
Hervé CouqueNEXTER Group | NEXTER · Faculty of Engineering
Hervé Couque
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Introduction
Publications
Publications (66)
The work engaged for many years between the ICB laboratory and the Nexter Munitions company has clearly shown the interest of combining a high-energy mechanical milling of the metallic powders followed by a SPS sintering. Indeed, commercial powders are mechanically activated (i.e. successive actions of fracture | cold welding of the powder particle...
The harmonic structure is a recently introduced concept paving the way for engineering metallic materials to achieve excellent mechanical performance. They consist of soft coarse-grained regions (Core) that are three-dimensionally surrounded by a connected network of hard ultra-fine grained regions (Shell). The interaction in these Core–Shell regio...
The influence of strain rate on the mechanical response of two different nanostructured pure coppers was investigated under uniaxial compression. The first nanostructured copper was elaborated by powder metallurgy using the Spark Plasma Sintering (SPS) process. The second nanostructured copper was elaborated by Severe Plastic Deformation (SPD). Con...
Three-point bend fracture tests have been conducted at different loading rates with a quadratic martensitic steel. The failure energy has been found to increase with loading rate. To get insights in this increase a numerical investigation has been undertaken with different strategies using ABAQUS and IMPETUS softwares in order to address quasi-stat...
Starting from micrometric pure nickel powder, high-energy ball milling process allows to produce nanostructured agglomerate powders associated with both a decrease of crystallite size and an increase of structural defects. With the help of a fast sintering process, such as the Spark Plasma Sintering technique, dense nanostructured materials can be...
Due to molybdenum’s Body-Centered Cubic (BCC) crystalline structure, its ductile–brittle transition temperature is sensitive to shaping, purity and microstructure. Dense molybdenum parts are usually shaped by the powder metallurgy process. The aim of this work concerns the spark plasma sintering of high-purity powders prepared by inductively couple...
This paper presents a comprehensive study of a direct impact compression loading on an Electrolytic Tough-Pitch copper (Cu-ETP). The aim of the study is to provide reliable experimental data in
this dynamic loading using high-speed video camera records and real time projectile deceleration profiles.
Test set-up has been optimized to ensure an eff...
Failure processes encountered in the Taylor impact specimen are of two kinds, Mode II cracks initiating within adiabatic shear bands and tensile cracks. Because friction occurring with the single Taylor configuration influences the failure processes, it is necessary to use the symmetric loading procedure providing frictionless interfaces. Numerical...
Dynamic fracture strength of tungsten alloys generated with a round bar Charpy specimen impacted at velocity ranging from 25 to 160 m/s has been reported to increase with loading rate. For as sintered alloys, the Charpy energy reached 1150 J/cm² at 145 m/s, which is about 4 times the conventional Charpy energy obtained at 5 m/s. The analysis of the...
Tensile properties and fracture characteristics of high purity polycrystalline nickel were investigated using micro-tensile specimens. The material response to the applied load was found to be sensitive to both geometry and sample size. The post-tests examination of the fracture surfaces revealed that the mechanisms leading to failure were associat...
Spark Plasma Sintering is a non-conventional process of the powder metallurgy field which uses a high electrical current to rapidly produce fully dense materials. In the present paper, a thermal-electrical-mechanical model developed on ABAQUS Software is proposed to simulate the densification of a nickel disk. A compaction model, studied in [Wolff,...
In this study, a combined experimental and numerical procedure that based on extended analysis of stresses, strains, and damage of the specimen notch region, was proposed to investigate high-strength steel and pure nickel metals. Miniaturized notched tensile specimens with different notch radii were used to generate various levels of triaxial stres...
The microstructure and the mechanical behavior of equimolar Ti20Hf20Zr20Ta20Nb20 high-entropy alloy in a wide range of initial strain rates between ∼ 10- 3 s- 1 and ~ 3.4 × 103 s- 1 were studied. A significant increment in the yield strength with increasing strain rate was observed. The yield strength at ~ 3.4 × 103 s- 1 was about 40% higher than t...
Taylor tests are a mean to investigate the dynamic plastic and failure behaviour of metals under compression. By taking in account the strengthening occurring at high strain rates, the Taylor final diameter of a pure nickel impacted at 453 m/s have been numerically reproduced by 13%. Through post-mortem observations of the specimen impacted at 453...
