Modelling probabilistic fatigue crack propagation rates for a mild structural steel

A class of fatigue crack growth models based on elastic–plastic stress–strain histories at the crack tip region and local strain-life damage models have been proposed in literature. The fatigue crack growth is regarded as a process of continuous crack initializations over successive elementary material blocks, which may be governed by smooth strain-life damage data. Some approaches account for the residual stresses developing at the crack tip in the actual crack driving force assessment, allowing mean stresses and loading sequential effects to be modelled. An extension of the fatigue crack propagation model originally proposed by Noroozi et al. (2005) to derive probabilistic fatigue crack propagation data is proposed, in particular concerning the derivation of probabilistic da/dN-?K-R fields. The elastic-plastic stresses at the vicinity of the crack tip, computed using simplified formulae, are compared with the stresses computed using an elasticplastic finite element analyses for specimens considered in the experimental program proposed to derive the fatigue crack propagation data. Using probabilistic strain-life data available for the S355 structural mild steel, probabilistic crack propagation fields are generated, for several stress ratios, and compared with experimental fatigue crack propagation data. A satisfactory agreement between the predicted probabilistic fields and experimental data is observed

A numerical study of two different specimen fixtures for the modified compact tension test – their influence on concrete fracture parameters

The modified compact tension test (MCT) may represent a new test configuration for the performance of static and other kinds of fatigue tests on concrete-like materials. Core drilling can be employed to obtain specimens which are cylindrical in shape and have a standard diameter of 150 mm, this being appropriate for the determination of the residual life of structures. This contribution focuses on the evaluation of MCT specimen fracture behavior during static tests. Cracks evolution are simulated numerically using ATENA finite element (FE) software, while the results are represented as L-COD diagrams, i.e. load vs. crack opening displacement measured on the loading axis. After numerical calculations, the results for two different fixtures are compared and the advantages or drawbacks for each solution are discussed

TP86 154. ¿Es útil el sistema de asistencia oxigenador de membrana extracorpórea en el tromboembolismo pulmonar masivo?

IntroducciónEn España, en series de autopsias se ha descubierto que la causa de muerte es un tromboembolismo pulmonar (TEP) en un 2,6%. La aparición de esta enfermedad está claramente aumentada en los pacientes oncológicos. En ocasiones las medidas clásicas del tratamiento del TEP no surgen efecto y el paciente puede fallecer; en estos casos está indicada la implantación de una asistencia cardiorrespiratoria.Material y métodosPaciente de 35 años con quimioterapia por un seminoma. Ingresa por disnea y dolor torácico con mala evolución. Se traslada a unidad de coronarias donde se diagnostica de un TEP bilateral masivo y disfunción importante ventricular derecha presentando dos episodios de parada cardiorrespiratoria. Tras las maniobras de resucitación se implanta oxigenador de membrana extracorpórea (ECMO) venoarterial que se mantiene 5 días, mientras tanto se hepariniza al paciente completamente.ResultadosEl paciente es extubado y dado de alta a los 21 días. Tras un seguimiento de 20 meses el paciente permanece vivo.ConclusiónEl sistema ECMO venoarterial es muy útil en pacientes con TEP, ya que realiza dos funciones: descargar el corazón derecho y oxigenar al paciente. Podemos heparinizar al paciente para revertir el cuadro, ya que las incisiones a nivel femoral son fácilmente controladas

A probabilistic interpretation of the Miner number for fatigue life prediction

The Miner number M, used as a tool for lifetime prediction of mechanical and structural components in most of the standards related to fatigue design, is generally accepted as representing a damage stage resulting from a linear progression of damage accumulation. Nonetheless, the fatigue and damage approach proposed by Castillo and Fernández-Canteli, permits us to reject this conventional cliché by relating M to the normalized variable V, which represents percentile curves in the S-N field unequivocally associated to probability of failure. This approach, allowing a probabilistic interpretation of the Miner rule, can be applied to fatigue design of mechanical and structural components subjected to variable amplitude loading. The results of an extensive test program on concrete specimens under compressive constant and load spectra, carried out elsewhere, are used. A parallel calculation of the normalized variable V and the Miner number M is performed throughout the damage progression due to loading allowing probabilities of failure to be assigned to any value of the current Miner number. It is found that significant probabilities of failure, say P=0.05, are attained for even low values of M, thus evidencing the necessity of a new definition of the safety coefficient of structural or machine components when the Miner rule is considered. The experimental and analytical probability distributions of the resulting Miner numbers are compared and discussed, the latter still providing a nonconservative prediction in spite of the enhancement. A possible correction is analyzed

