic material, 3. the use of pre-stress to cloak cylindrical cavities from incoming horizontally polarised shear waves, and 4. the scattering of shear waves from a spherical cavity in a pre-stressed, infinite, nonlinear elastic material.It is observed that waves in a hyperelastic material are significantly affected by pre-stress, and different results are obtained from those which would be obtained if the underlying stress was neglected and only geometrical changes were considered. In Chapter 3 we show that the dispersion curves for torsional waves propagating in an annular cylinder are strongly dependent on the pre-stress applied. A greater pressure on the inner surface than the outer causes the roots of the dispersion curves to be spaced further apart, whereas a greater pressure on the outer surface than the inner causes them to be spaced closer together. We also show that a longitudinal stretch causes the cut-on frequencies to move closer together and decreases the gradient of the dispersion curves, whilst a longitudinal compression causes the cut-on frequencies to move further apart and increases the gradient of the dispersion curves. In Chapter 4 we observe that pre-stress affects the scattering coefficients for shear waves scattered from a cylindrical cavity. It is shown that, for certain parameter values, the scattering coefficients obtained in a pre-stressed medium are closer to those that would be obtained in the undeformed configuration than those that would be obtained in the deformed configuration if the pre-stress were neglected. This result is utilised in Chapter 5 where the cloaking of a cylindrical cavity from horizontally polarised shear waves is examined. It is shown that neo-Hookean materials are optimal for this type of cloaking. A stonger dependence of the strain energy function on the second strain invariant leads to a less efficient cloak.We observe that, for a Mooney-Rivlin material, as S1 tends from 1 towards 0 (in other words, as a material becomes less dependent on the first strain invariant, and more dependent on the second strain invariant), there is more scattering from the cloaking region. For materials which are strongly dependent on the second strain invariant the pre-stress actually increases the scattering cross-section relative to the scattering cross-section for an unstressed material, hence these materials are unsuitable for pre-stress cloaking.Finally, in Chapter 6 we study the effect of pressure applied to the inner surface of a spherical cavity and at infinity on the propagation and scattering of shear waves in an unbounded medium. It is shown that the scattering coefficients and cross-sections for this problem are strongly dependent on the pre-stress considered. We observe that a region of inhomogeneous pre-stress can lead to some counterintuitive relationships between cavity size and scattering cross-sections and coefficients.

Utilisés dans de nombreux domaines, les matériaux piézoélectriques sont régulièrement soumis à des sollicitations externes ou internes qui modifient leurs propriétés. Dans le but de prévoir et d’anticiper ces altérations, ce travail étudie les propriétés de matériaux piézoélectriques soumis à une contrainte statique de type mécanique ou électrique. Dans un premier temps, nous développons les équations du mouvement d’un matériau piézoélectrique (non hystérétique) au second ordre, en tenant compte des déformations dynamiques, mais aussi statiques. L’étude numérique des vitesses et du coefficient de couplage est faite sur le niobate de lithium, dans différents plans de coupe et différents systèmes de coordonnées afin d’évaluer dans quelles configurations l’application d’une contrainte externe électrique ou mécanique améliore ou dégrade les propriétés du matériau. Nous caractérisons ensuite les comportements hystérétiques de piézocéramiques sous contraintes en modélisant l’évolution des polarisations et déformations rémanentes microscopiques via les mouvements de murs de domaines. La comparaison des résultats numériques avec des évolutions de 4 piézocéramiques nous permet de définir le domaine de validation de nos hypothèses et d’expliciter les comportements hystérétiques de piézocéramiques. Dans une dernière partie, nous mettons en place un dispositif expérimental de mesure de déformations et du déplacement électrique de structures piézoélectriques sous contrainte mécanique. Ces résultats nous permettent de dimensionner notre étude sur le niobate de lithium et apportent une meilleure compréhension de l’évolution des déformations transversales dans les piézocéramiques. / Used in many domains, piezoelectric materials are frequently submitted to external or internaI stresses which modify their properties. In order to prevent and anticipate these modifications, this work studies the properties of piezoelectric materials under static electrical or mechanical stress. First, the motion equations of a piezoelectric (non hysteretic) rnaterial are developed at the second order taking to account the static strain and the dynamic ones. The numerical study of plane wave velocities and coupling coefficients is performed on lithium niobate, in different cuts and different coordinate systems. Then, we evaluate in which configurations the application of an electrical or mechanical stress improves or degrades the material properties. In a second part, the hysteretic behaviours of piezocerarnic materials under electrical and mechanical stresses are characterized by modelling the evolutions of microscopic remanent polarization and strains through the movements of domain walls. Numerical results are compared to evolutions of 4 piezoceramics and allow us to define the validation domain of our hypothesis and to explain hysteretic behaviours of soft and hard piezoceramics. In the last part, an experimental device to measure strains and electrical displacements under mechanical stress is developped. Results allow study on lithium niobate to be planned and bring a better understanding of transversal strain evolutions in piezoceramics.

