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Void growth in ductile materials with realistic porous microstructures
(Elsevier, 2023-08-01) Ambikadevi Rajasekharan Nair, Vishnu; Vadillo Martin, Guadalupe; Rodríguez-Martínez, José A.; Rodriguez Martinez, Guillermo; Comunidad de Madrid; European Commission; Ministerio de Economía y Competitividad (España)
In this paper, we have investigated void growth in von Mises materials which contain realistic porous microstructures. For that purpose, we have performed finite element calculations of cubic unit-cells which are subjected to periodic boundary conditions and include porosity distributions representative of three additively manufactured metals. The initial void volume fraction in the calculations varies between 0.00564% and 1.75%, the number of actual voids between 14 and 5715, and the pores size from 2.3mum to 110mum. Several tests with different void sizes and positions have been generated for each of the three porous microstructures considered, and for each test we have performed several realizations with different spatial arrangement of the voids. The simulations have been carried out with random spatial distributions of pores and with clusters of the same size and different void densities. The macroscopic stress state in the unit-cell is controlled by prescribing constant triaxiality and Lode parameter throughout the loading. Calculations performed exchanging the loading directions for a given distribution of void sizes and positions have shown that the porous microstructure makes the macroscopic strain softening of the unit-cell (slightly) anisotropic. Moreover, the results obtained with the realistic porous microstructures have been compared with unit-cell calculations having an equivalent single central pore, and with calculations in which the material behavior is modeled with Gurson plasticity. It has been shown that both initial void volume fraction and spatial and size distribution of voids affect the macroscopic response of the porous aggregate and the void volume fraction evolution. Moreover, the calculations with random spatial distribution of voids have brought out that different tests of the same microstructure carry significant variations to the effective behavior of the porous aggregate, and that the interaction between neighboring pores dictates the volume evolution of individual voids, especially at higher macroscopic triaxiality. The calculations with clusters have shown that pores clustering promotes localization/coalescence due to increased interaction between the voids, which results in an increased growth rate of voids in clusters with large number of pores. Moreover, the results for the evolution of the distribution of plastic strains in the unit-cell have provided quantitative indications of the role of porous microstructure on the development of heterogeneous plastic strain fields which promote macroscopic strain softening. Namely, the accelerated growth rate of the plastic strains near the voids which indicates the onset of localization/coalescence has been shown to occur earlier as the number of voids in the microstructure increases.
High-velocity impact fragmentation of additively-manufactured metallic tubes
(Elsevier, 2023-05-01) Nieto Fuentes, Juan Carlos; Espinoza, J.; Sket, F.; RodrÍguez MartÍnez, José Antonio; European Commission; Universidad Carlos III de Madrid
In this paper, we have developed and demonstrated a novel high-velocity impact experiment to study dynamic fragmentation of additively-manufactured metals. The experiment consists of a light-gas gun that fires a conical nosed cylindrical projectile, that impacts axially on a thin-walled cylindrical tube fabricated by 3D printing. The diameter of the cylindrical part of the projectile is approximately twice greater than the inner diameter of the cylindrical target, which is expanded as the projectile moves forward, and eventually breaks into fragments. The experiments have been performed for impact velocities ranging from ≈ 180 m∕s to ≈ 390 m∕s, leading to strain rates in the cylindrical target that vary between ≈ 9000 s-1 and ≈ 23500 s−1. The cylindrical samples tested are printed by Selective Laser Melting out of aluminum alloy AlSi10Mg, using two printing qualities, with two different outer diameters, 12 mm and 14 mm, and two different wall thicknesses, 1 mm and 2 mm. A salient feature of this work is that we have characterized by X-ray tomography the porous microstructure of selected specimens before testing. Three-dimensional analysis of the tomograms has shown that the initial void volume fraction of the printed cylinders varies between 1.9% and 6.1%, and the maximum equivalent diameter of the 10 largest pores ranges from 143 μm to 216 μm, for the two different printing conditions. Two high-speed cameras have been used to film the experiments and thus to obtain time-resolved information on the mechanics of formation and propagation of fractures. Moreover, fragments ejected from the samples have been recovered, sized, weighted and analyzed using X-ray tomography, so that we have obtained indications on the effect of porous microstructure, specimen dimensions and loading velocity on the number and distribution of fragment sizes. To the authors’ knowledge, this is the first paper (i) providing a systematic experimental study (34 impact tests) on the fragmentation behavior of printed specimens, and (ii) including 3D reconstructions of dynamic cracks in porous additively-manufactured materials.
