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In this paper, the microbial degradation of methyl red (MR) was investigated by novel isolated bacteria S1 and K1 from wastewater effluent. Based on morphological, biochemical and molecular methods, strain S1 belonged to the Staphylococcus schleiferi subsp. coagulans GA 211 with 92.67% similarity (accession number MT573965) and strain K1 belonged to the Klebsiella michiganensis W14 with 99.85% similarity (accession number MT572941). To the best of our knowledge, this is the first report of dye degradation activity of these bacteria. The Taguchi Statistical Method (TSM) was used to optimize the experimental conditions. Taguchi’s L18 orthogonal array (OA) was performed, and the results were used to calculate signal-to-noise (S/N) ratios. According to S/N ratios, it was discovered that optimum experimental conditions are at pH 8, temperature 40°C, initial dye concentration 50 mg/l, carbon source glucose, rpm 75, inoculum 3% v/v and strain S1. Analyzing the results of the biodegradation assay by Minitab 19 statistical software, it was found that rpm and temperature are most and less effective in the MR biodegradation assay, respectively. Significant changes in the HPLC and UV-Vis spectrum of samples of strain S1 before and after biodegradation assay confirmed the complete biodegradation of MR by S1 strain.

In this paper, we have performed a microstructurally-informed finite element analysis on the effect of porosity on the formation of multiple necks and fragments in ductile thin rings subjected to dynamic expansion. For that purpose, we have characterized by X-ray tomography the porous microstructure of 4 different additively manufactured materials (aluminium alloy AlSi10Mg, stainless steel 316L, titanium alloy Ti6Al4V and Inconel 718L) with initial void volume fractions ranging from ≈0.0007% to ≈ 2%, and pore sizes varying between ≈ 6 μm and ≈110 μm. Three-dimensional analysis of the tomograms has revealed that the voids generally have nearly spherical shape and quite homogeneous spatial distribution in the bulk of the four materials tested. The pore size distributions quantified from the tomograms have been characterized using a Log-normal statistical function, which has been used in conjunction with a Force Biased Algorithm that replicates the experimentally observed random spatial distribution of the voids, to generate ring expansion finite element models in ABAQUS/Explicit (2016) which include actual porous microstructures representative of the materials tested. We have modeled the materials behavior using von Mises plasticity, and we have carried out finite element calculations for both elastic perfectly-plastic materials, and materials which show strain hardening, strain rate hardening and temperature softening effects. Moreover, we have assumed that fracture occurs when a critical value of effective plastic strain is reached. The finite element calculations have been performed for expansion velocities ranging from 50 m/s to 500 m/s. A key point of this investigation is that we have established individualized correlations between the main features of the porous microstructure (i.e. initial void volume fraction, average void size and maximum void size) and the number of necks and fragments formed in the calculations. In addition, we have brought out the effect of the porous microstrucure and inertia on the distributions of neck and fragment sizes. To the authors’ knowledge, this is the first paper ever considering actual porous microstructures to investigate the role of material defects in multiple localization and dynamic fragmentation of ductile metallic materials.

The mechanical behaviour of anisotropic, closed-cell PU foams has been studied from the experimental and simulation viewpoints. Anisotropic PU foams with different densities were tested in compression along the rising and the perpendicular directions to determine the mechanical properties (elastic modulus and plateau stress). In addition, the microstructure of the foams (cell size distribution and anisotropy, cell wall thickness, etc.) was carefully measured and the mechanical properties of the solid PU within the foam were determined by means of nanoindentation. This information was used to build realistic representative volume elements of the microstructure of the foams by means of Laguerre tessellations and the mechanical behaviour was obtained by means of the finite element simulation of these representative volume elements. The numerical simulations corroborated the large increase in the stiffness, in the stress at the onset of plastic instability and in the plateau stress of the foam along the rising direction, in agreement with the experimental results. In addition, the analysis of the elastic response in the representative volume elements showed that deformation along the rising direction (parallel to the longer axis of the cells) was dominated by the axial deformation of the struts while bending controlled the deformation perpendicular to the rising direction. The stress at the onset of plastic instability was controlled by the buckling of the struts, which was triggered at lower stresses in the direction perpendicular to the rising direction because of bending.

A modelling strategy based on micromechanical characterization and computational homogenization has been developed to determine the mechanical behavior of rigid, closed-cell PU foams taking into account the microstructural features. The macroscopic mechanical properties of the foam were obtained by means of the finite element simulation of a representative volume element of the PU foam. The foam microstructure in the model was obtained from the Laguerre tessellation of the space from a random close-packed sphere distribution, which followed the cell size distribution of the foam. The faces and edges of the polyhedra representing the cell walls and struts and were discretized using shell and beam elements, respectively, and their geometric features (shape, thickness, etc.) were carefully measured by means of X-ray computed tomography. Finally, the elastic modulus and yield strength of the solid polyurethane in the foam were measured using instrumented nanoindentation. The numerical simulations of the mechanical behavior of the foam in compression were in agreement with the experimental results of the elastic modulus and of the stress at the onset of instability, which was triggered by the localization of damage in a section of the microstructure by the elasto-plastic buckling of the struts.