Vibration structural health monitoring (VHM) uses the vibration properties to evaluate many civil structures during the design steps, building steps and service life.The whole function, expressed by stiffness and frequencies of tensegrity structures are primarily related to the level of pre-stress. The present work investigates the possibilities to use this relation in designing, constructing and evaluating the tensegrity structures.One of the aims of the thesis was to improve the current models for resonance frequency simulation of tensegrities. This has been achieved by introducing the bending behaviour of all components, and by a one-way coupling between the axial force and the stiffness.The environmental temperature effects on vibration properties of tensegrity structures have been also  investigated. Changes in dynamic characteristics due to temperature variations were compared with the changes due to decreasing pre-tension in one of the cables. In general, it is shown that the change in structural frequencies coming from temperature changes could of several magnitude as those from damage.Coinciding natural frequencies and low stiffness are known issues of tensegrity structures. The former can be an obstacle in VHM, while the later normally limits their uses in real engineering applications. It has been shown that the optimum self-stress vector of tensegrity structures can be chosen such that their lowest natural frequency is high, and separated from others.The environmental temperature effects on vibration properties of tensegrity structures were revisited to find a solution such that the natural frequencies of the tensegrity structures are not strongly affected by the changes in the environmental temperature. An asymmetric self-stress vector can be chosen so that the criterion is fulfilled as well as possible. The level of pre-stress can also be regulated to achieve the solution. The last part of this thesis, services as a summary of the work. / <p>QC 20160429</p>

The stiffness and frequency properties of tensegrity structures are functions of the pre-stress, topology, configuration, and axial stiffness of the elements. The tensegrity structures considered are tensegrity booms, tensegrity grids, and tensegrity power lines. A study has been carried out on the pre-stress design. It includes (i) finding the most flexible directions for different pre-stress levels, (ii) finding the pre-stress pattern which maximizes the first natural frequency. To find the optimum cross-section areas of the elements for triangular prism and Snelson tensegrity booms, an optimization approach is utilized. A constant mass criterion is considered and the genetic algorithm (GA) is used as the optimization method. The stiffness of the triangular prism and Snelson tensegrity booms are modified by introducing actuators. An optimization approach by means of a GA is employed to find the placement of the actuators and their minimum length variations. The results show that the bending stiffness improves significantly, but still an active tensegrity boom is less stiff than a passive truss boom. The GA shows high accuracy in searching the non-structural space. The tensegrity concept is employed to design a novel transmission power line .A tensegrity prism module is selected as the building block. A complete parametric study is performed to investigate the influence of several parameters such as number of modules and their dimensions on the stiffness and frequency of the structure. A general approach is suggested to design the structure considering wind and ice loads. The designed structure has more than 50 times reduction of the electromagnetic field and acceptable deflections under several loading combinations. A study on the first natural frequencies of Snelson, prisms, Micheletti, Marcus and X-frame based tensegrity booms has been carried out. The result shows that the differences in the first natural frequencies of the truss and tensegrity booms are significant and not due to the number of mechanisms or pre-stress levels. The tensegritybooms of the type Snelson with 2 bars and prism with 3 bars have higher frequencies among tensegrity booms. / <p>QC 20120904</p>

D’un point de vue mécanique, la peau est une structure multicouche complexeayant des propriétés viscoélastique, non-linéaire, quasi-incompressible, anisotrope eten état de précontrainte. Le travail présenté dans cette thèse associe expérimentation,modélisation et identification numérique et se distingue en particulier parl’utilisation d’un dispositif d’extensométrie développé au laboratoire et adapté à desmesures in vivo non invasives. Des tests ex vivo ont cependant été réalisés égalementà titre de comparaison et de validation. Une attention particulière a été portée à latension cutanée initiale (ou naturelle). Les essais in vivo ont permis d’obtenir desréponses force – déplacement sous différentes configurations angulaires, d’intensitéet pour diverses localisations corporelles. Les essais ex vivo ont quant à eux permisd’estimer l’état de contrainte initiale par la mesure des forces nécessaires à la remiseen tension d’explants. Ces différents essais expérimentaux ont été modélisés en utilisantdeux lois de comportement : la loi d’Ogden du premier ordre permettant dedécrire un comportement hyperélastique isotrope et la loi d’Holzapfel-Gasser-Ogden(HGO) décrivant un comportement hyper élastique anisotrope. Cette dernière a étéimplémentée sous l’interface utilisateur du logiciel ANSYS. Les paramètres caractéristiquesdes zones cutanées testées ont été identifiés par méthode inverse. L’influencede la compressibilité de la peau sur son comportement mécanique est mise en évidence.Au final, les travaux de cette thèse ont été appliqués au lambeau d’avancementde type V-Y qui est une technique de suture pratiquée pour combler les pertes desubstance.229 / From a mechanical point of view, the human skin is a complex multilayerstructure with viscoelastic, non-linear and anisotropic properties and a pre-stressstate. The work presented in this thesis combines experimentation, modeling andnumerical identification and distinguishes especially by the use of an extensometerdevice developed in the laboratory and suitable for non-invasive in vivo measurements.Ex vivo tests were however also performed for comparison and validation.Particular attention was paid to the initial skin tension. in vivo tests allowed theobtaining of load – displacement responses for different angular configurations, intensitiesand body locations. ex vivo tests in turn allowed the estimation of the stateof initial stress by measuring the forces necessary for the re-tension of the explants.These different experimental tests were modeled using two constitutive laws : thefirst order Ogden law allowing the description of an isotropic hyperelastic behavior,and the Holzapfel-Gasser-Ogden’ law (HGO) allowing the description of an anisotropichyperelastic behavior. The latter was implemented in the user interface ofANSYS software. The characteristics parameters of the skin areas tested were identifiedby the reverse method. The influence of the compressibility of the skin on itsmechanical behavior is highlighted. Finally, the work of this thesis were applied toan advancement flap of V-Y type.