The influence of strain rate over domains involving the thermal activation and the viscous drag behaviour of the dislocations is discussed. While it is recognized that the Koslky-Hopkinson technique or split Hopkinson pressure bar technique can generate data up to the upper strain-rate limit of the thermal-activated regime, it is necessary to use a...
The influence of strain rate over domains involving the thermal activation and the viscous drag behavior of the dislocations has been investigated for a series of face centered cubic metals. The effect of grain size on the strain rate threshold delimitating the two domains in compression was examined with grains sizes ranging from 0.1 to 100 μm for...
Spark plasma sintering of a blend of powders with nanometer and micrometer sized particles yielded to a composite-like nickel microstructure consisting of ultrafine-grained (UFG) and coarse-grained (CG) volumes with the fractions of 36% and 64%, respectively. Microstructure evolution and nanohardness distributions of specimens submitted to impact l...
Previous studies have demonstrated that it is possible to compensate for the absence of Co in a W-Ni-Fe liquid sintered composite alloy under industrial conditions by appropriate control of the process parameters, to achieve mechanical properties close to those of Co containing alloys. The present investigation offers an optimised description of th...
The deformation mechanisms in ultrafine-grained hexagonal close packed Zn were investigated at different strain rates and temperatures. The influence of grain size on the deformation mechanisms was revealed by comparing the results obtained on ultrafine-grained and coarse-grained Zn. It was found that for coarse-grained Zn at room temperature and s...
In this contribution, the effect associated with stress triaxiality on ductile damage evolution in high purity nickel has been investigated from both experimental and theoretical points of view. Tensile tests on smooth and notched round bar specimens were performed to calibrate the fracture strain in a wide range of stress triaxiality. The capabili...
An investigation was performed to evaluate the potential of the spark plasma sintering process in producing dense nanostructured materials. Microstructured and nanostructured nickel was sintered by SPS starting from nickel powder (APS 3–7 m) in the as-received state and after a mechanical activation using a high-energy ball mill. First, a sintering...
Spark plasma sintering was used to process bulk ultrafine-grained (average grain size of similar to 250 nm) samples from high purity nanocrystalline Zn powder. The microstructure of the consolidated samples was investigated and the mechanical behaviour was characterised by means of quasistatic and dynamic compression tests at room temperature and c...
The influence of the dynamic loading rate on the tensile failure
properties of a series of tungsten alloys and an austenitic stainless
steel were investigated by evaluating the flexural strength with
unnotched and notched Charpy specimens. These data were generated with a
newly developed Hopkinson pressure bar technique. The technique consists
in i...
Numerous micromechanical models have been proposed to describe the behavior of porous materials. Cocks, 1989, Ponte-Castaneda, 1991, Duva and Crow, 1992 and Sofronis and McMeeking, 1992 have proposed different elliptic strain rate potentials which are mainly valid for small porosity (below 10% in general). In the present contribution, the domain of...
Polycrystalline Zn with an average grain size of about 300 gm was deformed by direct impact Hopkinson pressure bar at a velocity of 29 m/s. An inhomogeneous grain structure was found consisting of a center region having large average grain size of 20 gm surrounded by a fine-grained rim with an average grain size of 6 gm. Transmission electron micro...
The tensile failure behaviour of tungsten alloys is analyzed based on data available in the literature and on Charpy failure energy data generated with alloys containing 60 and 91% of tungsten. These data were generated with round bar specimens using a newly developed Hopkinson pressure bars technique. The technique consists in impacting with a str...
High purity electrolytic nickel (99.99%) samples deformed dynamically in compression using a direct impact Hopkison pressure bar test at the velocities of 10.9, 28.2 and 70.6 m s(-1) were investigated. The dislocation density increased with increasing the impact velocity up to 28.2 m s(-1) resulting in an increase of nanohardness and quasi-static c...
The mechanical and microstructural characteristics of high purity nickel processed by electrodeposition tested in compression up to a dynamic strain rate of 1.5 x 105 s-1 using a direct impact Hopkinson pressure bar technique have been analyzed. The nickel exhibits thermal activated strain rate sensitivity up to about 103 s-1. At higher rates, a sh...
The absence of structural ordering in bulk metallic glasses (BMGs) leads to plastic deformation controlled by shear bands initiation, propagation, and interactions which differ most likely when quasi-static or dynamic loads are applied. Under quasi-static loading and high hydrostatic stress, the shear band initiation and propagation process is stab...
