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Selected AbstractsFracture analysis of strength undermatched Al-Alloy welds in edge cracked tensile panels using FITNET procedureFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 9 2008S. CICERO ABSTRACT This paper presents a methodology for the assessment of the remaining load carrying capacity of thin-walled components under tension containing highly strength undermatched welds and edge cracks. The analysis is based on the strength mismatch option of the fracture module, part of the newly developed European fitness-for-service (FFS) procedure FITNET. The mismatch option of the FITNET fracture module allows weld features such as weld tensile properties and weld geometry to be taken into account in the fracture analysis of cracked welded components. The methodology described was verified for centre cracked Al-alloy large tensile panels containing undermatched welds in Ref. [1] and hence the present work provides validation with experimental results of the single edge cracked (SEC) and double edge cracked (DEC) panels. The material used is an age-hardening aluminium alloy 6013 in T6 temper condition used in welded airframe components. The welds in the form of butt joints were produced using the CO2 laser beam welding process. The results show that by using the FITNET FFS methodology with an appropriate selection of the input parameters, safe acceptable predictions of the maximum load carrying capacity of the welded panels can be obtained. It should also be noted that one of the main difficulties that engineers encounter when applying mismatch analysis for first time is its apparent complexity. A step-by-step analysis is proposed here in order to provide guidance for this kind of assessments. [source] The normal and cancerous living cellINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 14 2006Janos Ladik Abstract We do not have a definition of the living and cancerous states; we can give only their main characteristics at the different levels of organization: cell, organ, and organism. A simple model is proposed for a normal eukaryotic cell based on Prigogine's equation of chemical kinetics with diffusion. In this model, possibly only a few hundred key biochemical reactions should be selected together with their rate and diffusion constants. To solve these coupled nonlinear partial differential equation systems, it is proposed that the model cell be subdivided into compartments and that the problem be worked out always for one compartment (finite element method). This is possible, since the most important biochemical reactions and reaction cycles occur in different parts of the cell. The solutions (concentrations) obtained in one compartment can be used as input to the other compartments (together with the components entering from the environment). As an example, the problem of 10 reactions and 3 compartments has been solved by discretizing the space coordinates and choosing time steps. The solutions obtained by solving the 10 differential equations directly and by the compartmentalization agree very well. The main obstacles to further progress lie in the right choice of reactions and compartments, as well as in the correct estimation of the rate and diffusion constants, which were measured in only a few cases. If such a model cell can be obtained, the solutions should be investigated to determine (i) for their stability (homeostasis); (ii) whether changing the input concentrations to a larger degree one would obtain a new stationary state showing the characteristics of a precancerous state; and (iii) a method of extracting those input concentrations, or functions of them, which are the most important regulatory parameters. If successful, this would provide a scientific definition of the living state in the normal and cancerous states, respectively, at least at the cell level. Finally, outline is provided showing how the model might be extended to multicellular cases, as well as the main difficulties of such a process. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] A review of methods for Centropomus spp. (snooks) aquaculture and recommendations for the establishment of their culture in Latin AmericaAQUACULTURE RESEARCH, Issue 7 2008Luis Alvarez-Lajonchère Abstract The positive features of Centropomids and Latids for cultivation are analysed and highlighted. The main life cycle characteristics as well as market prices and demands are presented. Good growth, highly efficient food conversion ratios and energy utilization allow very high biomass yields per unit volume in nursery and grow-out systems. They have shown notable tolerances to main environmental conditions, as well as considerable versatility in adapting to culture systems (cages, ponds and tanks), and culture intensity (extensive, semi-intensive, intensive and superintensive), especially in estuarine and coastal sites and ponds. These positive features provide them with a high culture potential. A general