Model glassy polymers, polymethyl methacrylate (PMMA) and polycarbonate (PC) are used to experimentally probe several aspects of polymer fracture. In Chapter 1, the method of pre-stress is employed as a means of improving the fracture properites of brittle PMMA. Samples are tested under equi-biaxial compression, simple shear and a combination of biaxial compression and shear. Equi-biaxial compression is shown to increase the threshold stress level for projectile penetration whereas shear pre-stress has a large effect on the overall energy absorbed during an impact. There is also an apparent interaction observed between compression and shear to dramatically increase the threshold stress. Pre-stressed laminates of PMMA and PC show an increase in damage area because of the unique formation of a secondary cone. In Chapter 2, the effect of stress state on stress relaxation in PMMA and PC is investigated. Direct comparisons are made between uniaxial and biaxial loading conditions. The experimental methods used highlight the effect of hydrostatic stress on the relaxation process. The data shows an increase in relaxation time and increase in the breadth of the relaxation spectrum with increases in hydrostatic stress. This suggests that the stress state can have a significant effect on the useful lifetime of pre-stressed articles. In Chapter 3, Mode I and II fracture studies are performed from quasi-static to low velocity impact rates on PMMA and PC. Mode II testing utilizes an angled double-edge notched specimen loaded in compression. The shear banding response of PMMA is shown to be highly sensitive to rate, with diffuse shear bands forming at low rates and sharp distinct shear bands forming at high rates. As the rate increases, shear deformation becomes more localized to the point where Mode II fracture occurs. PC is much less rate dependent and stable shear band propagation is observed over the range of rates studied with lesser amounts of localization. A new theory is formulated relating orientation in a shear band to intrinsic material properties obtained from true-stress true-strain tests. In a qualitative sense the theory predicts the high rate sensitivity of PMMA. A kinematic limit for orientation within a shear band is also derived based on entanglement network parameters. Mode II fracture in PMMA is shown to occur at this kinematic limit. For the case of PC, the maximum impact rates were not high enough to reach the kinematic limit. In Chapter 4, the deformation response, as observed in a shear band is interpreted through the characterization of the "intrinsic material properties" obtained from true stress - true strain 8compression tests. The relatively high rate sensitivity of PMMA deformed at room temperature is related to the proximity of the beta transition to the test temperature. This is also shown in corollary experiments on PC where deformation near the beta transition is accompanied by an increase in rate sensitivity. Physical aging results in a more narrow alpha transition and is shown to increase strain localization and decrease rate sensitivity at low strain rates.

No description available.

Structures acting as waveguides are quite common withexamples being, construction beams, fluid filled pipes, railsand extruded aluminium profiles. Curved structures like cartyres and pipe-bends may also be considered as waveguides. Wavesolutions in such structures may be found by a method calledthe Waveguide Finite Element Method or WFEM. This method uses afinite element approach on the cross-section of a waveguide tomodel the vibro-acoustic response as a set of linear, coupled,one dimensional, wave-equations. In this thesis six novel waveguide finite elements arederived and validated. These elements are, straight and curvedpre-stressed, orthotropic or anisotropic shell elements,straight and curved fluid elements, and straight and curvedfluid-shell coupling elements. Forced response and input power calculations for infiniteand periodic waveguides are presented. The assembled waveguidemodels can also serve as input for the Super Spectral FiniteElement Method, which enables forced response calculations formore complex boundaries. Furthermore, several properties ofdamped and undamped wave solutions are investigated. Finally, a car tyre model, encompassing for the highlyanisotropic material and the air cavity inside the tyre is setforth. A number of forced response calculations for this modelare presented and compared with measurements with goodagreement. Keywords:wave equation, wave solution, waveguide,finite element, spectral finite element, tyre noise, tyrevibration, input power, shells, pre-stress, fluid-shellcoupling axi-symmetric, two-and-half-dimensional