Dynamic compression failure properties of swaged 90 % tungsten alloys with strength ranging from 1100to 1500 MPa were investigated using the symmetric impact Taylor test. To study the influence of rod size on failure initiation, the tests were conducted with Taylor specimens of different diameter ranging from 6 to 23 mm and diiferent length ranging...
The dynamic compression failure and ballistic penetration characteristics of conventional tungsten alloys similar in strength were investigated. Dynamic compression failure properties were generated with a symmetric Taylor test technique and penetration characteristics were obtained with 44 mm kinetic penetrators against an 300 HB hardness steel ta...
Adiabatic shear banding is considered as one of failure modes. Few numerical studies were devoted to the investigation of the initiation and propagation of a ductile crack within an adiabatic shear band. In this paper, adiabatic shear banding is clearly separated from subsequent fracture. Both phenomena are handled separately in the present numeric...
Because numerical simulations with classical Johnson-Cook formulation of explosively formed projectiles induce non representative projectiles, a modified Johnson-Cook model taking into account the thermally activated and viscous regimes as well as the notion of saturation stress in the 10 4 s -1 regime has been developed. Using both models, numeric...
A modified Johnson-Cook model is proposed to take into account the thermally activated and viscous regimes that can be easily integrated in numerical codes. The model was calibrated using experimental data generated at temperatures ranging from 23 to 1000circC and strain rates ranging from 0.002 to 3 × 104 s-1 and validated through numerical simula...
Adiabatic shear banding, ASB, of conventional tungsten alloys involves strain localisation of the low strength matrix followed by large deformation of the high strength tungsten particles. To get insights of this failure process, investigations have been conducted with 90% tungsten alloys along with their associated matrix or gamma phases. These al...
An investigation was conducted to examine adiabatic shear band formation under a pressure of 1.5 GPa with a modified version of the hat specimen. Using tungsten alloy data generated with a symmetric Taylor test, the influence of pressure on the critical strain associated with adiabatic shear banding formation was found to increase with the increase...
The present investigation examines the dynamic plastic deformation of a moderate-strength high-ductility austenitic steel at strain rates ranging from 103 to 104 s-1 using Taylor tests. Single and symmetric Taylor testing was employed to generate large plastic deformation under different stress states. The dynamic plastic deformation was analyzed t...
Symmetric Taylor testing procedures to evaluate dynamic mechanical properties under uniaxial compression of a variety of materials including aluminum alloys, tungsten alloys and steels are described. For material strength greater than 800 MPa, symmetric Taylor testing involved Teflon sabots deforming along with the Taylor specimens. For low strengt...
During the 30-year lifetime of the Space Station, NASA is concerned that a large piece of orbital debris could strike one of the inhabited or laboratory modules. The modules are basically cylindrical pressure vessels, 4.3 meters in diameter and 9.1 meters long, made of Al 2219-T87. There is a potential for unstable crack growth (“unzipping”) in the...
Local loading conditions associated with shear banding formation in laboratory tungsten alloy specimens are examined. The specimens consist of compression specimens of large length to diameter ratio made of 91W-6Ni-3Co swaged tungsten alloys tested with the Hopkinson pressure bar and symmetric Taylor techniques. Experimental and numerical results p...
Because of the inevitable need for procedures that could be used by gas distribution engineers to preclude the occurrence of large scale rupture in the larger diameter and higher pressure polyethylene (PE) gas piping systems anticipated in the future, research was undertaken to develop and validate an appropriate methodology. The approach that was...
L'effet de la vitesse de chargement sur les propriétés de ténacité en contrainte plane de plaques de 3.2 mm d'épaisseur en alliage d'aluminium 2219-T87 a été examiné à l'aide d'échantillons de traction à fissure centrale. Des courbes J de résistance à la rupture ont été obtenues à des vitesses de déplacements de 10-6 et 5.7 m s-1. L'essai dynamique...
La réponse en compression d'un cuivre, d'un acier et d'alliages de tungstènes a été obtenue à l'aide de barres d'Hopkinson (split Hopkinson pressure bar) pour des vitesses de déformation variant de 3 x 103 à 7 x 103 s-1. L'hypothèse de déformation à vitesse constante a été examinée à l'aide de jauges de contrainte placées sur l'échantillon. Au débu...
The effects of specimen planar size and dynamic loading rate on the dynamic initiation and propagation toughness properties of a high-strength, rate-insensitive 4340 steel at room temperature were investigated. Simultaneous dynamic initiation and propagation testing of prefatigue-cracked, compact specimens was accomplished with a coupled pressure b...
