A thermodynamically consistent plastic-damage framework for localized failure in quasi-brittle solids: material model and strain localization analysis

Aiming for the modeling of localized failure in quasi-brittle solids, this paper addresses a thermodynamically consistent plastic-damage framework and the corresponding strain localization analysis. A unified elastoplastic damage model is first presented based on two alternative kinematic decompositions assuming infinitesimal deformations, with the evolution laws of involved internal variables characterized by a dissipative flow tensor. For the strong (or regularized) discontinuity to form in such inelastic quasi-brittle solids and to evolve eventually into a fully softened one, a novel strain localization analysis is then suggested. A kinematic constraint more demanding than the classical discontinuous bifurcation condition is derived by accounting for the traction continuity and the loading/unloading states consistent with the kinematics of a strong (or regularized) discontinuity. More specifically, the strain jumps characterized by Maxwell’s kinematic condition have to be completely inelastic (energy dissipative). Reproduction of this kinematics implies vanishing of the aforesaid dissipative flow tensorial components in the directions orthogonal to the discontinuity orientation. This property allows naturally developing a localized plastic-damage model for the discontinuity (band), with its orientation and the traction-based failure criterion consistently determined a posteriori from the given stress-based counterpart. The general results are then particularized to the 2D conditions of plane stress and plane strain. It is found that in the case of plane stress, strain localization into a strong (or regularized) discontinuity can occur at the onset of strain softening. Contrariwise, owing to an extra kinematic constraint, in the condition of plane strain some continuous inelastic deformations and substantial re-orientation of principal strain directions in general have to take place in the softening regime prior to strain localization. The classical Rankine, Mohr–Coulomb, von Mises (J2) and Drucker–Prager criteria are analyzed as illustrative examples. In particular, both the closed-form solutions for the discontinuity angles validated by numerical simulations and the corresponding traction-based failure criteria are obtained.Peer ReviewedPostprint (author's final draft

Effect of the tool tilt angle on the heat generation and the material flow in friction stir welding

This work studies the effect of the tool tilt angle on the generated heat and the material flow in the work pieces joint by Friction Stir Welding (FSW). An apropos kinematic framework together with a two-stage speed-up strategy is adopted to simulate the FSW problem. The effect of tilt angle on the FSWelds is modeled through the contact condition by modifying an enhanced friction model. A rotated friction shear stress is proposed, the angle of rotation depending on the process parameters and the tilt angle. The proposed rotation angle is calibrated by the experimental data provided for a tilt angle 2.5°. The differences of generated heat and material flow for the cases of tool with tilt angle of 0° and 2.5° are discussed. It is concluded that due to the higher temperature, softer material and greater frictional force in the trailing side of the tool, the material flow in the rear side of the FSW tool with the title angle is considerably enhanced, which assists to prevent the generation of defect.Peer ReviewedPostprint (published version

3D numerical modelling of twisting cracks under bending and torsion of skew notched beams

The testing of mode III and mixed mode failure is every so often encountered in the dedicated literature of mechanical characterization of brittle and quasi-brittle materials. In this work, the application of the mixed strain displacement e-ue-u finite element formulation to three examples involving skew notched beams is presented. The use of this FE technology is effective in problems involving localization of strains in softening materials. The objectives of the paper are: (i) to test the mixed formulation in mode III and mixed mode failure and (ii) to present an enhancement in terms of computational time given by the kinematic compatibility between irreducible displacement-based and the mixed strain-displacement elements. Three tests of skew-notched beams are presented: firstly, a three point bending test of a PolyMethyl MethaAcrylate beam; secondly, a torsion test of a plain concrete prismatic beam with square base; finally, a torsion test of a cylindrical beam made of plain concrete as well. To describe the mechanical behavior of the material in the inelastic range, Rankine and Drucker-Prager failure criteria are used in both plasticity and isotropic continuum damage formats. The proposed mixed formulation is capable of yielding results close to the experimental ones in terms of fracture surface, peak load and global loss of carrying capability. In addition, the symmetric secant formulation and the compatibility condition between the standard irreducible method and the strain-displacement one is exploited, resulting in a significant speedup of the computational procedure.Peer ReviewedPostprint (author's final draft