summary of their culture is presented, based on the commercial cultivation of the Asian sea bass or barramundi Lates calcarifer as well as the experimental and pilot-scale results from the main American species. The importance of hatchery fry production as an essential culture pre-requisite is emphasized, in addition to an analysis of the main difficulties and constraints for future development. [source] Modeling And Robust Pi Control Of A Fluidized Bed Combustor For Sewage SludgeASIAN JOURNAL OF CONTROL, Issue 4 2002Yingmin Jia ABSTRACT Based on experimental data, a fluidized bed combustor is modeled as an interval system. Severe model uncertainty and large time delays lead to the main difficulties in solving the control problem. The design uses the stability test of closed-loop systems as the main guideline for developing a robust PI controller. In particular, a new formula to compute the maximal magnitude of an edge rational function at a fixed frequency is derived, which provides a way to deal with time delay by transforming it into multiplicative uncertainty. Both the theoretical and the experimental results show that the designed PI controller can satisfy the desired performance requirements. [source] Smeared crack approach: back to the original trackINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 12 2006M. Cervera Abstract This paper briefly reviews the formulations used over the last 40 years for the solution of problems involving tensile cracking, with both the discrete and the smeared crack approaches. The paper focuses on the smeared approach, identifying as its main drawbacks the observed mesh-size and mesh-bias spurious dependence when the method is applied ,straightly'. A simple isotropic local damage constitutive model is considered, and the (exponential) softening modulus is regularized according to the material fracture energy and the element size. The continuum and discrete mechanical problems corresponding to both the weak discontinuity (smeared cracks) and the strong discontinuity (discrete cracks) approaches are analysed and the question of propagation of the strain localization band (crack) is identified as the main difficulty to be overcome in the numerical procedure. A tracking technique is used to ensure stability of the solution, attaining the necessary convergence properties of the corresponding discrete finite element formulation. Numerical examples show that the formulation derived is stable and remarkably robust. As a consequence, the results obtained do not suffer from spurious mesh-size or mesh-bias dependence, comparing very favourably with those obtained with other fracture and continuum mechanics approaches. Copyright © 2006 John Wiley & Sons, Ltd. [source] Unraveling the Genetic Component of Multifactorial Diseases: Dream or RealityINTERNATIONAL STATISTICAL REVIEW, Issue 1 2000F. Clerget-Darpoux Summary The etiology of many human diseases is complex and very likely involves a combination of genetic and environmental risk factors. A popular strategy to detect genetic risk factors is to perform a systematic screening of the genome searching for linkage. The power of such and approach depends very much on the unknown characteristics of the genetic factors and the main difficulty is to establish a good trade-off between false positives and false negatives. Besides, a precise localisation of the risk factor will generally not be obtained. The set up of a candidate gene stratery is necessary to go further in genetic factor identification. It is likely that for multicfactorioal diseases the only genetic risk factors that can be detected are those with fairly strong effect. Even in that case, it is important to design strategies which increase the power of detection and provide for a better evaluation of the associated risks. Résumé La majorité des maladies humaines ont une étiologic complexe et résultent, de I'interaction de facteurs génétiques, etd' environnment. Une stratégic, populaire pour détecter; des cacteurs de risque est la recherche systématique, de liaison sur le génome. La puissance d' une telle approch dépend essentiellement des caractéristiques, inconnues des facteurs génétiques, et la difficultéprincipale est d'établir un bon cornpromis entre faux positifs et faux négatifs. PPar ailleurs, elle ne permet généralement pas de locatiser de facon préciseles facteurs génétiques, impliqués. La misc en place d'une stratégic, de géne candidat est nécessaire pour avancer vers I; identificatin d' un facteur de risque génétique. IIest vraisemblable que pour les maladies multifactorielles, seuls les facteurs ayant un effet immportant pourront étre, détecté. Méme, dans ce cas, il est important de mettre enpalce des stratégies, qui donnent une pussance maximum de détection, et permettent d' évaluer au mieux les risques associés. [source] Advances in MR imaging of the skin,NMR IN BIOMEDICINE, Issue 7 2006Jacques Bittoun Abstract MR imaging of the skin is challenging because of the small size