Experimental and computational micromechanical study of fiber-reinforced polymers

Los materiales compuestos de matriz polimérica reforzados por fibras tienen propiedades mecánicas muy adecuadas para su utilización en aplicaciones estructurales. Estos materiales, debido a los altos valores de rigidez y resistencia específica, están desplazando a materiales convencionales en numerosas aplicaciones en las que se requiere la mejor relación entre las prestaciones mecánicas y el peso. Hoy en día, en el diseño de estructuras con materiales compuestos se utilizan criterios de fallo con base fenomenológica que requieren de un gran número de ensayos experimentales para obtener todos los parámetros del material necesarios y garantizar la seguridad de los componentes construidos. Este inconveniente en la aplicación de materiales compuestos se debe a la complejidad de su comportamiento mecánico, que está gobernado por diversos procesos que ocurren a nivel microscópico. Este es el caso, por ejemplo, de procesos como la concentración de tensiones y deformaciones, las decohesiones entre las distintas fases del compuesto o el fallo de los microconstituyentes, que determinan las propiedades finales del material. En esta tesis se estudia la influencia de la microestructura y las propiedades de los microconstituyentes en el comportamiento mecánico en la dirección perpendicular al refuerzo de un laminado unidireccional de fibras de vidrio embebidas en una matriz epoxi. Las propiedades mecánicas de la matriz polimérica y de las intercaras fibra/matriz se caracterizaron mediante técnicas de nanoindentación y pushout, respectivamente. Los mecanismos de deformación y daño desarrollados en la microestructura del material compuesto se analizaron mediante la realización de ensayos mecánicos de compresión y flexión en tres puntos en el interior de un microscopio electrónico de barrido. Como resultado, se encontró que el daño del material compuesto sometido a cargas en la dirección perpendicular al refuerzo comenzaba en las intercaras fibra/matriz y se propagaba a través de la unión de las intercaras dañadas mediante la rotura de los ligamentos de matriz entre fibras decohesionadas. El comportamiento mecánico del laminado hasta rotura se modeló mediante la simulación por elementos finitos de modelos micromecánicos que representaban la microestructura real del material. Siguiendo las observaciones experimentales, el modelo numérico incluía la decohesión de las intercaras y la rotura de la matriz. Los resultados de las simulaciones mostraron un buen acuerdo con los experimentos tanto en su respuesta macroscópica como en los mecanismos de deformación y daño microscópicos. El modelo numérico desarrollado sirvió para obtener la envolvente de fallo del material compuesto bajo cargas normales en la dirección perpendicular al refuerzo y cortante fuera de plano. Los resultados obtenidos de las simulaciones se compararon con las predicciones de los criterios de Hashin y Puck para el fallo de laminados. Fiber-reinforced polymers (FRPs) exhibit outstanding mechanical properties, ideal for their use in structural applications. Due to the high specific values of stiffness and strength, these composites are replacing conventional materials in applications where a superior mechanical performance in combination with weight saving is necessary. However, the lack of reliable and experimentally contrasted models to accurately predict the failure strength of FRPs makes the optimization process of composite structures to be carried out by means of a costly and time-consuming trial-and-error approach, which requires an immense burden of testing and restricts the promising improvements of the use of composites. This difficulty in the simulation of mechanical behavior and damage of composites is due to the fact that an accurate model should account for the phenomena which take place at the micron scale, which is in the size of the reinforcement. This is the case, for instance, of the stress and strain concentration, decohesion of the reinforcement and damage of the material constituents. This thesis studies the influence of the microstructure and the properties of the microconstituents in the mechanical behavior of a unidirectional laminate of glass fiber embedded in a matrix of epoxy resin, for the transverse loading cases. To this end, the mechanical properties of the polymeric matrix and the fiber/matrix interfaces have been experimentally characterized by means of nanoindentation and push-out techniques, respectively. The deformation and damage mechanisms developed in the microstructure of the composite have been analyzed through compression and threepoint bending tests carried out inside the scanning electron microscope. It was found that the damage process began by the decohesion of the fiber/matrix interfaces, and continues with the subsequent propagation of the damage by the link up of the interface fractures by breaking the matrix ligaments between debonded fibers. The mechanical behavior of the unidirectional laminate until final fracture was modeled by the finite element simulation of micromechanical models which represented the microstructure of the real material. Following the previous experimental observation, the interface decohesion and the failure of the matrix were included in the computational model. The simulation results were in very good agreement with the experiments in terms of the macroscopic response and of the microscopic mechanisms. Finally, the numerical model was also employed to obtain the failure envelope of the composite under transverse loads and out-of-plane shear. The model results were assessed with the predictions of the Hashin and Puck’s crieria for the failure of laminate

A method to determine the tau neutrino helicity using polarized taus

A method is presented to extract the tau neutrino helicity, or equivalently, the chirality parameter γVA, independent of any tau polarization which may be present. The method is thus well-suited to measurements using taus produced from the Zº and is complementary to analyses using tau correlations since it provides the sign of the chirality parameter which is otherwise unavailable without recourse to lower energy experiments where taus are unpolarized. Results of Monte Carlo studies and comments regarding the use of the technique in experiments are also included.Facultad de Ciencias Exacta

Mecanismos de Deformación en laminados de matriz polimérica correlación digital de imágenes y micromecánica computacional

Se ha realizado un estudio micromecánico experimental del comportamiento de laminados unidireccionales sometidos a compresión en la dirección perpendicular a las fibras. Se ha empleado la técnica de correlación digital de imágenes para observar la evolución de los campos de desplazamientos y deformaciones en la microestructura del material compuesto. En los contornos de deformación obtenidos experimentalmente se ha comprobado como las fibras tienen una deformación muy pequeña, mientras que las mayores deformaciones se concentran en las zonas de matriz de menor fracción volumétrica de fibras. Simulando por elementos finitos la microestructura estudiada se han reproducido los resultados experimentales, obteniendo distribuciones de campos de desplazamientos y deformaciones muy similares a las observadas experimentalmente