A probabilistic approach for multiaxial fatigue criteria

Models proposed to study the multiaxial fatigue damage phenomenon generally lack probabilistic interpretation due to their deterministic form. This implies failure compulsory happening at the plane exhibiting the maximum damage value, whereas the remaining planes are disregarded. Nevertheless, the random orientation of the predominant defect evidences the possibility of failure being initiated as a function of the predominant defect presence without requiring, necessarily, maximum values of the damage parameter, which emphasizes the need of introducing probabilistic concepts into the failure prediction analysis. In this paper, a probabilistic model is presented that enables the failure probability to be found for any selected plane orientation by considering the damage gradient as a parameter for both proportional and non-proportional loading. The applicability of the model is elucidated by means of an example. Assuming the cdf for the local failure of the material to be known, the probability of failure is calculated for a cross shaped specimen in which shift between the principal stresses sigma xx and sigma yy ranges from 0º to 180º

Determining fracture energy parameters of concrete from the modified compact tension test

The modified compact tension (MCT) test, though not yet recognized as a valid test for determining fracture energy of concrete, is believed to represent a plausible and suitable alternative versus other well established procedures, such as the wedge-splitting test (WST) and the three point (3PB) or four point bending (4PB) tests, due to its simplicity and low cost. The aim of the paper is twofold: Firstly, to demonstrate the necessary correspondence between the experimental MCT test setup and finite element simulations and secondly, to initiate the way of establishing the desirable conversion between the fracture energy parameter values resulting from the MCT test and the standard conventional procedures. MCT tests are carried out and compared with the numerical results from 2-D and 3-D finite element calculations using the commercial codesABAQUS and ATENA, the latter being specifically developed for applications on concrete structures andelements. In this way, the usability of the modified compact tension test for practical purposes is confirmed

Evaluación de resultados de fatiga con alto y ultra alto número de ciclos mediante un modelo de regresión de Weibull

El modelo de fatiga de regresión de Weibull, propuesto por Castillo-Canteli, representa una alternativa posible y adecuada para la evaluación y predicción de vidas de fatiga ultra altas (VHCF). Este modelo proporciona una definición probabilística del campo S-N para los posibles mecanismos de rotura determinantes, basada en distribuciones de Weibull de mínimos, así como la existencia de un límite de fatiga asintótico y la propiedad de reducir todo el campo SN a una única función de distribución mediante la variable normalizada V=(log N-B)(log Δσ-C). De este modo, el doble mecanismo de rotura, interno y superficial, que caracteriza las vidas altas (HCF) y ultra altas (VHCF) en fatiga puede ser satisfactoriamente tratado como distribuciones independientes en este particular y complejo caso de muestras con resultados concurrentes, conocido como problema de datos confundidos. Una vez estimados los parámetros de ambas funciones de distribución se procede a la combinación y reconversión de ambas como campo S-N conjunto. El modelo permite establecer una estrategia para optimizar la programación de los ensayos. Por último se presentan dos ejemplos de aplicación, uno de un programa externo experimental y otro de resultados simulados, ambos con ultra alto número de ciclos y dos posible mecanismos de rotura.The fatigue Weibull regression model proposed by Castillo-Canteli represents a possible and adequate alternative for the assessment and prediction of very high cycle fatigue (VHCF) lifetimes. This model provides a probabilistic definition of the S-N field for the two determining failure mechanisms based Weibull distributions for minima, as well as the existence of an asymptotic fatigue limit and the capability to reduce the S-N field to a single cumulative distribution function by considering the normalized variable V=(log N-B) (log Δσ). In this way, both dual fracture mechanisms, i.e. the internal and the surface ones, characterizing the HCF and VHCF data can be adequately interpreted and handled as independent distributions in such a particular and complex case of concurrent populations, known as a confounded data problem. Once the model parameters of both normalized cumulative distribution functions are independently estimated for both failure mechanisms and subsequently combined and reconverted to a joint S-N field whereby. The model allows a test strategy to be established for optimizing the the fatigue program planning. Finally, two examples of application are presented, the first related to an external experimental program, and the second to simulated data both for VCCF with twofold failure mechanisms