Characterization of low temperature high voltage axial insulator breaks for the ITER cryogenic supply line
(IOP Publishing Limited, 2017-07-09) Fernández Pisón, María del Pilar; Sgobba, Stefano; Avilés Santillana, Ignacio; Langeslag, S.A.E; Su, M.; Piccin, R.; Journeaux, J.Y.; Laurenti, A.; Pan, W.
Cable-in-conduit conductors of the ITER magnet system are directly cooled by supercritical helium. Insulation breaks are required in the liquid helium feed pipes to isolate the high voltage system of the magnet windings from the electrically grounded helium coolant supply line. They are submitted to high voltages and significant internal helium pressure and will experience mechanical forces resulting from differential thermal contraction and electro-mechanical loads. Insulation breaks consist essentially of stainless steel tubes overwrapped by an outer glass - fiber reinforced composite and bonded to an inner composite tube at each end of the stainless steel fittings. For some types of insulator breaks Glass - Kapton - Glass insulation layers are interleaved in the outer composite. Following an extensive mechanical testing campaign at cryogenic temperature combined with leak tightness tests, the present paper investigates through non-destructive and destructive techniques the physical and microstructural characteristics of the low temperature high voltage insulation breaks and of their individual components, thus allowing to correlate the structure and properties of the constituents to their overall performance. For all the tests performed, consistent and reproducible results were obtained within the range of the strict acceptance criteria defined for safe operation of the insulation breaks.
Improvement of wear resistance of low-cost powder metallurgy beta-titanium alloys for biomedical applications
(Elsevier, 2022-03-25) Chirico, C.; Vaz-Romero, Álvaro; Gordo Odériz, Elena; Tsipas, Sophia Alexandra; Comunidad de Madrid; Ministerio de Economía y Competitividad (España)
Low wear resistance and the relative high Young's modulus reduce the lifetime of the current biomedical Ti alloys for orthopaedic applications. In this study, two novel low-cost beta-Ti alloys (Ti-5Fe-25Nb and Tisingle bond40Nb in wt%), with reduced elastic modulus, are produced by powder metallurgy route, starting from TiH2 powder. In order to increase their wear resistance, two strategies are proposed: 1) addition of 5 vol% of TiN reinforcement particles and 2) gas nitriding surface treatment to produce a TiN coating. Wear resistance was evaluated by dry sliding reciprocating wear tests against alumina as counter material. Dry sliding tests were performed under unlubricated conditions, applying 10 N and 20 N load. Gas nitrided samples exhibit higher hardness than base alloys, while maintaining low elastic modulus. Both modification techniques improve wear resistance. The highest wear reduction was obtained for gas nitrided samples, reaching a wear rate reduction between 86% and 43%, compared to untreated alloys at 10 N, and between 4% to 15% at 20 N.
High-speed infrared thermal measurements of impacted metallic solids
(Elsevier, 2020) Nieto Fuentes, Juan Carlos; Osovski, S.; Rittel, D.; European Commission
The methodology used to measure transient temperature changes in impacted solids, using high-speed infrared detectors, is presented and discussed thoroughly. The various steps leading to a reliable measurement, namely selection of the sensing device, calibration of the setup, interfacing with the impact apparatus (Kolsky bar), and data reduction are presented. The outcome of the above methodology is illustrated in terms of the Taylor-Quinney factor, a well-known measure of the efficiency of the thermomechanical conversion. Selection of infrared detectors. / Importance of the calibration procedure. / Determination of the Taylor-Quinney factor.
Modeling dynamic formability of porous ductile sheets subjected to biaxial stretching: Actual porosity versus homogenized porosity
(Elsevier, 2022-11) Nieto Fuentes, Juan Carlos; Jacques, N.; Marvi Mashhadi, Mohammad; N Souglo, Komi Espoir; Rodríguez-Martínez, José A.; European Commission; Ministerio de Ciencia e Innovación (España)
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.