The mechanical behavior of closed- and open-cell foams was analyzed using a computational homogenization strategy based on the finite element simulation of a representative volume element of the foam microstructure. The representation of the foam took into account the main microstructural features, namely density, fraction of solid material in cell walls and struts, cell anisotropy, cell size and distribution and strut shape. A parametric study was carried out to ascertain the influence of these parameters on the elastic modulus and the plateau stress under compression. It was found that these properties mainly depended on the density, fraction of solid material in cell walls and struts and cell anisotropy. Building upon the results of the parametric study, surrogate models were developed to relate the influence of the microstructure on the mechanical properties of the foams. It is believed that these models can be used for the design of foams with optimized properties for particular applications.

The mechanical behavior of closed-cell foams in compression is analyzed by means of the finite element simulation of a representative volume element of the microstructure. The digital model of the foam includes the most relevant details of the microstructure (relative density, cell size distribution and shape, fraction of mass in the struts and cell walls and strut shape), while the numerical simulation takes into account the influence of the gas pressure in the cells and of the contact between cell walls and struts during crushing. The model was validated by comparison with experimental results on isotropic and anisotropic polyurethane foams and it was able to reproduce accurately the initial stiffness, the plateau stress and the hardening region until full densification in isotropic and anisotropic foams. Moreover, it also provided good estimations of the energy dissipated and of the elastic energy stored in the foam as a function of the applied strain. Based on the simulation results, a simple analytical model was proposed to predict the mechanical behavior of closed-cell foams taking into the effect of the microstructure and of the gas pressure. An example of application of the simulation tool is presented to design foams with an optimum microstructure from the viewpoint of energy absorption for packaging.

In this paper we have investigated, using finite element calculations performed in ABAQUS/Explicit [1], the effect of ab initio geometric imperfections in the development of multiple necking patterns in ductile rings subjected to dynamic expansion. Specifically, we have extended the work of Rodríguez-Martínez et al. [2], who studied the formation of necks in rings with sinusoidal spatial perturbations of predefined amplitude and constant wavelength, by considering specimens with random distributions of perturbations of varying amplitude and wavelength. The idea, which is based on the work of El Maï et al. [3], is to provide an idealized modeling of the surface defects and initial roughness of the rings and explore their effect on the collective behavior and spacing of the necks. The material behavior has been modeled with von Mises plasticity and constant yield stress, and the finite element simulations have been performed for expanding velocities ranging from 10 m/s to 1000 m/s, as in ref. [2]. For each speed, we have performed calculations varying the number of imperfections in the ring from 5 to 150. In order to obtain statistically significant results, for each number of imperfections, the computations have been run with five random distributions of imperfection wavelengths. For a small number of imperfections, the variability in the wavelengths distribution is large, which makes the imperfections play a major role in the necking pattern, largely controlling the spacing and growth rate of the necks. As the number of imperfections increases, the variability in the wavelengths distribution decreases, giving rise to an array of more regularly spaced necks which grow at more similar speed. A key outcome is to show that, for a large number of imperfections, the number of necks formed in the ring comes closer to the number of necks obtained in the absence of ab initio geometric imperfections. The research leading to these results has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme. Project PURPOSE, grant agreement 758056.

In order to evaluate the hot flow behaviour of AZ80 magnesium alloy containing 0.5 wt% rare earth elements, isothermal hot compression tests were conducted at the temperatures range of 250–450 °C under the strain rates of 0.001–1 s−1. The flow curves exhibited a peak stress at a small applied strain after which the flow stresses decrease gradually to a steady state flow stress at higher applied strain values representing the occurrence of dynamic recrystallization. Moreover, the material constants in the Zener-Hollomon constitutive equation were found to be strain dependent, and the relationships between these parameters and the applied true strain were well described by fourth-degree polynomial functions. Accordingly, a strain-dependent model was developed in order to predict the hot flow behaviour of AZ80+0.5RE magnesium alloy. The true stresses predicted by this model were found to be in good agreements with the experimental results. Furthermore, the constitutive model developed using the experimental results was incorporated in the finite element model with which the hot compressive flow curve of the investigated material was modeled. The results showed that the accuracy of flow curves calculated by the numerical simulations depends on the accuracy of the developed model.

The flow curves and failure modes of dual phase steels containing 18 and 44 vol.% martensite were predicted using a micromechanical-based finite element method. Actual microstructures of the dual phase steels obtained by scanning electron microscope (SEM) were used as the representative volume elements (RVEs) in the calculations. Ductile failure of the representative volume was predicted as plastic strain localization during deformation. Computations were conducted on the representative volume to quantitatively evaluate the effects of mechanical properties of the dual phase constituents and their volume fractions on the macroscopic mechanical properties of the dual phase steels. The computational results were compared with the experimentally obtained data. It was found that the computational method can predict well both strength and ductility of the studied dual phase steels. Moreover, based on the computational results, shear dominant failure mode occurs in both of the studied dual phase steels which correlate with experimental findings.

