Elastoplastic solids subjected to dynamic tension: new experimental and computational insights

This Doctoral Thesis provides new insights into the mechanisms which control flow localization in elastoplastic solids subjected to dynamic tension. For that task, we have a developed a methodology which combines experiments and numerical calculations. Dynamic tension tests have been performed in a high-speed testing machine using specimens with six different gauge lengths, ranging from 20 �� to 140 ��, that have been tested within a wide spectrum of loading velocities from 1 �/� to 7.5 �/�. The experiments show that variations in the applied velocity and the gauge length of the samples lead to the systematic motion of the fracture location along the specimen. A key outcome is that we have provided experimental evidences of the deterministic nature of the flow localization in dynamic tensile specimens. Finite element calculations have been conducted in ABAQUS/Explicit in order to complement our experimental findings. The finite elements predict, in agreement with the experiments, the interplay between fracture location, impact velocity and gauge length. Moreover, we have explored the role played by initial and boundary conditions in plastic flow localization. A salient feature is that we have demonstrated that the intervention of stress waves within the specimen is a limiting factor for the sample ductility. On the one hand we have observed that the strain to failure, instead of being a material property, is strongly dependent on the specimen size. On the other hand, we have shown that the topology of the localization pattern is closely connected to the post-uniform elongation of the specimen. Finite difference calculations have been conducted in MATLAB in order to rationalize the experimental and finite element outcomes. For that task, we have developed a simple one-dimensional model within a finite deformation framework. The key point of our finite difference computations is that, unlike the finite element calculations, we solved the kinematics, and thus obtained a complete control of the problem. We show that the intervention of wave propagation phenomena within the specimen is responsible for the interplay between flow localization, impact velocity and gauge length. Moreover, we have explored the role of selected material properties in the kinetics of flow localization. A key outcome is that we have shown that material flaws (may) play a secondary role within the mechanisms which govern plastic localization in dynamic tensile specimens. All in all, we have developed a comprehensive and innovative research to establish: (1) the deterministic nature of flow localization and (2) the material properties and the initial and boundary conditions which control the process at handAgradecer al Ministerio de Ciencia e Innovación de España por el soporte financiero necesario para la consecución de este trabajo a través del proyecto DPI/2011-24068.Programa Oficial de Doctorado en Ingeniería Mecánica y de Organización IndustrialPresidente: José Fernández Sáez.- Secretario: Sebastián Mercier.- Vocal: Daniel Maurice Ritte

On the interplay between material flaws and dynamic necking

In this paper we investigate the interplay between material defects and flow localization in elastoplastic bars subjected to dynamic tension. For that task, we have developed a 10 finite difference scheme within a large deformation framework in which the material is modelled using rate-dependent J(2) plasticity. A perturbation of the initial yield stress is introduced in each node of the finite difference mesh to model localized material flaws. Numerical computations are carried out within a wide spectrum of strain rates ranging from 500 s(-1) to 2500 s(-1). On the one hand, our calculations reveal the effect of the material defects in the necking process. On the other hand, our results show that the necking inception, instead of being a random type process, is the deterministic result of the interplay between the mechanical behaviour of the material and the boundary conditions. This conclusion agrees with the experimental evidence reported by Rittel et al. [1] and Rotbaum et al. [2].The authors are indebted to the Ministerio de Economía y Competitividad de España (Project DPI2014-57989-P) for the financial support received which allowed conducting 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

The deterministic nature of the fracture location in the dynamic tensile testing of steel sheets

This paper investigates the key mechanisms which determine the fracture location in the dynamic tensile testing of steel sheets. For that purpose we have conducted experiments and finite element simulations. Experiments have been performed using samples with six different gauge lengths, ranging from 20 mm to 140 mm, that have been tested within a wide spectrum of loading velocities, ranging from 1 m/s to 7.5 m/s. Three are the key outcomes derived from the tests: (1) for a given gauge length and applied velocity, the repeatability in the failure location is extremely high, (2) there is a strong interplay between applied velocity, gauge length and fracture location and (3) multiple, and largely regular, localization patterns have been observed in a significant number of the experiments performed using the samples with the shorter gauge lengths. Our experimental findings are explained using the finite element simulations. On the one hand, we have shown that variations in the applied velocity and the gauge length alter the processes of reflection and interaction of waves taking place in the sample during the test, which leads to the systematic motion of the plastic localization along the gauge (as experimentally observed). On the other hand, we have detected that the emergence of multiple localization patterns requires short and equilibrated specimens with uniform stress and strain distributions along the gauge. We conclude that the experimental and numerical results presented in this paper show that, in the absence of significant material and/or geometrical defects, the location of plastic strain localization in the dynamic tensile test is deterministic.The authors are indebted to the Ministerio de Ciencia e Innovación de España (Project DPI/2011-24068) for the financial support received which allowed conducting this work.Publicad

Necking evolution in dynamically stretched bars: new experimental and computational insights