Theoretical predictions of dynamic necking formability of ductile metallic sheets with evolving plastic anisotropy and tension-compression asymmetry
(Springer, 2022-07) Murlidhar, Anil Kumar; N Souglo, Komi Espoir; Hosseini, Navab; Rodríguez-Martínez, José A.; European Commission
In this paper, we have investigated necking formability of anisotropic and tension-compression asymmetric metallic sheets subjected to in-plane loading paths ranging from plane strain tension to near equibiaxial tension. For that purpose, we have used three different approaches: a linear stability analysis, a nonlinear two-zone model and unit-cell finite element calculations. We have considered three materials –AZ31-Mg alloy, high purity α-titanium and OFHC copper– whose mechanical behavior is described with an elastic-plastic constitutive model with yielding defined by the CPB06 criterion (15) which includes specific features to account for the evolution of plastic orthotropy and strength differential effect with accumulated plastic deformation (48). From a methodological standpoint, the main novelty of this paper with respect to the recent work of N’souglo et al. (42) –which investigated materials with yielding described by the orthotropic criterion of Hill (24)– is the extension of both stability analysis and nonlinear two-zone model to consider anisotropic and tension-compression asymmetric materials with distortional hardening. The results obtained with the stability analysis and the nonlinear two-zone model show reasonable qualitative and quantitative agreement with forming limit diagrams calculated with the finite element simulations, for the three materials considered, and for a wide range of loading rates varying from quasi-static loading up to 40000 s− 1, which makes apparent the capacity of the theoretical models to capture the mechanisms which control necking formability of metallic materials with complex plastic behavior. Special mention deserves the nonlinear two-zone model, as it does not need prior calibration –unlike the stability analysis– and it yields accurate predictions that rarely deviate more than 10% from the results obtained with the unit-cell calculations.
3D numerical simulations and microstructural modeling of anisotropic and tension compression asymmetric ductile materials
(Elsevier, 2022-12-01) Hashem Sharifi, Sarvnaz; Hosseini, Navab; Vadillo, Guadalupe; Ministerio de Ciencia e Innovación (España)
In the present work, we have analyzed the effect of anisotropy on void growth and stress-strain behavior for materials that exhibit remarkable tension-compression asymmetry (i.e., zirconium alloys). For that purpose, we have performed finite element simulations using a cubic 3D cell with a spherical void inside and subjected to periodic boundary conditions. Nonlinear kinematic constraints are also imposed as boundary conditions in order to maintain the values of macroscopic ratios constant during the whole loading history of the cell and account for a general (3D) stress state. The behavior of the matrix material is described by the CPB06 anisotropic criterion developed by Cazacu et al. (2006). The numerical results are compared to those considering 3D homogeneous (without void) cell with the same initial porosity as the voided one and governed by the anisotropic porous yield criterion developed by Stewart and Cazacu (2011). To investigate the influence of prescribed stress, strength differential parameter and strain hardening exponent on stress-strain behavior and void growth in the non-homogeneous (with void) and the homogeneous (without void) cells, we have used several stress ratios, three strength differential parameters and three strain hardening exponents. Finite element results obtained from different stress ratios show the strength differential parameter significantly affect void growth in both homogeneous and non-homogeneous cells. Moreover, comparison of two cells proves that both stress-strain behavior and porosity evolution are in good qualitative agreement for all three values of strength differential parameter. In contrast, as the value of strain hardening exponent increases, the agreement between results obtained from homogeneous and non-homogeneous cells is worse. An heuristic extension of the Stewart and Cazacu (2011)s model is proposed in this work in an attempt to improve the accuracy of the model
Shear band formation in porous thin-walled tubes subjected to dynamic torsion
(Elsevier, 2022-10-01) Ambikadevi Rajasekharan Nair, Vishnu; Nieto Fuentes, Juan Carlos; Rodríguez-Martínez, José A.; European Commission; Universidad Carlos III de Madrid
