Análisis experimental y numérico de la influencia del estado tensional en la deformación de fallo de elementos estructurales de aluminio

El diseño de componentes estructurales cuya misión es absorber energía en choques de baja y media velocidad (crashworthiness) es un área de gran relevancia en ingeniería (industria automovilística y aeronáutica) debido al incremento de los requerimientos de seguridad y fiabilidad de las estructuras de vehículos y aeronaves. La creciente aplicación de nuevos componentes estructurales más ligeros para reducir el consumo de combustible manteniendo la capacidad de absorción de energía, hace necesario optimizar su diseño utilizando, cada vez con mayor insistencia, herramientas numéricas que reduzcan el coste de los ensayos experimentales sobre prototipos completos. Para una correcta predicción del comportamiento completo (hasta rotura) del componente estructural metálico deben incluirse criterios de fallo que consideren la influencia del estado tensional. El objetivo principal de esta Tesis Doctoral ha sido investigar el efecto combinado de la triaxialidad y del parámetro de Lode sobre el comportamiento de metales en condiciones de fallo. A continuación, se recogen los objetivos específicos planteados: • Desarrollo de una ley de endurecimiento con fundamentación física y amplio intervalo de aplicación (velocidad de deformación y temperatura). • Desarrollo de un procedimiento experimental de obtención de la deformación de fallo en metales para diferentes valores del estado tensional. • Aplicación de un criterio de fallo que recoja la influencia del estado tensional (triaxialidad y parámetro de Lode) y que permita predecir el comportamiento en condiciones de fallo. • Análisis experimental y numérico del comportamiento de elementos estructurales para absorción de energía y protección frente a impacto en problemas de perforación. Para alcanzar los objetivos anteriores se han llevado a cabo las siguientes actividades: • Formulación e implementación de una ley de endurecimiento con fundamentación física basada en dislocaciones y aplicable a metales FCC. • Desarrollo de una metodología de ensayos combinados de tracción-torsión en probetas tubulares de doble entalla. • Calibración del criterio de fallo de Bai y Wierzbicki (dependiente de la triaxialidad y del parámetro de Lode) a partir de los resultados obtenidos en los ensayos de tracción-torsión combinada. • Realización de ensayos de perforación sobre metales utilizando diferentes formas de impactador (cónico, hemisférico y plano) que dan lugar a distintos modos de fallo del componente estructural. • Desarrollo de simulaciones numéricas del proceso de perforación que permitan conocer la influencia de la triaxialidad y del parámetro de Lode en la deformación de fallo. Los estudios se han llevado a cabo sobre las aleaciones de aluminio: 2024-T351, 5754-H111 y 6082- T6, que son habitualmente empleadas en la industria aeronáutica y de automoción. El conocimiento del comportamiento mecánico asociado al estado tensional (triaxialidad y parámetro de Lode) debe dar lugar a nuevos modelos numéricos, que incorporen adecuados criterios de fallo de los elementos estructurales de absorción de energía y protección frente a perforación e impacto. Los resultados proporcionarán herramientas para análisis y la simulación de prototipos físicos y virtuales de alto interés para la industria automovilística y aeronáutica.Design and optimization of structures considered for energy absorption during low and intermediate velocity impacts (crashworthiness), such as those taking place in traffic or railway accidents, is a principal topic in engineering science due to the increasing requests of safety and reliability for the structures in vehicles and aircrafts. In order to achieve an important reduction of the fuel consumption and the air pollution, lighter vehicle structures are used. The need to keep at the same time, or even improve, their energy absorption capacity encourages the use of numerical simulation tools for optimization design and reduced design costs of the prototypes. For a reliable prediction of the behaviour of the structural element and of its energy absorption capacity until breakage, the numerical tools have to consider a failure criteria for the material. The main goal has been to examine the effect of triaxiality and Lode parameter on metal behaviour at failure. The specific objectives of this doctoral Thesis have been the following: • Development of a physical-based flow stress model within wide range of application (strain rate and temperature). • Development of an experimental procedure in order to obtaine the failure strain in metals for different values of the stress state. • Application of failure criterion which considers the influence of the stress state (triaxiality and Lode parameter) and it can predict the behaviour under failure conditions . • Experimental and numerical analysis of the behaviour of structural elements for energy absorption, protection and perforation problems. In order to achieve the previous objectives, the following tasks have been developed: • Formulation and implementation of flow stress model with physical foundation based in dislocations and applicable to metals FCC. • Development of a empirical methodology for combined tension-torsion tests on circumferentially double notched tube specimens. • Calibration of Bai and Wierzbicki failure criterion (triaxiality and Lode parameter) as from the results obtained in combined tension-torsion tests. • Implementation of perforation tests on metals with different nose shape of projectile (conical, hemispheric and blunt) that cause several modes of failure of the structural component. • Development of numerical simulations on the perforation process and analysis of influence of triaxiality and Lode parameter on failure strain. The studies of this doctoral Thesis have been carried out on AA 2024-T351, AA 5754-H111 y AA 6082-T6, structural alloys commonly used in the aerospace and automotive industry. The knowledge of these mechanisms will lead to the development of damage models that could be implemented in finite viii Abstract element commercial codes, allowing predicting the mechanical behaviour of structural elements for impact protection and energy absorption. The results will provide tools for the analysis and simulation of virtual prototypes, of high interest for the automotive and aeronautic industries

