Assessments of structures with postulated or existing defects are generally carried out using standards and engineering assessment procedures. Assessments of this type involve comparing an applied force, in this case the crack driving force, with a material property, which in this case is the material's resistance to fracture, its fracture toughness. The crack driving force J can be calculated directly or implicitly by using a failure assessment diagram. Assessments can be based on either the initiation of the growth of a crack or, when dealing with ductile fracture, on an amount, e.g. 2 mm, of ductile tearing. Material fracture toughness values are obtained by testing high constraint specimens such as deeply cracked compact tension specimens and single edge notched bend specimens under uniaxial loading conditions. The high constraint of the test specimens provides conservative measurements of the fracture toughness for most applications. However, this assumption of conservatism is not necessarily applicable where there are biaxial loading conditions. The literature concerning assessments of such components mainly discusses whether uniaxial loading conditions provide conservative estimates of fracture toughness. Crack driving forces under biaxial loading can be overestimated, leading to a loss of conservatism. Conversely, biaxial loading could be beneficial and thus an approach that is consistently conservative has implications for the cost and time involved in the consequences of prematurely assessing or predicting the failure of a structure or component. This research considers the effects of biaxial loading on all the parameters involved in the integrity assessment of structures, components and specimens with defects. These parameters include the crack driving force, material fracture toughness, internal stresses and limit loads. It will address their relative effects on the determination of failure when compared with the assumption of uniaxial loading. The methods used will be analytical, using the equations and theories of standard solid mechanics, fracture mechanics and existing advice in R6 and the literature, and numerical using finite element analyses. Experimental, analytical and numerical work in the literature will be assessed and discussed and their outcomes compared with the findings of this research. The overall aim is to provide more explicit advice on the assessment of defects in components under biaxial loading in the R6 procedure.

Ce travail revisite la problématique des transitions de phases cristallines induites thermiquement et mécaniquement dans les polymères semi-cristallins à liaisons fortes. Ce point revêt une importance primordiale dans des matériaux où l'anisotropie de liaisons intermoléculaires (van der Waals vs hydrogène) conduit à l'existence de phases cristallines présentant une structure en feuillets des liaisons hydrogène. L'originalité principale réside dans le recours à l'expérimentation in situ pour le suivi de l'évolution structurale de ces matériaux polymorphes par diffraction et diffusion des rayons X sous rayonnement synchrotron. L'étude concerne les polyamides 11 et 6 et une attention particulière est portée à l'influence de ces transitions sur le comportement mécanique sous sollicitations uniaxiale et biaxiale. La caractérisation structurale confirme l'existence d'une transition de phase, la transition de Brill, vers une phase HT de symétrie hexagonale dans le cas de certaines structures cristallines (Alpha' et Delta' du PA11, Alpha défectueuse et Beta du PA6). Les formes stables (Alpha' du PA11 et Alpha du PA6) se reforment au refroidissement. Sous sollicitation uniaxiale, les polyamides se révèlent ductiles, quelle que soit la forme cristalline; une transformation ordre vers désordre intervient à T<TBrill et favorise l'apparition des mésophases (Delta’ du PA11 et Beta du PA6), tandis que la phase HT est stable mécaniquement. En étirage biaxial les phases présentant une organisation en feuillets des liaisons H sont fragiles, contrairement aux phases désordonnées et HT. Ceci souligne la nécessité de maîtriser l'évolution structurale dans la stratégie de mise en œuvre industrielle par biétirage à l'état solide. / This work revisits the problem of thermally and mechanically-induced crystal phase transitions in semi-crystalline polymers bearing H bonds. This point is of prime importance in materials where anisotropic intermolecular interactions (van der Waals vs H-bonding) gives rise to the existence of crystal phases with sheet-like H-bonded structures. The main originality of the present study lies in the use of in situ characterization of these polymorphic materials by wide angle and small angle X Ray scattering from a synchrotron source. Materials under concern are polyamides 11 and 6, and specific attention is paid to the role of these transitions on the mechanical behavior under uniaxial and biaxial drawing. The structural characterization confirms the existence of a phase transition, the Brill transition, towards a HT phase with hexagonal symmetry for some specific crystal structures (Alpha' and Delta' in PA11, Alpha defective and Beta in PA6). The most stable forms (Alpha' in PA11 and Alpha in PA6) rebuild upon cooling. Under uniaxial drawing, polyamides exhibit ductility, whatever the original crystal form is; an order towards disorder transition occurs for T<TBrill, which favors mesophase development (Delta’ in PA11 and Beta in PA6), while above TBrill the HT phase is mechanically stable. Upon biaxial stretching, phases with H-bonded sheet-like organization are brittle, contrary to mesomorphic and HT phases. These findings underline the need to monitor the structural evolution in order to achieve proper solid-state biaxial stretching in industrial processing.