In this paper, we have performed 3D finite element calculations of thin-walled tubes subjected to dynamic twisting to investigate the effect of porous microstructure on the formation of shear localization bands under simple shear conditions. For that purpose, we have incorporated into the finite element model the porous microstructures of four different additive manufactured metals – aluminium alloy AlSi10Mg, stainless steel 316L, titanium alloy Ti6Al4V and Inconel 718 – for which the void volume fraction varies from ≈ 0.001% to ≈ 2 %, and the voids size between ≈ 6 μm and ≈ 110 μm (Marvi-Mashhadi et al., 2021). For each microstructure, we have created up to 10 realizations varying the spatial location of the voids and the distribution of voids size. The matrix material is elastic/plastic, with yielding defined by the von Mises yield criterion and associated flow rule. The yield stress evolution is considered to be dependent on strain, strain rate and temperature, with parameters corresponding to Titanium and HY-100 Steel, taken from Molinari (1997) and Batra and Kim (1990), respectively. Moreover, we have assumed the deformation process to be adiabatic. The calculations have been performed for shear strain rates ranging from 100 s−1 to 10000 s−1. To the authors’ knowledge, this is the first study ever that simulates dynamic torsion testing of porous materials with actual representation of voids, providing new results which bring to light the influence of porosity on dynamic shear banding under simple shearing. Namely, the numerical calculations have shown that both the location of the shear band and the critical strain leading to the shear band formation depend on the spatial and size distribution of the voids in the specimen, evidencing the influence of material defects on the localization pattern. Notably, the shear band nucleation strain decreases with both the void volume fraction in the specimen and the size of the voids, the size of the largest pore being the main microstructural feature controlling the loss of load carrying capacity of the specimen. In addition, we have carried out a parametric analysis varying the temperature and strain rate sensitivities of the material, and the loading rate. For the strain rates investigated, increasing the loading speed leads to a mild decrease of the shear strain leading to shear band formation, while the strain rate sensitivity is shown to stabilize material behavior and delay localization. Moreover, the numerical results have made apparent that for the hardening materials considered, thermal softening is essential to trigger the shear band formation, so that the porous microstructure alone does not lead to shear localization.
Secondary Phases Quantification and Fracture Toughness at Cryogenic Temperature of Austenitic Stainless Steel Welds for High-Field Superconducting Magnets
(2018-02-26) Avilés Santillana, Ignacio; Fernández Pisón, María del Pilar; Langeslag, Stefanie; Sgobba, Stefano; Lunt, Alexander; Boyer, Christelle; Ruiz Navas, Elisa María
The ITER magnet system is based on the "cable-inconduit" conductor concept, which consists of various types of stainless steel jackets filled with superconducting strands. The jackets provide high strength and fracture toughness to counteract the high stress imposed by, amongst others, electromagnetic loads at cryogenic temperature. Material properties of austenitic stainless steel at cryogenic temperature are known to some extent, but only partial information is available for their welds, particularly in combination with weld fillers envisaged for cryogenic service. When a full inspection of the welded components is not possible, it becomes of special interest an assessment of its fracture toughness under close-to-service conditions if a fracture mechanics' design approach is to be adopted. In absence of defects, brittle secondary phases are generally held responsible of the loss of ductility and toughness which is to be expected after postweld heat treatments. Their quantification becomes thus essential in order to explain the negative impact in fracture toughness after unavoidable thermal treatments. This paper investigates fracture toughness behavior at 7 K of AISI 316L and AISI 316LN tungsten inert gas welds using two fillers adapted to cryogenic service, EN 1.4453 and JK2LB. Additionally, the effect of such an aforementioned heat treatment, here the Nb3Sn reaction heat treatment (650 degrees for 200 h) on fracture toughness of the welds is evaluated. A correlation between the evolution of properties and the quantity of secondary phases as a result of the above treatment is provided.
