Constitutive modelling and mechanical characterization of aluminium-based metal matrix composites produced by spark plasma sintering

Spark plasma sintering has been applied to the production of aluminium-based functionally graded material systems to be used in abrasive and high temperature conditions. The overall mechanical properties of these metal matrix composites were determined during the optimization phases of the production process by a fast and reliable identification procedure based on instrumented indentation, which can be easily performed on small specimens. The experimental information gathered from conical (Rockwell) indentation was used as input data for the calibration of the material parameters entering the elastic–plastic Drucker–Prager constitutive model. Eventually, the so identified material parameters were used to predict the result of pyramidal (Vickers) indentation, in order to validate the model selection and the output of the identification procedure. The good matching between modelling and experimental results for the different test configurations confirmed the soundness of the considered approach, especially evidenced on the light of the strong influence on the overall mechanical characteristics of the material microstructure and defectiveness resulting from the production process, which prevent the use of classical homogenization rules to evaluate the macroscopic material properties

Automated non-destructive integrity assessment of metal structures

Offshore windmills and pipeline networks are examples of strategic infrastructures used for the production of clean energy and for the storage and long-distance transportation of hydrocarbons, hydrogen and water. The relevant structural elements are mainly made of welded portions of steel pipes, which often interact with aggressive fluids and hostile environments. Material aging is thus accelerated and localized damage processes are promoted, harming the design safety factors. The structural health of such components can be monitored in operation, throughout their lifetime, by non-destructive testing performed by portable devices. The equipment at present available on the market permits to develop fully automated testing campaigns, overcoming the difficulties associated to large extension and difficult accessibility. The data collected on site can be transferred through virtual networks, to be evaluated and processed in order to permit the quantitative evaluations required by the optimization and the planning of repair and retrofit operations. This contribution discusses the potential offered by the current practice and illustrates the methodological adaptations that produce effective diagnostic tools in the outlined context

Childish Gambino y la serie "Atlanta": el uso de Internet para una experiencia inmersiva

O seguinte artigo tem o intuito de analisar a série estadunidense Atlanta, mais especificamente as estratégias transmidiáticas que seu criador, Donald Glover, utiliza para ampliar o universo narrativo da série, assim como a influência do conceito de cibercultura na criação do programa. Junto a isso, analisaremos o trabalho de Glover, desde o seu projeto como músico, chamado Childish Gambino, com foco no seu disco “Because the Internet” (Por causa da Internet), de 2013. A fundamentação teórica do trabalho abordará conceitos de narração transmídiatica, comunidades virtuais e ARG (alternate reality game). Foram usados autores como Henry Jenkins, Pierre Lévy, Janet Muray, Jason Mittell, Newton Cannito, entre outros, para a estruturação da pesquisa.El siguiente artículo tiene el propósito de analizar la serie estadounidense Atlanta, más específicamente las estrategias transmidiáticas que su creador, Donald Glover, utiliza para ampliar el universo narrativo de la serie, así como la influencia del concepto de cibercultura en la creación del programa. En el caso de Glover, desde su proyecto como músico, llamado Childish Gambino, centrándose en su disco “Because the Internet” (Debido a la Internet), de 2013. La fundamentación teórica del trabajo abordará conceptos de narración transmidiatica, comunidades virtuales y ARG (alternate reality game). Se utilizaron autores como Henry Jenkins, Pierre Lévy, Janet Muray, Jason Mittell, Newton Cannito, entre otros, para la estructuración de la investigación

Machine Learning tools applied to the prediction and interpretation of the structural behavior of existing dams

The safety of existing dams is mainly ensured by the correct interpretation of monitoring data recorded during the whole lifetime of these structures. In this context, an increasing number of devices are being installed to provide more and more frequent measurements. Several Machine Learning tools have emerged as possible alternatives to traditional prediction approaches in recent years. Neural Networks have shown the ability to adapt to complex interactions and, therefore, to reach greater accuracy than conventional methods. However, this technique is susceptible to parameter tuning and difficult to generalize. Other recent studies have focused on Boosted Regression Trees. Less frequently used in dam engineering, they have proved to be equally accurate compared to Neural Networks, simpler to implement, and not sensitive to noisy and low relevant predictors. However, applications are limited to a few specific cases. The present contribution aims to evaluate the performances of this novel approach on dam data with a different specificity from previous research. The case study corresponds to a double-curvature arch dam introduced as a benchmark test by the International Commission on Large Dams. The input data include raw environmental variables, some derived variables, and time-related variables. Predictions of displacements under varying environmental conditions are performed, and relative influence indices are identified to determine the strength of each input-output relationship

