From S-N to the Paris Law with a New Mixed-Mode Cohesive Fatigue Model

The relationship between fatigue life and fatigue crack propagation rate is explored with a new cohesive damage model. The parameters of the model are obtained from idealizations of S-N diagrams used in engineering design. The model is based on the hypothesis that both stable tearing damage and damage due to cyclic loading are representations of a density of microcracks and, therefore, a single damage variable can describe the state of damage. This assumption implies that the quasi-static cohesive law that describes tearing is also the envelope of the fatigue damage. Fatigue damage within the cohesive envelope is assumed to accumulate at a rate that depends on the displacement jumps. The fatigue model was implemented as a UMAT subroutine for Abaqus cohesive elements by adding fatigue damage accumulation to a cohesive model based on the Turon mixed-mode cohesive laws. The analyses were conducted using a simplified cyclic loading procedure in which the maximum applied load is kept constant and the computational expense of cycling the load is avoided. The predicted propagation rates in mode I and mixed mode were compared to experimental results for IM7/8552 graphite/epoxy tape. Several aspects of the results were investigated, including the effect of R-curves, the stress ratio R, and the difference between displacement control and force control

Assessment of the Damage Tolerance of Postbuckled Hat-Stiffened Panels Using Single-Stringer Specimens

A procedure is proposed for the assessment of the damage tolerance and collapse of stiffened composite panels using a single-stringer compression specimen. The dimensions of the specimen are determined such that the specimen s nonlinear response and collapse are representative of an equivalent multi-stringer panel in compression. Experimental tests are conducted on specimens with and without an embedded delamination. A shell-based finite element model with intralaminar and interlaminar damage capabilities is developed to predict the postbuckling response as well as the damage evolution from initiation to collapse

Análisis comparativo técnico y económico entre diseño de pavimentos delgados de concreto (TCP) y losas de pavimentación tradicional con el método AASHTO 93

En el Perú la brecha del sector transporte representa el 36% de la brecha de infraestructura del país, de estos las carreteras constituyen el 20%, reflejando la urgencia de implementar mecanismos para contribuir al desarrollo de la infraestructura y cerrar las brechas de las mismas. Además, los diseños de pavimentos en el país se efectúan mediante la metodología tradicional o empírico, AASTHO 93, basados en criterios de serviciabilidad y ensayos experimentales de la AASHO donde se utilizó un solo tipo de subrasante, un clima específico (Ottawa, Illinois, EE. UU.) y materiales de diseño limitado. En virtud de lo cual surge la necesidad de proponer metodologías de diseño de pavimento que sean viables y factibles a las condiciones locales y que permitan realizar el comparativo con otros diseños, esperando implementar su uso en la construcción de pavimentos rígidos. Así, la presente investigación realiza el análisis comparativo técnico y económico de pavimentos rígidos diseñados con metodología tradicional (AASTHO 93) y la metodología de pavimentos delgados de concreto (TCP). Estos pavimentos fueron sometidos a análisis y cálculo de esfuerzos producto de ejes de carga actuando en centro y borde del pavimento, posteriormente los pavimentos son evaluados mediante un modelo de falla propuesto por el PCA. Obteniendo que los pavimentos TCP soportan con menor espesor los mismos ejes de carga que los pavimentos AASTHO 93 y resultan más económica frente a estos.In Peru, the gap in the transportation sector represents 36% of the country's infrastructure gap, of which roads constitute 20%, reflecting the urgency of implementing mechanisms to contribute to the development of infrastructure and close the gaps in the same. In addition, pavement designs in the country are carried out using the traditional or empirical methodology, AASTHO 93, based on serviceability criteria and experimental tests of the AASHO where a single type of subgrade was used, a specific climate (Ottawa, Illinois, EE. USA) and limited design materials. By virtue of which, the need arises to propose pavement design methodologies that are viable and feasible to local conditions and that allow comparison with other designs, hoping to implement their use in the construction of rigid pavements. Thus, this research carries out the technical and economic comparative analysis of rigid pavements designed with traditional methodology (AASTHO 93) and the thin concrete pavement methodology (TCP). These pavements were subjected to analysis and calculation of stresses as a result of load axes acting in the center and edge of the pavement, subsequently the pavements are evaluated using a failure model proposed by the PCA. Obtaining that TCP pavements support the same load axes with less thickness than AASTHO 93 pavements and are more economical compared to these.Tesi