Depleted uranium (DU) is currently the top-performing kinetic energy penetrator, followed closely (but not matched) by several tungsten heavy alloy variants. Because of the adverse environmental impact associated with DU, it is extremely desirable to determine a means of upgrading the ballistic performance of heavy alloys to a level at least compar...
The influence of loading rate and microstructure on the tensile and compressive failure properties of three microstructurally dissimilar tungsten alloys has been investigated. Dynamic tensile fracture properties were characterized through fracture toughness tests performed at a stress intensity loading rate of $10^6$ MPa $\sqrt{{\rm m}}$ s$^{-1}$,...
The upper-shelf dynamic fracture properties of a low-strength high-toughness steel, A533 Grade B Class 1, have been investigated with a coupled pressure bars techniques, using small compact specimens. Toughness data were generated from numerical simulations with a dynamic viscoplastic finite-element fracture code. The dynamic ductile fracture proce...
The tensile strength and associated failure micromechanisms have been characterized for a SiC fibrereinforced ceramic matrix
composite subject to strain rates approaching 1000 s−1. It is found that behaviour under such conditions is not described by the current matrix fracture/fibre pull-out models.
This is a consequence of the rapid and extreme fr...
The use of coupled pressure bars (CPB) to induced dynamic fracture in tough materials using small specimens is investigated. CPB experiments were performed with a nuclear pressure vessel steel. A533 Grade B Class 1, over the temperature range 37 to 100°C. The dynamic fracture initiation toughness at a stress intensity loading rate. K1, of 2 × 106 M...
The many practical situations in which the initiation of rapid crack propagation cannot be absolutely precluded and where the consequences of a large scale fracture would be catastrophic mandate the development and use of crack arrest fracture mechanics technology. Most currently applied procedures utilize linear elastic analyses and corresponding...
There are many practical situations in which the initiation of rapid crack propagation cannot be absolutely precluded and where the consequences of a large scale fracture would be catastrophic. For these cases crack arrest fracture mechanics technology is needed. The currently available procedures utilize elastodynamic analyses and material fractur...
An investigation was conducted into the effects of temperature, loading rate, and various micro-structural parameters on the initiation of plane strain fracture of a plain carbon AISI 1020 steel. Ferrite and prior austenite grain sizes were chosen as the principal microstructural features to be in-vestigated. The microstructural variations were acc...
Une méthode expérimentale est introduite pour la détermination de la ténacité dynamique reliée aux procédés d'initiation et de propagation d'une fissure utilisant des petites éprouvettes compactes de rupture. Un couplage de barres d'Hopkinson préchargées en tension est employé dans l'expérience. Le relachement rapide de l'énergie emmagasinée dans l...
Une investigation a été conduite sur les effets de température, de vitesse de chargement et de plusieurs paramètres microstructuraux sur l'initiation de rupture en déformation plane d'un acier doux AISI 1020. Les températures d'essai ont porté sur un intervalle de -150 à 150 °C couvrant les ruptures de type de clivage transgranulaire et de type fib...
In order to perform structural reliability assessments, utilities need to characterize fracture properties from small specimens-not only for economic reasons but because in many situations only a small quantity of material is available. This report presents a novel experimental-analytic procedure to measure dynamic fracture toughness properties of...
The principal objective of the program has been to establish the loading rate dependence of compressive deformation and fracture mechanisms in whisker and fiber-reinforced ceramic matrix composites, and in fiber-reinforced polymeric matrix composites. Work during the last year has emphasized several types of composite: Silicon Carbide fiber and Sil...
Depleted uranium (DU) is currently the top-performing kinetic energy penetrator, followed closely (but not matched) by several tungsten heavy alloy variants. Because of the adverse environmental impact associated with DU, it is extremely desirable to determine a means of upgrading the ballistic performance of heavy alloys to a level at least compar...
Computer-produced copy. Thesis (Sc. M.)--Brown University, 1984. Bibliography: leaves 31-32.
A modified Johnson-Cook model is proposed to take into account the thermally activated and viscous regimes that can be easily integrated in numerical codes. The model was calibrated using experimental data generated at temperatures ranging from 23 to 1000$^{\circ}$C and strain rates ranging from 0.002 to $3 \times 10^{4}$ s$^{-1}$ and validated thr...