Ajuste de la curva de fractura de hormigón como función de densidad perteneciente a la familia generalizada de extremos

En este trabajo, la curva de fractura P-δ, registrada durante el proceso de fractura del hormigón en ensayos de flexión a tres puntos, se identifica con un fenómeno estadístico que se describe adecuadamente mediante una función de densidad de la familia de distribuciones generalizadas de valores extremos (DGVE), que demuestra ser de Fréchet de máximos, como caso particular entre las distribuciones de cola pesada. Dado que la función analítica propuesta ajusta todo el registro del ensayo, cabe esperar que permita estimar el trabajo de fractura no medido, correspondiente a la cola asintótica superior de la curva experimental de fractura P-δ, con mayor precisión, fiabilidad y sencillez que otros modelos reconocidos. El parámetro general de escala, Ω, identificado como el área bajo la curva de fractura y los tres parámetros de la función de densidad de Fréchet, se estiman por optimización del ajuste al registro de la curva P-δ del ensayo mediante un programa de Matlab específico. La idoneidad del modelo se confirma mediante su aplicación al ajuste de datos experimentales de un extenso programa de ensayos y su comparación con los resultados obtenidos con otros modelos reconocidos.In this work, the load-displacement curve P-δ, recorded during the fracture process of concrete under 3-PB tests is identified as a statistical phenomenon, which is adequately described as a density function of the generalized extreme value distributions (GEVD) which proves to be maximal Fréchet, as a particular case of heavy tail distributions. Since the proposed analytical function fits throughout the test record, one expects that the non-measured fracture work, corresponding to the upper asymptotic tail of the fracture curve P-δ, will be estimated with higher accurateness, reliability and easier than using other models currently recognized. The general scale parameter Ω, identified as the area under the fracture curve, and the three parameters of the Fréchet density function are estimated by optimizing the fitting of recorded data to the experimental P-δ curve using a specific Matlab program. The suitability of the model is confirmed after being applied to fit an extensive program of experimental data and the results being compared with those provided by other recognized models.Los autores agradecen el apoyo obtenido a través del Proyecto de investigación BIA2013-48352-P del Ministerio Español de Economía y Competitividad, así como del Proyecto SV-PA-11-012 y el apoyo obtenido por las Ayudas Predoctorales del Programa Severo Ochoa recibidas por el Gobierno Regional del Principado de Asturias

An active tectonic field for CO2 storage management: the Hontomín onshore case study (Spain)

One of the concerns of underground CO2 onshore storage is the triggering of induced seismicity and fault reactivation by the pore pressure increasing. Hence, a comprehensive analysis of the tectonic parameters involved in the storage rock formation is mandatory for safety management operations. Unquestionably, active faults and seal faults depicting the storage bulk are relevant parameters to be considered. However, there is a lack of analysis of the active tectonic strain field affecting these faults during the CO2 storage monitoring. The advantage of reconstructing the tectonic field is the possibility to determine the strain trajectories and describing the fault patterns affecting the reservoir rock. In this work, we adapt a methodology of systematic geostructural analysis to underground CO2 storage, based on the calculation of the strain field from kinematics indicators on the fault planes (ey and ex for the maximum and minimum horizontal shortening, respectively). This methodology is based on a statistical analysis of individual strain tensor solutions obtained from fresh outcrops from the Triassic to the Miocene. Consequently, we have collected 447 fault data in 32 field stations located within a 20 km radius. The understanding of the fault sets’ role for underground fluid circulation can also be established, helping further analysis of CO2 leakage and seepage. We have applied this methodology to Hontomín onshore CO2 storage facilities (central Spain). The geology of the area and the number of high-quality outcrops made this site a good candidate for studying the strain field from kinematics fault analysis. The results indicate a strike-slip tectonic regime with maximum horizontal shortening with a 160 and 50◦ E trend for the local regime, which activates NE–SW strike-slip faults. A regional extensional tectonic field was also recognized with a N–S trend, which activates N–S extensional faults, and NNE–SSW and NNW– SSE strike-slip faults, measured in the Cretaceous limestone on top of the Hontomín facilities. Monitoring these faults within the reservoir is suggested in addition to the possibility of obtaining a focal mechanism solutions for microearthquakes (M < 3)This work has been partially supported by the European Project ENOS: ENabling Onshore CO2 Storage in Europe, H2020 Project ID: 653718 and the Spanish project 3GEO, CGL2017-83931-C3-2-P, MICIU-FEDE