Hipertensión arterial como factor asociado a mortalidad por COVID-19, Hospital II-2 Tarapoto - MINSA, marzo 2020 - febrero 2021

Introducción: La epidemia de COVID-19 que se inició en China se propagó rápidamente en el mundo, reportándose más de 180 millones de casos, en nuestro país se han notificado aproximadamente dos millones casos y cerca de 200 mil fallecimientos. La duración de esta enfermedad varía, en los casos leves es de dos semanas en los casos graves de 3 a 6 semanas. El agente causal al ocasionar la infección origina varias interacciones, tales como interacción con el sistema renina-angiotensina-aldosterona, con el sistema inmunitario, la coagulación y el sistema micro vascular, lo cual repercute en la gravedad de los casos. La presencia de comorbilidades como enfermedad cardiovascular, diabetes, hipertensión arterial, obesidad y cáncer, entre las más frecuentes, se están asociando a la gravedad de la COVID-19 y a la fatalidad, la información sobre la gravedad sigue variando en el tiempo, lo que generalmente ocurre en la aparición de enfermedades emergentes, al inicio se presentan casos graves y fatales y luego casos más leves. Se refieren varios factores que influyen en la mortalidad por COVID-19, entre estos tenemos, la edad mayor de 60 años, diabetes mellitus, obesidad, hipertensión arterial, cáncer e inmunosupresión. La hipertensión arterial, enfermedad cardiovascular, diabetes y obesidad son frecuentes en los pacientes que tienen una peor evolución. En el país y en el nivel local son escasas las publicaciones y no se ha estudiado la mortalidad por COVID-19 asociada a hipertensión arterial en el hospital donde se realizará el estudio. El presente proyecto de investigación tiene como objetivo general, identificar si la hipertensión arterial es factor asociado a la mortalidad por COVID-19, los objetivos específicos son, describir la población en estudio según edad y sexo, determinar la frecuencia de hipertensión arterial en pacientes fallecidos y no fallecidos por COVID-19 y establecer la asociación entre la hipertensión arterial y la mortalidad por COVID-19. Para cumplir con los objetivos, se seleccionará una muestra de la población. Respecto al diseño del estudio, será un trabajo de 4 investigación observacional, retrospectivo, no experimental de casos controles. En la recolección de los datos se utilizarán las historias clínicas y la ficha de recolección elaborada por el autor. El resultado que se espera lograr es determinar si la hipertensión arterial es un factor asociado a la mortalidad por COVID-19. Los datos serán procesados utilizando el Microsoft Excel y el programa estadístico SPSS v20, en español para Windows 8; para determinar la asociación de las variables se utilizará el odds ratio con intervalo de confianza al 95%, el resultado se presentará en una tabla.Tesis de segunda especialida

Simplified Models for the Study of Postbuckled Hat-Stiffened Composite Panels

The postbuckling response and failure of multistringer stiffened panels is analyzed using models with three levels of approximation. The first model uses a relatively coarse mesh to capture the global postbuckling response of a five-stringer panel. The second model can predict the nonlinear response as well as the debonding and crippling failure mechanisms in a single stringer compression specimen (SSCS). The third model consists of a simplified version of the SSCS that is designed to minimize the computational effort. The simplified model is well-suited to perform sensitivity analyses for studying the phenomena that lead to structural collapse. In particular, the simplified model is used to obtain a deeper understanding of the role played by geometric and material modeling parameters such as mesh size, inter-laminar strength, fracture toughness, and fracture mode mixity. Finally, a global/local damage analysis method is proposed in which a detailed local model is used to scan the global model to identify the locations that are most critical for damage tolerance

"Analyse des facteurs de migration mexique-États-Unis et des impacts sociaux, économiques"

Abstract Mexico decided to internationalize its economy, entering the globalizing process of a quasi-total opening of its economy. Foreign trade was defined as the basis for the development of the country, which is why 12 free trade agreements were negotiated that allow access to preferential products to 43 countries. The evolution of the relationship with all of them, with the exception of the United States and Central America, has been unsatisfactory because most of them have a growing deficit in the trade balance. The basis of this problem is the growing lack of productivity and competitiveness of the Mexican economy, caused by the absence of reforms to reduce transaction costs in our country. This is aggravated by the absence of a regulatory policy and Mexico also managed to increase its economy by being able to have trade agreements with different countries

