Showing papers by "Marc A. Meyers published in 2004"

Self organization of shear bands in stainless steel

Abstract: The spatial distribution of shear bands was investigated in 304L stainless steel through the radial collapse of a thick-walled cylinder under high-strain-rate deformation (∼10 4 s −1 ). The shear-band initiation and propagation were also examined. Self-organization of multiple adiabatic shear bands was observed. The effect of grain size on spacing of shear bands was investigated at four different grain sizes: 30 m, 50m, 140m and 280m. A single crystal with a similar composition was also tested. The experimental results show only a modest variation of shear-band spacing within the investigated grain size range. Three principal mechanisms are considered to be active in initiation: (a) momentum diffusion by stress unloading, (b) perturbation in the stress/strain/temperature fields, (c) microstructural inhomogeneities. The observed shear-band spacing is compared with existing theories; Grady–Kipp and Wright–Ockendon–Molinari theories. These are one-dimensional theories that do not consider the evolution in spacing as the shear bands grow. A discontinuous growth mode for shear localization under periodic perturbation is applied and predicts spacings in good agreement with observations. Self-organized initiation and propagation modes are discussed in relation to the interaction among the nucleus and well-developed shear bands. © 2004 Elsevier B.V. All rights reserved.

Materials science under extreme conditions of pressure and strain rate

Abstract: Solid-state dynamics experiments at very high pressures and strain rates are becoming possible with high-power laser facilities, albeit over brief intervals of time and spatially small scales. To achieve extreme pressures in the solid state requires that the sample be kept cool, with Tsample 103 GPa (10 Mbar), on the National Ignition Facility (NIF) laser at Lawrence Livermore National Laboratory.

Modeling the elastic properties and damage evolution in Ti-Al3Ti metal-intermetallic laminate (MIL) composites

Abstract: The mechanical performance of Ti–Al3Ti metal–intermetallic laminate (MIL) composites synthesized by a reactive foil sintering technique was evaluated. The elastic properties and anisotropy of the laminates were calculated and successfully compared with resonant ultrasonic spectroscopy (RUS) measurements. The effect of internal stresses due to differences in the thermal expansion coefficient on fracture toughness was analyzed. The principal mechanisms of damage initiation and accumulation were identified experimentally. The compressive strength was modeled by FEM using the Johnson–Holmquist constitutive equation. The computed results were successfully compared with experiments.

Laser-induced shock compression of copper: Orientation and pressure decay effects

Abstract: Pure copper monocrystals with [001] and [\(\bar 1\)34] orientations were subjected to ultrashort shock pulses ranging in initial duration from 2.5 to 10 ns, induced by a laser at energies ranging from 10 to 70 MJ/m2. The deformation structure was significantly dependent on the crystallographic orientation and depth from the laser-impacted surface, as characterized by transmission electron microscopy (TEM). The threshold pressure for twinning in the [001] direction was observed to be in the range of 20 to 40 GPa compared with 40 to 60 GPa for the [\(\bar 1\)34] orientation. Dislocation densities were also different for the two orientations, under similar shock conditions. The [\(\bar 1\)34] dislocation density was systematically lower. This is attributed to the activation of fewer slip systems resulting in a lower rate of hardening. The different results found for [001] and [\(\bar 1\)34] copper single crystals are described and effects of pressure decay in [\(\bar 1\)34] specimens are discussed. Differences in the mechanical response between the two orientations are responsible for differences in the shear stress in the specimens at the imposed pressures and associated strains. The [\(\bar 1\)34] orientation is initially subjected to deformation by single slip, (111)[101], which has a Schmid factor of 0.4711 and a well-defined easy glide region followed by a cross-slip regime with secondary slip. The [001] orientation has eight slip systems {111}〈110〉 with identical Schmid factors of 0.4082, which lead to immediate work hardening. At an imposed and prescribed pressure (that establishes the strain), the [\(\bar 1\)34] orientation exhibits a lower shear stress. The orientation dependence of the twinning stress is much lower, as expressed by Schmidt factors. This higher stress for [001] predisposes the onset of twinning in this orientation. The results are interpreted in terms of a criterion in which slip and twinning are considered as competing mechanisms. A constitutive description using a modified mechanical threshold stress (MTS) model is applied to the two orientations, incorporating both slip and twinning. The threshold pressure for twinning is calculated, considering the effect of shock heating. The constitutive description provides a rationale for the experimental results: the calculated thresholds are 17 GPa for [001] and 25 GPa for [\(\bar 1\)34].