of the structures to be visualized. By increasing the gradient amplitude and/or duration, skin layers can be visualized with a voxel size of the order of 20,µm, clearly the smallest obtained for in vivo images in a whole-body imager. Currently, the gradient strength of most commercial systems enables acquisition of such a small voxel size, and the main difficulty has thus become to achieve sufficient detection sensitivity. The signal-to-noise ratio (SNR) can be increased either by increasing the magnetic field strength or by minimizing noise with small coils; cooling copper coils or superconducting coils can enhance the SNR by a factor of 3 or more. MR imaging, because of the large number of parameters it is able to measure, can provide more than the microscopic architecture of the skin: physical parameters such as relaxation times, magnetization transfer or diffusion, and chemical parameters such as the water and fat contents or phosphorus metabolism. In spite of the amount of information they have provided to date, MR imaging and spectroscopy have had limited clinical applications, mainly because cutaneous pathologies are easily accessible to the naked eye and surgery. However, MR technologies indeed represent powerful research tools to study normal and diseased skin. Copyright © 2006 John Wiley & Sons, Ltd. [source] The ontogeny of handling hard-to-process food in wild brown capuchins (Cebus apella apella): evidence from foraging on the fruit of Maximiliana maripaAMERICAN JOURNAL OF PRIMATOLOGY, Issue 11 2010Noëlle Gunst Abstract We examined age-related differences in wild brown capuchins' foraging efficiency and the food-processing behaviors directed toward maripa palm fruit (Maximiliana maripa). A detailed comparison of the different foraging techniques showed that plucking the fruit from the infructescence constituted the main difficulty of this task. Foraging efficiency tended to increase with age, with a threshold at which sufficient strength allowed immatures by the age of three to reach adult-level efficiency. Youngsters spent more time than older individuals browsing the infructescence and pulling the fruit in an attempt to harvest it. Infants tried to compensate for their inability to pluck fruit by adopting alternative strategies but with low payback, such as gnawing unplucked fruit and opportunistically scrounging others' partially processed food. Although around 2 years of age, young capuchins exhibited all of the behaviors used by adults, they did not reach adult-level proficiency at feeding on maripa until about 3 years (older juveniles). We compared this developmental pattern with that of extractive foraging on beetle larvae (Myelobia sp.) hidden in bamboo stalks, a more difficult food for these monkeys [Gunst N, Boinski S, Fragaszy DM. Behaviour 145:195,229, 2008]. For maripa, the challenge was mainly physical (plucking the fruit) once a tree was encountered, whereas for larvae, the challenge was primarily perceptual (locating the hidden larvae). For both foods, capuchins practice for years before achieving adult-level foraging competence, and the timeline is extended for larvae foraging (until 6 years) compared with maripa (3 years). The differing combinations of opportunities and challenges for learning to forage on these different foods illustrate how young generalist foragers (i.e. exploiting a large number of animal and plant species) may compensate for their low efficiency in extractive foraging tasks by showing earlier competence in processing less difficult but nutritious foods, such as maripa fruit. Am. J. Primatol. 72:960,973, 2010. © 2010 Wiley-Liss, Inc. [source] Existence and convergence for quasi-static evolution in brittle fractureCOMMUNICATIONS ON PURE & APPLIED MATHEMATICS, Issue 10 2003Gilles A. Francfort This paper investigates the mathematical well-posedness of the variational model of quasi-static growth for a brittle crack proposed by Francfort and Marigo in [15]. The starting point is a time discretized version of that evolution which results in a sequence of minimization problems of Mumford and Shah type functionals. The natural weak setting is that of special functions of bounded variation, and the main difficulty in showing existence of the time-continuous quasi-static growth is to pass to the limit as the time-discretization step tends to 0. This is performed with the help of a jump transfer theorem which permits, under weak convergence assumptions for a sequence {un} of SBV-functions to its BV-limit u, to transfer the part of the jump set of any test field that lies in the jump set of u onto that of the converging sequence {un}. In particular, it is shown that the notion of minimizer of a Mumford and Shah type functional for its own jump set is stable under weak convergence assumptions. Furthermore, our analysis justifies numerical methods used for computing the time-continuous quasi-static evolution. © 2003 Wiley Periodicals, Inc. 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