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Pelvic organ prolapse, the decent of the pelvic organs from their normal anatomical position, is a common condition among women that is associated with mechanical alterations of the vaginal wall. In order to characterize the complex mechanical behavior of the vagina, we performed planar biaxial tests of vaginal specimens in both the passive (relaxed) and active (contracted) states. Specimens were isolated from virgin, female Long-Evans rats (n=16) and simultaneously stretched along the longitudinal direction (LD) and circumferential direction (CD) of the vagina. Tissue contraction was induced by electric field stimulation (EFS) at incrementally increasing values of stretch and, subsequently, by KCl. On average, the vagina was stiffer in the CD than in the LD (p<0.001). The mean maximum EFS-induced active stress was significantly higher in the CD than in the LD (p<0.001). On the contrary, the mean KCl-induced active stress was lower in the CD than in the LD (p<0.01). When comparing the mean maximum EFS-induced active stress to the mean KCl-induced active stress, no differences were found in the CD (p=0.404) but, in the LD, the mean active stress was much higher in response to the KCl stimulation (p<0.001). Collectively, these results demonstrate that the anisotropic behavior of the vaginal tissue is determined not only by the collagen and smooth muscle fiber organization but also by the innervation. The findings of this study may contribute to the development of more effective treatments for pelvic organ prolapse. / MS

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Tese de mestrado integrado. Engenharia Civil (Estruturas). Universidade do Porto. Faculdade de Engenharia. 2010

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Characterization of multiaxial mechanical properties of cardiovascular soft tissue is essential in order to better understand their growth and remodeling in homeostatic conditions and in response to injury or pathological conditions. Though numerous phenomenological models have been proposed to characterize such multiaxial mechanical behavior, the approach has certain drawbacks regarding experimental determination of the model coefficients. We propose a method that aims to overcome these drawbacks. The approach makes use of orthogonal polynomials to fit the biaxial test data and suggests a way to derive the strain energy function from these analytical fits by way of minimizing the deviation of the behavior from hyperelastic ideal. Using the proposed method, a strain energy function for a lymphatic vessel is derived and the method is compared with traditional ones that used non-orthogonal polynomials as independent variables in the functional form for strain energy. The unique coefficient values obtained using the proposed method, for the first time gives us an opportunity to attribute a physical characteristic of the material to the coefficient values. The method also provides a way to assess two different material behaviors by way of comparing their deviation from the hyperelastic behavior when a similar test protocol is used to collect the data, over a similar deformation range and the order of polynomial function is chosen so as to give a similar error of fit. The behavior of mesenteric lymph vessels from normal cows, cows subjected to sham surgery and those subjected to 3 days of edematous conditions by venous occlusion are compared using this method. To be able to better understand the changes in mechanical behavior, morphological analysis of the vessels was carried out and the geometric and structural changes in these vessels were studied. We found that the behavior of bovine mesenteric lymph vessels subjected to a high flow condition shows a small difference in their mechanical behavior as compared to the vessels from normal a cow and a cow subjected to sham surgery. The geometry and structure of these vessels also showed marked differences from the other two. The thickness to radius ratio increased and a rise in percentage of area occupied by smooth muscle cells and medial collagen was observed. Though not all the differences were statistically significant, we conclude that the behavior and the morphology are suggestive of the remodeling of the vessel in response to altered hemodynamic conditions and require further investigation.