A simple and computationally efficient stress integration scheme based on numerical approximation of the yield function gradients: Application to advanced yield criteria
(Elsevier, 2021-09-15) Hosseini, Navab; Rodríguez-Martínez, José A.; European Commission
In this paper, we have modified the stress integration scheme proposed by Choi and Yoon [1]; which is based on the numerical approximation of the yield function gradients, to implement in the finite element code ABAQUS three elastic isotropic, plastic anisotropic constitutive models with yielding described by Yld2004-18p [2], CPB06ex2 [3] and Yld2011-27p [4] criteria, respectively. We have developed both VUMAT and UMAT subroutines for the three constitutive models, and have carried out cylindrical cup deep drawing test simulations and calculations of dynamic necking localization under plane strain tension, using explicit and implicit analyses. An original feature of this paper is that these finite element simulations are systematically compared with additional calculations performed using (i) the numerical approximation scheme developed by Choi and Yoon [1]; and (ii) the analytical computation of the first and second order yield functions gradients. This comparison has shown that the numerical approximation of the yield function gradients proposed in this paper facilitates the implementation of the constitutive models, and in the case of the implicit analyses, it leads to a significant decrease of the computational time without impairing the accuracy of the finite element results. In addition, we have demonstrated that there is a critical loading rate below which the dynamic implicit analyses are computationally more efficient than the explicit calculations.
Novel high entropy alloys as binder in cermets: From design to sintering
(Elsevier, 2021-09) Prieto Muñoz, Estela María; Vaz-Romero, Álvaro; Gonzalez Julian, J.; Alvaredo Olmos, Paula; Comunidad de Madrid
In recent years a new group of alloys has emerged breaking with the classical alloying concepts of physical metallurgy, high entropy alloys (HEA). Their main characteristic is that these alloys present 4 or 5 main elements increasing the entropy of the system and favouring the formation of a single phase. The disordered solid solution leads to develop an alloy with improved properties, in particular high thermal stability, hardness and strength. These properties make this group of alloys attractive as potential candidates for alternative binders in hard materials. In this work, two new compositions have been designed with the aim of obtaining a single BCC phase, reducing the cost and minimizing the presence of critical elements using elements that can present good potential properties for a cermet and with low toxicity and price such as Al, Cr, Mo, Ni, Fe and Ti. The design has been made based on the composition calculation applying the HEA phase formation empirical rules from literature in combination with thermodynamic simulations by Calphad method. The viability of the compositions has been studied through the processing of the compositions by casting and the study of wettability and solubility at high temperature on the hard phase of TiCN. Once the chosen compositions have been validated as competitive binders, cermets have been consolidated by spark plasma sintering (SPS) and the influence of the compositions on the mechanical properties of the compound materials has been studied.
Size effects on the plastic shock formation in steady-state cavity expansion in porous ductile materials
(Elsevier, 2021-04) Dos Santos, Tiago; Rodriguez Martinez, Jose Antonio; European Commission
In this paper, we have studied the hypervelocity expansion of a spherical cavity in an infinite medium modeled with the extension of the porous plasticity criterion of Gurson (1977) developed by Chen and Yuan (2002) to account for plastic strain gradient induced size effects. Following the self-similar, steady-state solution derived by Cohen and Durban (2013) for size-independent porous materials, we have computed the critical cavity expansion velocity which leads to the emergence of plastic shock waves for a wide range of initial void volume fractions and different values of the length scale parameter that controls the effect of size. We have shown that size effects hinder the emergence of plastic shock waves, so that as the length scale parameter increases, the expansion velocity required for the plastic shock to be formed increases. In addition, while porosity favors the formation of plastic shocks, as shown by Cohen and Durban (2013), our results indicate that the effect of initial void volume fraction on plastic shock wave formation decreases for size-dependent materials.