An orthotropic mesh corrected crack model

This paper recovers the original spirit of the continuous crack approaches, where displacements jumps across the crack are smeared over the affected elements and the behaviour is established through a softening stress–(total) strain law, using standard finite element displacement interpolations and orthotropic localconstitutive models. The paper focuses on the problem of shear locking observed in the discrete problem when orthotropic models are used. The solution for this drawback is found in the form of a mesh corrected crack model where the structure of the inelastic strain tensor is linked to the geometry of the cracked element. The discrete model is formulated as a non-symmetric orthotropic local damage constitutive model, in which the softening modulus is regularized according to the material fracture energy and the element size. The resulting formulation is easily implemented in standard non-linear FE codes and suitable for engineering applications. Numerical examples show that the results obtained using this crack model do not suffer from dependence on the mesh directional alignment, comparing very favourably with those obtained using related standard isotropic or orthotropic damage models

An orthotropic mesh corrected crack model

This paper recovers the original spirit of the continuous crack approaches, where displacements jumps across the crack are smeared over the affected elements and the behaviour is established through a softening stress–(total) strain law, using standard finite element displacement interpolations and orthotropic localconstitutive models. The paper focuses on the problem of shear locking observed in the discrete problem when orthotropic models are used. The solution for this drawback is found in the form of a mesh corrected crack model where the structure of the inelastic strain tensor is linked to the geometry of the cracked element. The discrete model is formulated as a non-symmetric orthotropic local damage constitutive model, in which the softening modulus is regularized according to the material fracture energy and the element size. The resulting formulation is easily implemented in standard non-linear FE codes and suitable for engineering applications. Numerical examples show that the results obtained using this crack model do not suffer from dependence on the mesh directional alignment, comparing very favourably with those obtained using related standard isotropic or orthotropic damage models

A smeared‐embedded mesh‐corrected damage model for tensile cracking

Traditional smeared orthotropic models display an unacceptable dependence of the solution on the alignment of the mesh, which usually manifests as stress locking. A solution for this drawback is proposed in this paper by adopting the concept of embedded inelastic strains, rather than displacement jumps, and by linking the structure of the inelastic strain to the geometry of the cracked element. The resulting model, applicable to linear 3‐noded triangles, is formulated as a non‐symmetric orthotropic local damage constitutive model, with the softening modulus regularized according to the material fracture energy and the element size. Analytical and numerical results show that this approach is effective in removing the locking problem as well as efficient from the computational point of view

Adopting test automation at Effizency to Improve Agility and Software Quality

Digital solutions have long been used as a means to solve everyday problems. Over time these solutions have been improved and refined. These solutions have emerged to help humans, primarily with tasks that can be cumbersome or repetitive. The demand for repetitive tasks and process optimization through digital means is peaked. For this reason, many companies in the software development area have adopted the use of automated tests capable of doing autonomously and quickly the tasks that previously required a lot of resources to perform, thus jumping on the "Automation Bandwagon”. By applying this approach, companies have the goal to improve the quality standards of the software offered by reducing the number of bugs and identifying them as early as possible in the development process. To observe the applicability, optimization, and efficiency of the automation of autonomous testing in a specific system, the concepts and technologies proposed here were applied in a professional scenario of a young company, Effizency. This company aims to facilitate the sale of energy services and electrical optimization. Effizency currently works using an agile approach and is constantly looking for ways to improve its development process. The company is currently facing the challenge of increasing the quality of its software and at the same time reducing the repetitiveness of its validation processes. Through this dissertation, it is expected that an improvement will be identified in terms of both a reduction of process repetition, time consumption and an increase in the test coverage performed. The main objective of this dissertation is to improve the quality of a company's software and the agility of its development process. This objective will be achieved using automated testing.As soluções digitais são usadas para resolver problemas do dia a dia há muito tempo. Ao longo do tempo, estas têm sido melhoradas e aperfeiçoadas. Estas soluções surgiram com o objetivo de ajudar o ser humano nas suas tarefas, maioritariamente tarefas que podem ser pesadas ou repetitivas. A procura pela automatização de tarefas repetitivas e de otimização de processos através de meios digitais está no seu auge. Por essa razão, muitas empresas na área de desenvolvimento de software adotaram o uso de testes automáticos capazes de fazer de forma autónoma e rápida as tarefas que anteriormente necessitariam de muitos recursos para realizar, entrando assim no “Vagão da Automatização”. Ao aplicar esta abordagem, as empresas têm o objetivo de melhorar os padrões de qualidade do software oferecidos reduzindo o número de bugs e identificando-os o mais cedo possível no processo de desenvolvimento. Com o intuito de observar a aplicabilidade, otimização e eficiência da automação de testes autónomos num sistema em concreto foi realizada a aplicação dos conceitos e tecnologias aqui propostos num cenário profissional de uma empresa jovem, Effizency. A Effizency trabalha atualmente utilizando uma abordagem ágil e está constantemente à procura de formas de melhorar o seu processo de desenvolvimento. Atualmente a empresa, enfrenta o desafio de aumentar a qualidade do seu software e, ao mesmo tempo reduzir o a repetitividade dos seus processos de validação. Através desta dissertação, espera-se que seja identificada uma melhoria tanto em termos de uma redução da repetição do processo, do consumo de tempo e de um aumento da cobertura dos testes realizados. O principal objetivo desta dissertação é melhorar a qualidade do software de uma empresa e a agilidade do seu processo de desenvolvimento. Este objetivo será alcançado através de testes automatizados