Análisis numérico del comportamiento frente a impacto de alumnio 2024-T351 sometido a ensayo de Taylor

Con este proyecto se ha realizado un estudio numérico de impacto sobre aluminio 2024-T351 empleando el Ensayo de Taylor. Para realizar dicho estudio se ha empleado el programa de elementos finitos ABAQUS/ Explicit. El estudio sobre el Ensayo de Taylor se ha estructurado en primer lugar analizando la influencia del criterio de fallo y los parámetros asociados a los distintos mecanismos daño dúctil. Se ha comprobado que los parámetros de fallo influyen de manera determinante en la predicción de la respuesta del material. Igualmente se ha analizado la influencia de la ecuación constitutiva, en el que la elección de los distintos parámetros produce variaciones en el cálculo del límite elástico. Por último se ha realizado un estudio comparativo de los distintos tipos de contacto para simular el impacto donde la variación del límite elástico es de un 3-7 %.Ingeniería Industria

A dislocation-based constitutive description for modeling the behavior of FCC metals within wide ranges of strain rate and temperature

In this work a dislocation based constitutive description for modeling the thermo visco plastic behavior of FCC metals has been developed. The constitutive description, which is founded on the concepts of thermal activation analysis and dislocation dynamics, assumes the plastic flow additively decomposed into internal stress and effective stress. The internal stress represents the applied stress required for the transmission of plastic flow between the polycrystal grains and it is defined by the Hall Petch relationship. The effective stress formulation, which is the main innovative feature of this work, represents the thermally activated deformation behavior. This is defined taking into account the interrelationship between strain rate and temperature, and gathers structural evolution dependence. This structural evolution is described as a function of dislocations density, which acts as internal state variable in the material deformation behavior. A systematic procedure for identifica tion of the material parameters is developed and the model is applied to define the behav ior of annealed OFHC copper. The analytical predictions of the constitutive description are compared with the experimental data reported by Nemat Nasser and Li (Nemat Nasser, S., Li, Y., (1998). Flow stress of FCC polycrystals with application to OFHC Copper. Acta Mater. 46, 565 577). Good correlation between experiments and analytical predictions is found within wide ranges of strain rate and temperature.The researchers of the University Carlos III of Madrid are indebted to the Comunidad Autónoma de Madrid (Project CCG10 UC3M/DPI 5596) and to the Ministerio de Ciencia e Innovación de España (Project DPI/2008 06408) for the financial support received which allowed conduct ing part of this work.Publicad