The combined effect of size, inertia and porosity on the indentation response of ductile materials
(Elsevier, 2021-02) Santos, T. dos; Srivastava, Ankit; Rodríguez-Martínez, José A.; European Commission
Herein, we present a self-similar cavity expansion model that follows from the work of Cohen and Durban (2013b) to analyze the dynamic indentation response of elasto-plastic porous materials while accounting for the plastic strain gradient induced size effect. The incorporation of the plastic strain gradient induced size effect in the dynamic cavity expansion model for elasto-plastic porous materials is the key novelty of our model. The material hardness predicted using the cavity expansion model for a wide range of indentation depths and speeds is compared against the available experimental results for OFHC copper, for strain rates varying from 10−4 s−1 to 108 s−1. We note that despite several simplifying assumptions, the predictions of our cavity expansion model show a reasonable agreement with the experimentally measured material hardness over a wide range of indentation depths and speeds. In addition, we have also carried out parametric analysis to elucidate the specific roles of indentation speed, size effect and initial porosity, on the material hardness and cavitation fields that develop during the indentation process. In particular, our parametric analysis shows that there exists a critical value of the indentation speed beyond which the contribution of inertial effect becomes extremely important and the material hardness increases rapidly. While the influence of the initial porosity on the material hardness is found to increase with increasing indentation speed and decrease with increasing size effect.
Assessment of residual stresses in ITER CS helium inlet welds fatigue tested at cryogenic temperature
(IOP Publishing Ltd., 2019-06) Sgobba, Stefano; Avilés Santillana, Ignacio; Langeslag, Stefanie A. E.; Fernández Pisón, María del Pilar; Castillo Rivero, S.; Libeyre, P.; Jong, C.; Everitt, D.; Freudenberg, K.
The ITER Central Solenoid (CS) consists of six independent wound modules. The cooling of the cable-in-conduit conductor is assured by a forced flow of supercritical He at 4.5 K supplied by He inlets located at the innermost radius of the coil. The inlets consist of a racetrack-shaped boss welded to the outer conduit wall through a full penetration Tungsten Inert Gas (TIG) weld. They are critical structural elements submitted to severe cyclic stresses due to the electro-magnetic forces acting on the coils. The weld contour is shape-optimised and locally processed by Ultrasonic Shot Peening (USP), conferring large compressive residual stresses on a subsurface layer of several millimetres thickness to improve fatigue strength. The distribution of the residual stresses and the effect of USP on microstructure and mechanical properties is assessed, with reference to the results of a cryogenic fatigue test campaign, performed on peened and as-welded inlets for comparison.
Dynamics of necking and fracture in ductile porous materials
(ASME, 2019-12-23) Zheng, Xinzhu; N Souglo, Komi Espoir; Rodríguez-Martínez, José A.; Srivastava, Ankit; European Commission
The onset of necking in dynamically expanding ductile rings is delayed due to the stabilizing effect of inertia, and with increasing expansion velocity, both the number of necks incepted and the number of fragments increase. In general, neck retardation is expected to delay fragmentation as necking is often the precursor to fracture. However, in porous ductile materials, it is possible that fracture can occur without significant necking. Thus, the objective of this work is to unravel the complex interaction of initial porosity and inertia on the onset of necking and fracture. To this end, we have carried out a series of finite element calculations of unit cells with sinusoidal geometric perturbations and varying levels of initial porosity under a wide range of dynamic loading conditions. In the calculations, the material is modeled using a constitutive framework that includes many of the hardening and softening mechanisms that are characteristics of ductile metallic materials, such as strain hardening, strain rate hardening, thermal softening, and damage-induced softening. The contribution of the inertia effect on the loading process is evaluated through a dimensionless parameter that combines the effects of loading rate, material properties, and unit cell size. Our results show that low initial porosity levels favor necking before fracture, and high initial porosity levels favor fracture before necking, especially at high loading rates where inertia effects delay the onset of necking. The finite element results are also compared with the predictions of linear stability analysis of necking instabilities in porous ductile materials.
Dynamic cylindrical cavity expansion in orthotropic porous ductile materials
(Elsevier Ltd., 2019-10) Santos, T. dos; Vaz-Romero, Álvaro; Rodríguez-Martínez, José A.; European Commission
This paper investigates the steady-state elastoplastic fields induced by a pressurized cylindrical cavity expanding dynamically in an anisotropic porous medium. For that task, we have developed a theoretical model which: (i) incorporates into the formalism developed by Cohen and Durban [4] the effect of plastic anisotropy using the constitutive framework developed by Benzerga and Besson [1] and (ii) uses the artificial viscosity approach developed by Lew et al. [24] to capture the shock waves that emerge at high cavity expansion velocities. We have shown that while the development of the shock waves is hardly affected by the material anisotropy, the directionality of the plastic properties does have an effect on the elastoplastic fields that evolve near the cavity. The importance of this effect is strongly dependent on the cavity expansion velocity, the initial porosity and the strain hardening of the material. In addition, the theoretical model has been used in conjunction with the Recht and Ipson [33] formulas to assess the ballistic performance of porous anisotropic targets against high velocity perforation.