Paleodieta : un acercamiento al estudio de la alimentación en las poblaciones del pasado

Mitjançant l'anàlisi química d'elements traça en restes òssies humans, la Química contribueix al coneixement de les poblacions passades, col·laborant amb l'Arqueologia i complementant l'estudi antropològic i paleopatològic, ajudant-nos així a un millor enteniment de la vida, economia i subsistència d'antigues civilitzacions.By means of the chemical analysis of trace elements in human bones, Chemistry contributes to the knowledge of the passed populations, collaborating with Archaeology and complementing the anthropological and paleopathological study, helping us thus to a better understanding of the life, economy and subsistence of old civilizations.Mediante el análisis químico de elementos traza en restos óseos humanos, la Química contribuye al conocimiento de las poblaciones pasadas, colaborando con la Arqueología y complementando el estudio antropológico y paleopatológico, ayudándonos así a un mejor entendimiento de la vida, economía y subsistencia de antiguas civilizaciones

Explicit mixed strain–displacement finite elements for compressible and quasi-incompressible elasticity and plasticity

The final publication is available at Springer via http://dx.doi.org/ 10.1007/s00466-016-1305-zThis paper presents an explicit mixed finite element formulation to address compressible and quasi-incompressible problems in elasticity and plasticity. This implies that the numerical solution only involves diagonal systems of equations. The formulation uses independent and equal interpolation of displacements and strains, stabilized by variational subscales. A displacement sub-scale is introduced in order to stabilize the mean-stress field. Compared to the standard irreducible formulation, the proposed mixed formulation yields improved strain and stress fields. The paper investigates the effect of this enhancement on the accuracy in problems involving strain softening and localization leading to failure, using low order finite elements with linear continuous strain and displacement fields (P1P1 triangles in 2D and tetrahedra in 3D) in conjunction with associative frictional Mohr–Coulomb and Drucker–Prager plastic models. The performance of the strain/displacement formulation under compressible and nearly incompressible deformation patterns is assessed and compared to analytical solutions for plane stress and plane strain situations. Benchmark numerical examples show the capacity of the mixed formulation to predict correctly failure mechanisms with localized patterns of strain, virtually free from any dependence of the mesh directional bias. No auxiliary crack tracking technique is necessary.Peer ReviewedPostprint (author's final draft

Breve reseña sobre métodos para datación arqueológica

La arqueología es una ciencia que permite, por medio de los vestigios naturales de las épocas pasadas, comprobar datos históricos y sustentar los datos prehistóricos, obteniendo así el conocimiento de las artes, técnicas y modos de vida de los distintos pueblos y épocas. El rápido desarrollo de las ciencias experimentales y sus aplicaciones prácticas ha prestado a la incipiente ciencia arqueológica una base práctica y metodológica, tan importante como aquella que, en el plano teórico, le habían prestado la historiografía y las ciencias sociales.Archaeology is a science that allows,by means of the natural vestiges of the last times, to verify the historical data and to sustain the prehistoric data, collecting therefore the knowledge of the arts, techniques and ways of life of the different towns and different times.The fast development of experimental sciences and its practical applications has lent to incipient archaeological science a practical and methodological base as historiography and social sciences had done before in a theoretical plane