Explosive Welding of Aluminum to Aluminum: Analysis, Computations and Experiments

Abstract: 6061 T0 aluminum alloy was joined to 6061 T0 aluminum alloy by explosive welding. This is a process in which the controlled energy of a detonating explosive is used to create a metallic bond between two similar or dissimilar materials. The welding conditions were tailored to produce both wavy and straight interfaces. A three‐pronged study was used to establish the conditions for straight weld formation: (a) analytical calculation of the domain of weldability; (b) characterization of the explosive welding experiments carried out under different conditions, and (c) 2D finite differences simulation of these tests using the explicit Eulerian hydrocode Raven with a Johnson‐Cook constitutive equation for the Al alloy. The numerical simulation and the analytical calculations confirm the experimental results and explain the difficulties met for obtaining a continuous straight interface along the entire weld.

Shear localization-martensitic transformation interactions in Fe-Cr-Ni monocrystal

Abstract: A Fe-15 wt pet Cr-15 wt pet Ni alloy monocrystal was deformed dynamically (strain rate similar to10(4) s(-1)) by the collapse of an explosively driven thick-walled cylinder under prescribed initial temperature and strain conditions. The experiments were carried out under the following conditions: (a) alloy in austenitic state, temperature above transformation temperature; (b) alloy in transformed state; and (c) alloy at temperature slightly above g,, propitiating concurrent shear-band propagation and martensitic transformation. The alloy exhibited profuse shear-band formation, which was a sensitive function of the deformation condition. Stress-assisted and strain-induced martensitic transformation competes with shear localization. The alloy deformed at a temperature slightly above g, shows a significantly reduced number of shear bands. The anisotropy of plastic deformation determines the evolution of strains and distribution of shear bands. The different conditions showed significant differences that are interpreted in terms of the microstructural anisotropy. Calculated shear-band spacings based on the Grady-Kipp (GK) and Wright-Ockendon (WO) theories are compared with the observed values. The microstructure within the shear bands was characterized by transmission electron microscopy. Regions of sub-micron grain sizes exhibiting evidence of recrystallization were observed, as well as amorphous regions possibly resulting from melting and rapid resolidification.

Effect of low-temperature shock compression on the microstructure and strength of copper

Abstract: Copper with two purities (99.8 and 99.995 pct) was subjected to shock compression from an initial temperature of 90 K. Shock compression was carried out by explosively accelerating flyer plates at velocities generating pressures between 27 and 77 GPa. The residual microstructure evolved from loose dislocation cells to mechanical twins and, at the 57 and 77 GPa pressures, to complete recrystallization, with a grain size larger than the initial one. The shock-compressed copper was mechanically tested in compression at a strain rate of 10−3 s−1 and temperature of 300 K; the conditions subjected to lower pressures (27 and 30 GPa) exhibited work softening, in contrast to the conventional work-hardening response. This work softening is due to the uniformly distributed dislocations and the formation of loose cells, evolving, upon plastic deformation at low strain rates, into well-defined cells, with a size of approximately 1 µm. The 99.995 pct copper subjected to the higher shock-compression pressures (57 and 77 GPa) exhibited a stress-strain response almost identical to the unshocked condition. This indicates that the residual temperature rise was sufficient to completely recrystallize the structure and eliminate the hardening due to shock compression. Thermodynamic calculations using the Hugoniot-Rankine conservation equations predict residual temperatures of 570 and 1000 K for the 57 and 77 GPa peak pressures, respectively.

Laser-Induced Shock Compression of Copper and Copper Aluminum Alloys

Abstract: Single crystal copper and copper 2‐wt% aluminum alloy with [134] and [001] orientations are compressed by means of a high energy short pulse laser. Pressures ranging from 20 GPa to 60 GPa are achieved. The shocked samples are recovered and the residual defect substructure is analyzed by transmission electron microscopy. Results show systematic differences depending on orientation and stacking fault energy. Samples with orientations [001] are symmetrical with simultaneous activation of eight slip systems. This leads to a higher work hardening rate. The [134] orientation is asymmetrical with one dominating slip system, and thus a reduced work hardening rate due to a prolonged easy glide region for dislocations. These differences in work hardening response affect the stresses required to achieve the twinning threshold pressure. The effects of stacking fault energy on the defect substructure and threshold twinning are also characterized. Experimental results are rationalized in terms of a constitutive description of the slip‐twinning transition using a modified MTS equation. Differences in the mechanical response of the orientations and the chemical compositions are responsible for differences in the shear stress in the specimens at the imposed pressures and associated strains.

Large Deformation Simulations of Nanocrystalline Materials

Abstract: The role of grain size in the deformation of nanocrystalline materials was investigated by modeling a representative volume element of grains using a multi‐material Eulerian finite element formulation. Individual grains are modeled with single crystal plasticity, with the interior and boundary of the grain having different hardening properties. The predicted response is compared to experiments on copper with grain sizes of 1 μm and 26 nm.