Aplicación de la mecánica de la fractura a la rotura frágil de fibras de sémola

La rotura por tracción de fibras frágiles ha sido estudiada tradicionalmente por métodos estadísticos, entre los que destaca el Modelo de Weibull. En este trabajo se estudia la aplicabilidad de este modelo y de la Mecánica de la Fractura Elástica Lineal a la rotura por tracción de fibras frágiles, empleándolos para analizar la rotura de fibras de sémola de trigo duro, ordinariamente conocidas como espaguetis. Se ha comprobado que los modelos estadísticos de rotura, entre ellos el de Weibull, no reproducen adecuadamente el efecto de tamaño en la carga de rotura al variar la longitud de la fibra cuando los defectos no tienen una distribucion estrictamente aleatoria, como sucede en el caso de las fibras de sémola. Por el contrario, este trabajo demuestra que la Mecánica de la Fractura Elástica Lineal es aplicable a la rotura de este tipo de fibras, en particular en presencia de defectos internos de muy diferentes posiciones y tamaños

Propiedades mecánicas y resistencia de aortas ascendentes patológicas

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Relationship between fibre orientation and tensile strength of natural collagen membranes for heart valve leaflets

Heart valve prostheses are used to replace native heart valves which that are damaged because of congenital diseases or due to ageing. Biological prostheses made of bovine pericardium are similar to native valves and do not require any anticoagulation treatment, but are less durable than mechanical prostheses and usually fail by tearing. Researches are oriented in improving the resistance and durability of biological heart valve prostheses in order to increase their life expectancy. To understand the mechanical behaviour of bovine pericardium and relate it to its microstructure (mainly collagen fibres concentration and orientation) uniaxial tensile tests have been performed on a model material made of collagen fibres. Small Angle Light Scattering (SALS) has been also used to characterize the microstructure without damaging the material. Results with the model material allowed us to obtain the orientation of the fibres, relating the microstructure to mechanical performanc

Efecto de la temperatura en la rotura por despegue de una placa de ateroma

Las placas de ateroma de las paredes arteriales son acumulaciones de lípidos y otras sustancias que modifican las propiedades de la pared vascular. El principal problema que presentan estas placas es su rotura y despegue de la pared, con la posibilidad de obstruir la circulación de la sangre. En este trabajo se analiza, mediante un modelo animal en conejos, el efecto que una variación de temperatura (por ejemplo en un proceso febril) puede tener en la aparición de tensiones tangenciales entre la placa y la pared. Para ello se han obtenido las propiedades termomecánicas de la pared arterial y de la placa de ateroma. Asimismo, se ha elaborado un modelo para estimar las tensiones generadas por un incremento de temperatura. Como ejemplo se muestra que un aumento de la temperatura de 4ºC, que correspondería a una fiebre de algo más de 40ºC, puede generar elevadas tensiones capaces de provocar el despegue de la placa. - Atheroma plaques are accumulations of lipids and other substances attached to the inner side of the artery walls, which modify the mechanical behaviour of the artery wall. Their main problem comes when the plaque breaks and incorporates to the blood flow, because it can eventually block blood circulation and damage to the surrounding tissues and organs. This work explores, by means of a rabbit model, the effect of temperature variation on the development of interfacial shear stresses between plaque and wall. The thermomechanical properties of arterial wall and atheroma plaques have been obtained. A simplified model has also been developed in order to estimate the stress level, which has shown that due to a moderate temperature increment of 4ºC interfacial shear stresses can reach high values, enough to provoke plaque rupture and detachment

Fractional calculus applied to model arterial viscoelasticity

Arterial viscoelasticity can be described using stress-relaxation experiments. To fit these curves, models with springs and dashpots, based on differential equations, were widely studied. However, uniaxial tests in arteries show particular shapes with an initial steep decay and a slow asymptotic relaxation. Recently, fractional order derivatives were used to conceive a new component called spring-pot that interpolates between pure elastic and viscous behaviors. In this work we modified a standard linear solid model replacing a dashpot with a spring-pot of order α. We tested the fractional model in human arterial segments. Results showed an accurate relaxation response during 1-hour with least squares errors below 1%. Fractional orders α were 0.2-0.4, justifying the extra parameter. Moreover, the adapted parameters allowed us to predict frequency responses that were similar to reported Complex Elastic Moduli in arteries. Our results indicate that fractional models should be considered as real alternatives to model arterial viscoelasticity

Strength and stiffness of repaired tendons

Rotator cuff tears of the shoulder are a common cause of pain and disability. Although surgery is frequently beneficial, re-tearing of the tendons is likely to re-occur. In many cases even if the reparation is successful it will still generate discomfort, problems with mobility, as well as a sharp pain. This project is funded in the cooperation with the Hospital Clinico San Carlos de Madrid. The purpose of this work is to analyze the effect of the surgical repair and the application of different therapies, including mesenchymal stem cell therapy on the biomechanical properties (strength and stiffness) of the repaired tendon. An animal model of rotator cuff tendon reparations has been developed on laboratory rats.To obtain the mechanical response of the healthy and repaired tendons, it was necessary to develop an experimental set up to reproduce the in-vivo working conditions of the tendons (37 ºC, immersed in physiological serum), and especially the load transfer. The biomechanical properties (maximum load and stiffness) have been measured in healthy and repaired tendons. A total of 70 rats are used in this particular study. It has been found that the repaired tendon is stronger than the original on. However, the repaired tendons demonstrate less flexibility than the healthy (original) ones prior to the damag