Failure behavior of 2024-T3 aluminum under tension-torsion conditions

Experimental and numerical investigations of the failure strain of aeronautical 2024-T3 aluminum were conducted. Experiments on the Double notched tube (DNT) specimen loaded in combined tension and torsion were applied to an aluminum alloy for the first time. Numerical analysis showed that the specimen exhibited uniformity in stress-strain as plastic strain developed. Low triaxiality values and a wide range of Lode parameter values were obtained at failure conditions. The failure strain of 2024-T3 aluminum showed strong dependence on the Lode parameter in agreement with the observations reported by other authors. The use of the DNT specimen was proven to be efficient in calibrating the ductile failure model of aluminum alloys.The researchers are indebted to the Ministerio de Ciencia e Innovación (DPI/2011-24068) for the financial support received, which enabled this work to be conducted

Investigation of mechanical impact behavior of short carbon-fiber-reinforced PEEK composites

This paper describes the results of an experimental and numerical investigation of the impact behavior of short carbon fiber reinforced polyether-ether-ketone (SCFR PEEK) composites. The biocompatibility of PEEK and its short fiber composites, their rapid processing by injection molding and suitability for modern imaging have supported technological advances in prosthetic implants used in orthopedic medicine. Surgical implants, including hip and cranial implants, can experience clinically significant impact loading during medical installation and useful life. While the incorporation of short fibers in a thermoplastic matrix can produce significant improvements in stiffness and strength, it can also cause a marked reduction in ductility, making study of their energy absorption capability essential. In this work, the mechanical impact behavior of PEEK composites reinforced with polyacrylonitrile (PAN) short carbon fibers 30% in weight is compared with unfilled PEEK. The perforation tests conducted covered an impact kinetic energy range from 21 J to 131 J, equivalent to the range observed in a fall, the leading cause of hip fractures. Energy absorption capability, damage extension and failure mechanism have been quantified and reported. A numerical modeling that includes homogenization of elastic material and anisotropic damage is presented and validated with experimental data. At all impact energies, SCFR PEEK composites showed a brittle failure and their absorption energy capability decreases drastically in comparison with unfilled PEEK. (C) 2015 Elsevier Ltd. All rights reserved.The researchers of the University Carlos III of Madrid are indebted to the Ministerio de Ciencia e Innovación de España (Project DPI/2011-24068) and to the Ministerio de Economía y Competitividad de España (Project DPI/2014-57989-P) for financial support towards part of this work. The researchers are indebted to LATI Company for PEEK material supplie

Low temperature effect on impact energy absorption capability of PEEK composites

This paper describes the results of an experimental investigation which analyses the impact behavior at low temperature of polyether ether ketone (PEEK) and its short carbon fiber reinforced composite (SCFR PEEK). These polymer materials are widely employed in aeronautical applications subjected to impact loadings in which the energy absorption capability is an aspect that should be taken into account. The energy absorption capability can drastically decrease if temperatures near to the ductile-to-brittle transition temperature of polymeric matrix are reached. In this work, a set of perforation tests has been conducted covering a testing temperature range from -75 degrees C to +25 degrees C and an impact kinetic energy range from 11 J to 175 1 including typical values considered in impact loadings at aeronautical flight speeds. Energy absorption capability, damage extension and failure mechanisms have been quantified and reported. At low temperatures, a ductile-to-brittle transition was found in PEEK unfilled resulting in a suddenly change of its mechanical impact behavior affecting the energy absorption capability. In case of SCFR PEEK composite, a brittle behavior was observed for the whole temperature range considered and its energy absorption capability decreases drastically at lower temperatures. The brittleness of PEEK and SCFR PEEK at low temperature will limit the application of this composite in aeronautical structures exposed to impact.The researchers of the University Carlos III of Madrid are indebted to the Ministerio de Ciencia e Innovación de España (Project DPI/2011-24068) and to the Ministerio de Economía y Competitividad de España (Project DPI/2014-57989-P) for financial support towards part of this work

Influence of stress state on the mechanical impact and deformation behaviors of aluminum alloys