Parametric failure analysis of metal-based composites

Metal-based composites can fail as a consequence of the growth and coalescence of micro-voids introduced with the manufacturing process. These detrimental phenomena influence the overall performance of the material to different extents since the macroscopic characteristics depend on both the local constitutive properties and geometry patterns, which promote various stress concentration and strain localization effects. The understanding of the different situations that arise in this context is often assisted by numerical simulations based on the GNT constitutive model, first proposed by Gurson (1977) and then refined by Needleman and Tvergaard (1984). However, exploring the influence of the most relevant material parameters on the composite response can be excessively time consuming. Therefore, traditional simulations based on non-linear finite element methods can be replaced by surrogate analytical approximations, which do not involve large computing costs but exhibit accuracy and sensitivity to the model parameters consistent with the practical applications. Some examples are presented in this contribution

3D Scan of Hardness Imprints for the Non-destructive In-Situ Structural Assessment of Operated Metal Components

The structural integrity of operated components can be assessed by non-destructive mechanical tests performed in-situ with portable instruments. Particularly promising in this context are small scale hardness tests supplemented by the mapping of the residual imprints left on metal surfaces. The data thus collected represent the input of inverse analysis procedures, which determine the material characteristics and their evolution over time. The reliability of these estimates depends on the accuracy of the geometry scans and on the robustness of the data filtering and interpretation methodologies. The objective of the present work is to evaluate the accuracy of the 3D reconstruction of the residual deformation produced on metals by hardness tests performed at a few hundred N load. The geometry data are acquired by portable optical microscopes with variable focal distance. The imperfections introduced by the imaging system, which may not be optimized for all ambient conditions when used in automatic mode, are analysed. Representative examples of the output produced by the scanning tool are examined, focusing attention on the experimental disturbances typical of onsite applications. Proper orthogonal decomposition and data reduction techniques are applied to the information returned by the instrumentation. The essential features of the collected datasets are extracted and the main noise is removed. The results of this investigation show that the accuracy achievable with the considered equipment and regularization procedures can support the development of reliable diagnostic analyses of metal components in existing structures and infrastructures

An indentation based investigation on the characteristics of artificially aged pipeline steels

The decay of the mechanical properties of structural components operating in an aggressive environment can be detected by nondestructive indentation tests. The effectiveness of this approach has been verified on artificially aged pipeline steel. Indentation tests have been performed at different scales to verify the transferability of the laboratory results to the field conditions, in view of the possible development of in-situ diagnostic procedures

Modeling water penetration at dam-foundation joint

When fracture occurs in a concrete dam, the crack mouth is typically exposed to water. Very often this phenomenon occurs at the dam-foundation joint and is driven also by the fluid pressure inside the crack. Since the joint is the weakest point in the structure, this evolutionary process determines the load bearing capacity of the dam. In this paper the cracked joint is analyzed through the cohesive model proposed by Cocchetti et al. [Cocchetti G, Maier G, Shen X. Piecewise linear models for interfaces and mixed mode cohesive cracks. J Engng Mech (ASCE) 2002;3:279-98.], which takes into account the coupled degradation of normal and tangential strength. The water pressure inside the crack, which reduces fracture energy and increases the driving forces, is analyzed through the model proposed by Reich et al. [Reich W, Bruhwiler E, Slowik V, Saouma VE. Experimental and computational aspects of a water/fracture interaction. In: Bourdarot E, Mazars J, Saouma V, editors, Dam Fracture and Damage, The Netherlands: Balkema; 1994. p. 123-31.] and Bruhwiler and Saouma [Bruhwiler E, Saouma VE. Water fracture interaction in concrete. Part II: Fracture properties. Am Concr Inst J 1995;92:296-303; Bruhwiler E, Saouma VE. Water fracture interaction in concrete. Part II: Hydrostatic pressure in cracks. Am Concr Inst J 1995;92:383-90.]. Some numerical results are presented which refer to the benchmark problem proposed in 1999 by the International Commission On Large Dams. During the evolutionary process the horizontal dam crest displacement has been found to be a monotonic increasing function of the external load multiplier. As the fictitious process zone moves from the upstream to the downstream edge a transition occurs in the path of crack formation: the initial phase is dominated by the opening displacement, on the contrary afterwards the shear displacement dominates. Therefore, crack initiation does not depend on dilatancy. On the contrary the load carrying capacity depends on dilatanc

Numerical simulation of non-standard tensile tests of thin metal foils

The evolution of the fracture processes occurring in thin metal foils can be evidenced by tensile tests performed on samples of non-standard dimensions. The load versus displacement record of these experiments does not return directly the local stress-strain relationship and the fracture characteristics of the investigated material. In fact, the overall response of thin foils is sensitive to local imperfections, size and geometric effects. Simulation models of the performed tests can support the interpretation of the experimental results, provided that the most significant physical phenomena are captured. The present contribution focuses on the role of modelling details on the numerical output that can be obtained in this context