Generalización del modelo probabilístico de Weibull mediante un parámetro energético basado en el gradiente de deformación

En este trabajo se analiza la aplicabilidad del parámetro generalizado de referencia GP = E(&#963;max (d&#949;/d&#963;)max - &#963;min (d&#949;/d&#963;)min), como sustitutivo al rango de tensiones convencional, &#916;&#963; = &#963;max - &#963;min , en un intento de superar las limitaciones evidenciadas en el modelo básico probabilístico de Weibull propuesto por Castillo y Fernández Canteli, en particular en lo referente a la superación de la tensión última, ahora evitada, y a la ambigua interpretación del parámetro B, que define la vida mínima garantizada. Los gradientes de deformación se calcularían a partir del diagrama cíclico tensión-deformación del material, definido, en principio, analíticamente. Con esta aproximación, se trata de representar el fenómeno de fatiga más adecuadamente que mediante el parámetro convencional &#916;&#963; , pero sobre todo de extender la aplicabilidad del modelo al dominio de fatiga de bajo número de ciclos. El resultado final como campo GPN puede reconvertirse, opcionalmente, a la forma tradicional del campo &#916;&#963; ? N que debería adoptar ahora una forma sigmoidal típica del campo experimental S-N, permitiendo el ajuste en la zona de bajo número de ciclos. Se discute la adecuación física del parámetro generalizado propuesto y se presenta un ejemplo de aplicación del nuevo modelo en la evaluación de resultados experimentales. El resultado no satisfactorio permite, sin embargo, algunas conclusiones de interés práctico.In this work, the applicability of the energetic reference parameter GP= E(&#963;max (d&#949;/d&#963;)max - &#963;min (d&#949;/d&#963;)min) is analyzed as an alternative to the conventional stress range &#916;&#963; = &#963;max - &#963;min , aiming at overcoming the limitations evidenced by the basic version of the Weibull probabilistic model of Castillo and Fernández Canteli. In this way, overpassing of the ultimate stress and the necessity of considering the dubious interpretation of the B parameter representing the minimum possible lifetime, is eluded. The strain gradients are calculated from the cyclic stress-strain curve of the material. With this approach a more reliable description of the fatigue progress, as that using the conventional &#916;&#963; parameter, is pursued but foremost an extension of the applicability of the probabilistic model to the low cycle fatigue domain. The final result as a GP-N field can be optionally reconverted to the conventional of the &#916;&#963; ? N field, which should speculatively adopt the typical sigmoidal shape fitting the experimental S-N field in the LCF region. The physical adequacy of the generalized parameter proposed is discussed and an example of application of the new model is introduced for the assessment of experimental data. The non-satisfactory result allows some conclusions of practical interest to be drawn.Los autores agradecen el apoyo obtenido prestado a través de Proyecto de investigación BIA2013- 48352-P del Ministerio Español de Economía y Competitividad, así como del Proyecto SV-PA-11- 012 y el apoyo obtenido por las Ayudas Predoctorales del Programa Severo Ochoa recibidas por el Gobierno Regional del Principado de Asturias