Dual-phase (DP) steels have a composite-type microstructure consisting mainly of hard martensite islands embedded in a soft ferrite matrix. DP steels exhibit a characteristic combination of high strength, high work hardening rate and good ductility. Mechanical behaviour of DP steels is closely related to their microstructures. Hence, it is necessary to take into account their microstructural parameters in any attempt to estimate their flow behaviour. In this study, the flow curves of low carbon DP steels with 26.4 and 52% martensite produced by intercritical annealing processes at different temperatures were calculated using the finite element method (FEM). According to the results of microscopical observations of steel microstructures, martensite islands were assumed in the model to be spherical in shape. Moreover, in agreement with the experimental results in the literature clearly showing the possibility of martensite plastic deformation in similar steels during straining when its volume fraction is greater than about 30%, the plasticity of martensite islands in the steel microstructures was taken into account in the model by using their experimentally obtained stress-strain relations as a function of their estimated carbon contents. Having estimated the stress-strain behaviour of the ferrite phase in all steel samples using the microstructural parameters corresponding to the starting ferrite+pearlite steel, the flow curves of different DP steel samples with elastic martensite (in low martensite content steels) and elasto-plastic martensite (in high martensite content samples) were calculated. The calculated stress-strain curves exhibited reasonable agreements with the experimental stress-strain curves obtained from the tensile tests.

Instrumented sharp indentation experiments using both conical and Vickers diamond pyramidal indenters were carried out to study deformation characteristics of Zr55Cu30Al10Ni5 bulk metallic glass. Finite element simulations of instrumented indentation were also performed to formulate an overall constitutive response. Comparing the experimentally obtained results with the finite element predictions, it can be stated that mechanical deformation of the bulk metallic glass can be described well by both Mohr–Coulomb and Drucker–Prager constitutive criteria. Using these criteria, the extent of material pile-up observed around the indenter was also estimated very well.

This paper investigates the effect of porous microstructure on the necking formability of ductile sheets subjected to dynamic in-plane stretching. We have developed an original approach in which finite element calculations which include actual void distributions obtained from additively manufactured materials are compared with simulations in which the specimen is modeled with the Gurson–Tvergaard continuum plasticity theory (Gurson, 1977; Tvergaard, 1982) which considers porosity as an internal state variable. A key point of this work is that in the calculations performed with the continuum model, the initial void volume fraction is spatially varied in the specimen according to the void distributions included in the simulations with the actual porous microstructure. The finite element computations have been carried out for different loading conditions, with biaxial strain ratios ranging from 0 (plane strain) to 0.75 (biaxial tension) and loading rates varying between 10000 s −1 and 60000 s −1. We have shown that for the specific porous microstructures considered, the necking forming limits obtained with the Gurson–Tvergaard continuum model are in qualitative agreement with the results obtained with the calculations which include the actual void distributions, the quantitative differences for the necking strains being generally less than ≈ 25% (the calculations with actual voids systematically predict greater necking strains). In addition, the spatial distribution of necks formed in the sheets at large strains is very similar for the actual porosity and the homogenized porosity models. The obtained results demonstrate that the voids promote plastic localization, acting as preferential sites for the nucleation of fast growing necks. Moreover, the simulations have provided individualized correlations between void volume fraction, maximum void size and necking formability, and highlighted the influence of the heterogeneity of the spatial distribution of porosity on plastic localization. The research leading to these results has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme. Project PURPOSE, grant agreement 758056. J. C. Nieto-Fuentes acknowledges support from the CONEX-Plus programme funded by Universidad Carlos III de Madrid and the European Union's Horizon 2020 research and innovation programme, under the Marie Sklodowska-Curie grant agreement 801538. J. A. Rodríguez-Martínez acknowledges the financial support provided by the Spanish Ministry of Science and Innovation under the programme Proyectos I+D Excelencia 2017. Project APPLIED, DPI2017-88608-P.

A simulation strategy based in the finite element model was developed to model the corrosion and mechanical properties of biodegradable Mg scaffolds manufactured by laser power bed fusion after immersion in simulated body fluid. Corrosion was simulated through a phenomenological, diffusion-based model which can take into account pitting. The elements in which the concentration of Mg was below a certain threshold (representative of the formation of Mg(OH)2) after the corrosion simulation were deleted for the mechanical simulations, in which Mg was assumed to behave as an isotropic, elastic-perfectly plastic solid and fracture was introduced through a ductile failure model. The parameters of the models were obtained from previous experimental results and the numerical predictions of the strength and fracture mechanisms of WE43 Mg alloy porous scaffolds in the as-printed condition and after immersion in simulated body fluid were in good agreement with the experimental results. Thus, the simulation strategy is able to assess the effect of corrosion on the mechanical behavior of biodegradable scaffolds, which is critical for design of biodegradable scaffolds for biomedical applications.