This paper presents new results on dynamic neck evolution in steel bars of varying diameters. Dynamic tensile tests were carried out in a Kolsky apparatus using cylindrical steel specimens with various cross-section diameters ranging from 1.5 mm to 4 mm. A high speed digital camera was used to record the deformation of the specimen during the loading process. Video recording of the tests enabled accurate experimental measurements of the necking evolution, specifically its growth rate as a function of the diameter. The experiments show that increasing the specimen cross-section slows down the neck development. This behavior has been further investigated using two different kinds of numerical calculations: (1) axisymmetric finite element simulations and (2) one-dimensional finite difference computations.The authors of the University Carlos III of Madrid are indebted to the Ministerio de Economía y Competitividad de España (Projects DPI2014 57989-P and EUIN2015-62556) for the financial support which permitted to conduct part of this work.Publicad

On the relation between shape imperfections of a specimen and necking growth rate under dynamic conditions

In this work, the growth rate of necks formed in dynamically loaded tensile steel samples is investigated. For that purpose, a combined experimental-numerical approach, in which the experimental results are systematically compared with finite element calculations, has been developed. The specimens have a machined sinusoidal geometrical imperfection that covers the whole gauge, introducing a characteristic wavelength in the samples. For a given cross-section diameter, specimens with 6 different gauge lengths (i.e. 6 different specimen wavelengths) were tested. Using a high-speed camera, we measured the time evolution of the radial contraction of the central section of the samples (central section of the neck), thus obtaining the growth rate of the necks. The experiments show that the speed of growth of the necks increases non-linearly with the specimen wavelength (concave-downward shape) until saturation is reached for the longest tested specimens. Numerical simulations performed for the strain rates attained in the experiments (from 900 s−1 to 2100 s−1) confirm this trend and demonstrate that the damping of short specimen wavelengths is caused by stress multiaxiality effects. Numerical simulations performed for strain rates greater than those attained in the experiments (above 7500 s−1) show that long specimen wavelengths become damped by inertia effects at sufficiently high strain rates. For strain rates greater than 7500 s−1, the maximum growth rate of the neck corresponds to an intermediate specimen wavelength defined by the joint action of stress multiaxiality and inertia on damping short and long wavelengths, respectively. Altogether, our experimental and numerical results suggest the existence of a specimen wavelength that, when inertia effects become important, determines the maximum growth rate of dynamic necks, in agreement with the predictions of the dynamic stability analyses developed by Molinari and co-workers (Fressengeas and Molinari, 1985, 1994; Mercier and Molinari, 2003, 2004).AVR and JARM are indebted to the Ministerio de Economía y Competitividad de España (Projects EUIN2015-62556 and DPI2014-57989-P) for the financial support which permitted to conduct part of this work. The research leading to these results has received funding from the European Union’s Horizon2020 Programme (Excellent Science, Marie-Sklodowska-Curie Actions) under REA grant agreement 675602 (Project OUTCOME)

Multiple necking pattern in nonlinear elastic bars subjected to dynamic stretching: the role of defects and inertia

In this paper we explore the inception and development of multiple necks in incompressible nonlinear elastic bars subjected to dynamic stretching. The goal is to elucidate the role played by a spatial-localized defect of the strain rate field in the necking pattern that emerges in the bars at large strains. For that task, we have used two different approaches: (1) finite element simulations and (2) linear stability analyses. The finite element simulations have revealed that, while the defect of the strain rate field speeds up the development of the necking pattern in the late stages of the localization process, the characteristic (average) neck spacing is largely independent of the defect within a wide range of defect amplitudes. The numerical results have been rationalized with the linear stability analyses, which enabled to explain the average spacing characterizing the necking pattern at high strain rates. Moreover, the numerical calculations have also shown that, due to inertia effects, the core of the localization process occurs during the post-uniform deformation regime of the bar, at strains larger than the one based on the Considère criterion. This phenomenon of neck retardation is shown to have a meaningful influence on the necking pattern.AVR and JARM are indebted to the Ministerio de Economía y Competitividad de España (Projects EUIN2015-62556 and DPI2014- 57989-P ) for the financial support which permitted to conduct part of this work. AM and JARM acknowledge the support by the French State through the program Investment in the future operated by the National Research Agency (ANR) and referenced by ANR-11-LABX- 0 0 08-01 (LabEx DAMAS). The research leading to these results has received funding from the European Union’s Horizon2020 Programme (Excellent Sci- ence, Marie Sklodowska-Curie Actions) under REA grant agreement 675602 (Project OUTCOME)

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

Understanding the links between the composition-processing-properties in new formulations of heas sintered by sps

This work presents two new compositions of high entropy alloys (HEAs) that were designed with the aim of obtaining a body-centered cubic (BCC) phase with high hardness values and a moderate density. Sintering was performed using Spark Plasma Sintering (SPS) with different heating rates to determine the influence of the processing parameters on the phase formation. The microstructural study revealed that the presence of Ni in the composition promoted phase separation, and the mechanical study confirmed a clear influence on the mechanical properties of both the composition and heating rate. The combination of microscopy with compression and nanoindentation tests at room and high temperature made it possible to advance our understanding of the relationships between the composition, processing, and properties of this emerging group of alloys

Outcomes from elective colorectal cancer surgery during the SARS-CoV-2 pandemic

This study aimed to describe the change in surgical practice and the impact of SARS-CoV-2 on mortality after surgical resection of colorectal cancer during the initial phases of the SARS-CoV-2 pandemic