Application of digital image correlation at the microscale in fiber-reinforced composites

Digital image correlation (DIC) is applied to analyzing the deformation mechanisms under transverse compression in a fiber-reinforced composite. To this end, compression tests in a direction perpendicular to the fibers were carried out inside a scanning electron microscope and secondary electron images obtained at different magnifications during the test. Optimum DIC parameters to resolve the displacement and strain field were computed from numerical simulations of a model composite and they were applied to micrographs obtained at different magnifications (250_, 2000_, and 6000_). It is shown that DIC of low-magnification micrographs was able to capture the long range fluctuations in strain due to the presence of matrix-rich and fiber-rich zones, responsible for the onset of damage. At higher magnification, the strain fields obtained with DIC qualitatively reproduce the non-homogeneous deformation pattern due to the presence of stiff fibers dispersed in a compliant matrix and provide accurate results of the average composite strain. However, comparison with finite element simulations revealed that DIC was not able to accurately capture the average strain in each phase

A method to determine the tau neutrino helicity using polarized taus

A method is presented to extract the tau neutrino helicity, or equivalently, the chirality parameter γVA, independent of any tau polarization which may be present. The method is thus well-suited to measurements using taus produced from the Zº and is complementary to analyses using tau correlations since it provides the sign of the chirality parameter which is otherwise unavailable without recourse to lower energy experiments where taus are unpolarized. Results of Monte Carlo studies and comments regarding the use of the technique in experiments are also included.Facultad de Ciencias Exacta

Intraply fracture of fiber-reinforced composites: microscopic mechanisms and modeling

The fracture behavior parallel to the fibers of an E-glass/epoxy unidirectional laminate was studied by means of three-point tests on notched beams. Selected tests were carried out within a scanning electron microscope to ascertain the damage and fracture micromechanisms upon loading. The mechanical behavior of the notched beam was simulated within the framework of the embedded cell model, in which the actual composite microstructure was resolved in front of the notch tip. In addition, matrix and interface properties were independently measured in situ using a nanoindentor. The numerical simulations very accurately predicted the macroscopic response of the composite as well as the damage development and crack growth in front of the notch tip, demonstrating the ability of the embedded cell approach to simulate the fracture behavior of heterogeneous materials. Finally, this methodology was exploited to ascertain the influence of matrix and interface properties on the intraply toughness

Effect of Cilastatin on Cisplatin-Induced Nephrotoxicity in Patients Undergoing Hyperthermic Intraperitoneal Chemotherapy

Cisplatin is one of the most widely used chemotherapeutic agents in oncology, although its nephrotoxicity limits application and dosage. We present the results of a clinical study on prophylaxis of cisplatin-induced nephrotoxicity in patients with peritoneal carcinomatosis undergoing cytoreduction and hyperthermic intraperitoneal intraoperative chemotherapy (HIPEC-cisplatin). Prophylaxis was with imipenem/cilastatin. Cilastatin is a selective inhibitor of renal dehydropeptidase I in the proximal renal tubule cells that can reduce the nephrotoxicity of cisplatin. Unfortunately, cilastatin is not currently marketed alone, and can only be administered in combination with imipenem. The study has a retrospective part that serves as a control (n = 99 patients receiving standard surgical prophylaxis) and a prospective part with imipenem/cilastatin prophylaxis corresponding to the study group (n = 85 patients). In both groups, we collected specific data on preoperative risk factors of renal damage, fluid management, hemodynamic control, and urine volume during surgery (including the hyperthermic chemotherapy perfusion), as well as data on hemodynamic and renal function during the first seven days after surgery. The main finding of the study is that cilastatin may exert a nephroprotective effect in patients with peritoneal carcinomatosis undergoing cytoreduction and hyperthermic intraperitoneal cisplatin perfusion. Creatinine values remained lower than in the control group (ANOVA test, p = 0.037). This translates into easier management of these patients in the postoperative period, with significantly shorter intensive care unit (ICU) and hospital stay

Examining the immune signatures of SARS-CoV-2 infection in pregnancy and the impact on neurodevelopment: Protocol of the SIGNATURE longitudinal study.

The COVID-19 pandemic represents a valuable opportunity to carry out cohort studies that allow us to advance our knowledge on pathophysiological mechanisms of neuropsychiatric diseases. One of these opportunities is the study of the relationships between inflammation, brain development and an increased risk of suffering neuropsychiatric disorders. Based on the hypothesis that neuroinflammation during early stages of life is associated with neurodevelopmental disorders and confers a greater risk of developing neuropsychiatric disorders, we propose a cohort study of SARS-CoV-2-infected pregnant women and their newborns. The main objective of SIGNATURE project is to explore how the presence of prenatal SARS-CoV-2 infection and other non-infectious stressors generates an abnormal inflammatory activity in the newborn. The cohort of women during the COVID-19 pandemic will be psychological and biological monitored during their pregnancy, delivery, childbirth and postpartum. The biological information of the umbilical cord (foetus blood) and peripheral blood from the mother will be obtained after childbirth. These samples and the clinical characterisation of the cohort of mothers and newborns, are tremendously valuable at this time. This is a protocol report and no analyses have been conducted yet, being currently at, our study is in the recruitment process step. At the time of this publication, we have identified 1,060 SARS-CoV-2 infected mothers and all have already given birth. From the total of identified mothers, we have recruited 537 SARS-COV-2 infected women and all of them have completed the mental health assessment during pregnancy. We have collected biological samples from 119 mothers and babies. Additionally, we have recruited 390 non-infected pregnant women