Extreme temperature events alter stream ecosystem functioning

Extreme temperature events have increased in intensity, duration and frequency in the last century, with potential consequences on organisms and ecosystems. In many streams, leaf litter of terrestrial origin is a key resource for microorganisms and some detritivores, and its decomposition has a main role on ecosystem functioning and is often used as an indicator of ecological integrity. As litter is often exposed to atmospheric conditions before entering the stream, extreme warming and freezing events may alter its physicochemical structure and affect decomposition and associated detritivores. We tested this prediction in a microcosm experiment by exposing litter of three tree species (in single-species treatments and the 3-species mixture) to different temperature pre-treatments: heating (40 degrees C), freezing ( - 20 degrees C) and both (heating followed by freezing). We then examined changes in litter traits due to leaching (72 h), litter decomposition in the absence and presence of detritivores, and detritivore growth (28 d), with focus on mass and nutrient (nitrogen and phosphorus) changes. Nutrient leaching was promoted mostly by the heating pre-treatment, which apparently produced lower-quality litter. However, microbial activity mostly resulted in litter mass and nutrient gain, which were reinforced by the heating pre-treatment, while freezing had the opposite effect. When detritivores were present, decomposition showed high variation among litter types but, again, the heating and freezing pre-treatments tended to reduce and enhance nutrient loss, respectively. The greatest and more consistent effects occurred for detritivore growth, which was reduced by temperature pre-treatments, particularly in the highest-quality litter type. In general, the sequential application of heating and freezing pre-treatments showed no synergistic effect, and the litter mixture showed similar responses to single-species treatments. Our results demonstrate that short-term extreme temperatures can modify litter quality in riparian soils and have subsequent effects on its decomposition within the stream and associated fauna, potentially altering stream food webs, ecosystem functioning and biogeochemical cycles.This study was funded by the Spanish Ministry for Science, Innovation and Universities and FEDER (project BioLoss, Ref. RTI2018-095023-B400), Basque Goverment funds (Ref. IT951-16) and Initiation Fondecyt Project (Ref. 11170390). I. Diaz and U. Apodaka-Etxebarria contributed to the sample processing

Effects of gamma irradiation on instream leaf litter decomposition

Leaf litter decomposition is a key process in stream ecosystems, the rates of which can vary with changes in litter quality or its colonization by microorganisms. Decomposition in streams is increasingly used to compare ecosystem functioning globally, often requiring the distribution of litter across countries. It is important to understand whether litter sterilization, which is required by some countries, can alter the rates of decomposition and associated processes. We examined whether litter sterilization with gamma irradiation (25 kGy) influenced decomposition rates, litter stoichiometry, and colonization by invertebrates after weeks of instream incubation within coarse-mesh and fine-mesh litterbags. We used nine plant species from three families that varied widely in litter chemistry but found mostly consistent responses, with no differences in decomposition rates or numbers of invertebrates found at the end of the incubation period. However, litter stoichiometry differed between irradiated and control litter, with greater nutrient losses (mostly phosphorus) in the former. Therefore, the effects of irradiation on litter chemistry should be taken into account in studies focused on stoichiometry but not necessarily in those focused on decomposition rates, at least within the experimental timescale considered here

Complementary regioselectivity in the synthesis of iminohydantoins: remarkable effect of amide substitution on the cyclization

Complementary regioselective synthesis of iminohydantoins from isocyanoacetamides controlled by the substituent on the amide group has been described. 4-Iminohydantoins were the major products when the starting materials were N-alkyl isocyanoamides, whereas 2-iminohydantoins were the major products with N-aryl isocyanoamides.Ministerio de Ciencia e Innovación, Spain (Project CTQ2009-12631- BQU), Junta de Castilla y León, Consejería de Educación y Cultura y Fondo Social Europeo (Projects BU023A09 and GR170)

Estudio mecánico sobre la resistencia de la pared de la aorta humana como predictor de riesgo de rotura de aneurisma

Definir el riesgo de rotura de un aneurisma, se considera un factor básico para elegir el momento adecuado de la intervención quirúrgica. Uno de los parámetros clásico utilizados es el diámetro del aneurisma (Ley de la Place). Sin embargo, otro factor mecánico como es el cociente entre la tensión máxima que soporta la pared (depende del diámetro de la aorta y de la morfología) y de la resistencia del material (pared de la aorta) es un elemento poco conocido. La razón estriba en comparar aortas “sanas” con “patológicas”. Este estudio lo hemos realizado en colaboración con el Departamento de Ciencias de Materiales de la Universidad Politécnica de Madri