Cohesive Laws for Analyzing Through-Crack Propagation in Cross Ply Laminates

The laminate cohesive approach (LCA) is a methodology for the experimental characterization of cohesive through-the-thickness damage propagation in fiber-reinforced polymer matrix composites. LCA has several advantages over other existing approaches for cohesive law characterization, including: visual measurements of crack length are not required, structural effects are accounted for, and LCA can be applied when the specimen is too small to achieve steady-state fracture. In this work, the applicability of this method is investigated for two material systems: IM7/8552, a conventional prepreg, and AS4/VRM34, a non-crimp fabric cured using an out-of-autoclave process. The compact tension specimen configuration is used to propagate stable Mode I damage. Trilinear cohesive laws are characterized using the fracture toughness and the notch tip opening displacement. Test results are compared for the IM7/8552 specimens with notches machined by waterjet and by wire slurry saw. It is shown that the test results are nearly identical for both notch tip preparations methods, indicating that significant specimen preparation time and cost savings can be realized by using the waterjet to notch the specimen instead of the wire slurry saw. The accuracy of the cohesive laws characterized herein are assessed by reproducing the structural response of the test specimens using computational methods. The applicability of the characterization procedure for inferring lamina fracture toughness is also discussed

Experimental and Numerical Analysis of Skin-Stiffener Separation Using a Seven-Point Bend Configuration

Skin-stiffener separation in stiffened composite panels consists of a complex interaction between multiple scales of progressive damage and failure mechanisms. This work used a superposed cohesive element method of modeling resistance curve effects that represents the structural interface of a unidirectional tape skin and a fabric stiffener. Finite element models using cohesive elements with input properties obtained from tape-to-fabric interface characterization tests were compared to experimental results of a stable skin-stiffener separation characterization test using a seven-point bend fixture. This fixture deformed the stiffened panel specimens into a buckled configuration which induced mixed-mode interlaminar stress states at the skin-stiffener interface. The advantage of this fixture was the potential for more stable damage initiation and delamination growth compared to a stringer-stiffened panel under axial compression. Use of the superposed cohesive elements showed promise, but the characterization of interface material properties as input to the cohesive elements remains a crucial component to be investigated to ensure accurate prediction of skin-stiffener separation

Modeling Transverse Cracking in Laminates With a Single Layer of Elements Per Ply

The objective of the present paper is to investigate the ability of mesolevel X-FEM models with a single layer of elements per ply to capture accurately all aspects of matrix cracking. In particular, we examine whether the model can predict the insitu ply thickness effect on crack initiation and propagation, the crack density as a function of strain, the strain for crack saturation, and the interaction between delamination and transverse cracks. Results reveal that the simplified model does not capture correctly the shear-lag relaxation of the stress field on either side of a crack, which leads to an overprediction of the crack density. It is also shown, however, that after onset of delamination many of the inserted matrix cracks close again, and that the density of open cracks becomes similar to the density predicted by the detailed model. The degree to which the spurious cracks affect the global response is quantified and the reliability of the mesolevel approach with a single layer of elements per ply is discussed

Guidelines and Parameter Selection for the Simulation of Progressive Delamination

Turon s methodology for determining optimal analysis parameters for the simulation of progressive delamination is reviewed. Recommended procedures for determining analysis parameters for efficient delamination growth predictions using the Abaqus/Standard cohesive element and relatively coarse meshes are provided for single and mixed-mode loading. The Abaqus cohesive element, COH3D8, and a user-defined cohesive element are used to develop finite element models of the double cantilever beam specimen, the end-notched flexure specimen, and the mixed-mode bending specimen to simulate progressive delamination growth in Mode I, Mode II, and mixed-mode fracture, respectively. The predicted responses are compared with their analytical solutions. The results show that for single-mode fracture, the predicted responses obtained with the Abaqus cohesive element correlate well with the analytical solutions. For mixed-mode fracture, it was found that the response predicted using COH3D8 elements depends on the damage evolution criterion that is used. The energy-based criterion overpredicts the peak loads and load-deflection response. The results predicted using a tabulated form of the BK criterion correlate well with the analytical solution and with the results predicted with the user-written element