Crack propagation in TRIP assisted steels modeled by crystal plasticity and cohesive zone method
(Elsevier, 2018-08-01) Dakshinamurthy, Manjunath; Ma, Anxin
The influence of transformation induced plasticity (TRIP) on materials mechanical behaviours, as well as failure phenomena including crack propagation and phase boundary debonding in multiphase steels (e.g. dual phase steels, TRIP steels) are studied by using an advanced crystal plasticity finite element method. We have coupled the crystal plasticity model Ma and Hartmaier (2015), which explicitly considers elastic-plastic deformation of ferrite and austenite, austenite-martensite phase, with a cohesive zone model designed for crack propagation, to study the deformations of several representative microstructural volume elements (RVE). Results shows that, the transformation induced plasticity enhances materials strength and ductility, hinders crack propagation and influences interface debonding. Furthermore, the martensitic transformation kinetics in TRIP steels was found depending on the crystallographic orientation and the stress state of a retained austenite grain. The current simulation results helps to investigate and design multiphase steels with improved mechanical properties.
Nonlinear axisymmetric vibrations of a hyperelastic orthotropic cylinder
(Elsevier, 2018-03-01) Aranda Iglesias, Francisco Damián; Rodríguez-Martínez, José A.; Rubin, M. B.; Ministerio de Economía y Competitividad (España)
In this paper we investigate the large-amplitude axisymmetric free vibrations of an incompressible nonlinear elastic cylindrical structure. The material behavior is described as orthotropic and hyperelastic using the physically-based invariants proposed by Rubin and Jabareen (2007, 2010). The cylinder is modeled using the theory of a generalized Cosserat membrane, which allows for finite deformations that include displacements along the longitudinal axis of the structure. The bi-dimensional approach represents a significant contribution with respect to most works published in this field, which approach the problem at hand assuming plane strain conditions along the axis of the cylinder. We have carried out a systematic analysis of the parameters that govern the dynamic behavior of the structure, paying specific attention to those describing the orthotropy of the material and the dimensions of the cylinder. Using Poincare maps, we have shown that the motion of the structure can turn from periodic to quasi-periodic and chaotic as a function of the initial conditions, the elastic and kinetic energy supplied to the specimen, the dimensions of the cylinder and the degree of mechanical orthotropy of the material.
Nonlinear resonances of an idealized saccular aneurysm
(Elsevier, 2017-12-01) Aranda Iglesias, Francisco Damián; Ramón Lozano, Clara; Rodríguez-Martínez, José A.; Ministerio de Economía y Competitividad (España)
This paper investigates the occurrence of dynamic instabilities in idealized intracranial saccular aneurysms subjected to pulsatile blood flow and surrounded by cerebral spinal fluid. The problem has been approached extending the original 2D model of Shah and Humphrey (1999) to a 3D framework. The justification for using a 3D formulation arises from the works of Suzuki and Ohara (1978), MacDonald et al. (2000) and Costalat et al. (2011) who showed experimental evidences of intracranial aneurysms with a ratio between wall thickness and inner radius larger that 0.1. Two different material models have been used to describe the mechanical behaviour of the aneurysmal wall: Neo-Hookean and Mooney-Rivlin. To the authors' knowledge, for the first time in literature, the dynamic response of the aneurysm has been analysed using complete nonlinear resonance diagrams that have been obtained from a numerical procedure specifically designed for that purpose. Our numerical results show that, for a wide range of wall thicknesses and both constitutive models considered, the saccular aneurysms are dynamically stable within the range of frequencies associated to the normal heart rates, which confirms previous results of Shah and Humphrey (1999). On the other hand, our results also show that the geometric and material nonlinearities of the problem could bring closer than expected the resonance frequencies of the aneurysm to the frequencies of the pulsatile blood flow.

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