Under impact loading conditions, the stress state derived from the contact between the projectile and the target, as well as from the subsequent mechanical waves, is a variable of great interest. The geometry of the projectile plays a dertermining role in the resulting stress state in the targeted structure. In this regard, different stress states lead to different failure modes. In this work, we analyze the influence of the stress state on the deformation and failure behaviors of three aluminum alloys that are commonly used in the aeronautical, naval, and automotive industries. To this purpose, tension-torsion tests are performed covering a wide range of stress triaxialities and Lode parameters. Secondly, the observations from these static tests are compared to failure mode of the same materials at high impact velocities tests with the aim of analysing the role of stress state and strain rate in the mechanical response of the aluminum plates. Experimental impacts are conducted with different projectile geometries to allow for the analysis of stress states influence. In addition, these experiments are simulated by using finite element models to evaluate the predictive capability of three failure criteria: critical plastic deformation, Johnson-Cook, and Bai-Wierzbicki.The researchers of the University CarlosIII are indebted to the Ministerio de Economía y Competitividad de España (Project DPI2014-57989-P) and Vicerrectorado de Política Científica UC3M (Project 2013-00219-002) for the financial suppor

Development of numerical model for ballistic resistance evaluation of combat helmet and experimental validation

Modern designing process of combat helmets requires both numerical modeling and experimental validation in order to achieve exigent requirements combining impact resistance and reasonable weight. In this work a finite element model of a combat helmet is presented. Mechanical behaviour of the shell aramid composite under impact conditions was analyzed from experimental Fragment Simulating Projectile (FSP) and Full-Metal Jacketed (FMJ) impact tests on aramid flat plates. Numerical modeling based on finite elements method was used to simulate both impacts in simple plates of the composite and also the simulation of ballistic impact involving real ammunition and the complex geometry of the helmet including inner foam. Experimental work involving impact tests on composite plates and also ballistic test on the helmet with a dummy provided real data for comparison with models predictions and proved the accuracy of the numerical models developed.The authors acknowledge the Ministry of Economy and Competitiveness of Spain and FEDER program under the Project RTC-2015-3887-8 for the financial support of the work

Experimental Study on the Perforation Process of 5754-H111 and 6082-T6 Aluminium Plates Subjected to Normal Impact by Conical, Hemispherical and Blunt Projectiles

This paper presents an experimental investigation on the perforation behaviour of 5754-H111 and 6082-T6 aluminium alloys. The mechanical response of these materials has been characterized in compression with strain rates in the range of . Moreover, penetration tests have been conducted on 5754-H111 and 6082-T6 plates of thickness using conical, hemispherical and blunt projectiles. The perforation experiments covered impact velocities in the range of . The initial and residual velocities of the projectile were measured and the ballistic limit velocity obtained for the two aluminium alloys for the different nose shapes. Failure mode and post-mortem deflection of the plates have been examined and the perforation mechanisms associated to each projectile/target configuration investigated. It has been shown that the energy absorption capacity of the impacted plates is the result of the collective role played by target material behaviour, projectile nose shape and impact velocity in the penetration mechanisms.The researchers of the University Carlos III of Madrid are indebted to the Comunidad Autónoma de Madrid (Project CCG10-UC3M/DPI-5596) and to the Ministerio de Ciencia e Innovación de España (Project DPI/2011-24068) for the financial support received which allowed conducting part of this work

Ballistic performance of aramid composite combat helmet for protection against small projectiles

This paper focuses on the ballistic performance of aramid composite combat helmet commonly worn by military and security corps, against small projectiles threat. We propose a numerical finite element model for aramid composite protections, considering a multi-layer architecture, able to predict its ballistic behaviour and damage extension. The aim is determining the minimum number of layers required for a correct protection against a given ballistic thread. The constitutive aramid behaviour has been calibrated by means of experimental tests with FSP (Fragment Simulate Projectiles) projectiles and steel spheres on aramid flat plates. Once calibrated, a predictive numerical model of the helmet against different small projectiles and impacted localisations was developed and compared with experimental tests performed in the real head protection. The results calculated for the absorbed impact energy by the helmet and the induced damage due to small projectiles at different impact location, are in good agreement with experimental results and postmortem helmet analysis, validating the proposed numerical model. The numerical model is thus validated for the design of optimized head protections based on aramid compositeThe authors acknowledge the Ministry of Economy and Competitiveness of Spain and the European Regional Development Fund, (FEDER) program under the Project RTC-2015-3887-8 and the Project DPI2017-88166-R for the financial support of the work