Showing papers by "Marc A. Meyers published in 2003"

Constitutive response of welded HSLA 100 steel

Abstract: The dynamic mechanical behavior of a welded joint of high-strength, low alloy (HSLA) 100 steel was investigated by both quasistatic (103 s � 1 ) and high strain rate (103 s � 1 ) tension loadings at ambient and low temperatures. The constitutive responses for the microstructurally different weld and base steels, along with the interface, which included the heat-affected zone (HAZ), were analyzed and compared. This response is successfully modeled by the mechanical threshold stress (MTS) constitutive equation for different regions of the welded joint, which shows qualitatively different behavior. The necking and failure occurred uniformly within the weld metal but not in the HAZ. The main mechanism for the failure of the welded joints is void growth. Microstructural characterization revealed that the nucleation of voids occurred mainly at the interface between the base and the weld metal, and initiated at inclusions. Measurements of damage distributions across HAZ were made to evaluate the contribution of porosity variation to the constitutive response. In both the quasi-static and dynamic tests, the deformation localization in the form of necking first appeared in the weld metal. Fractographic observation demonstrates that void evolution is a dominant factor in the macroscopic mechanical response. The Gurson � /Tvergaard model was included in the modeling effort to incorporate the effect of void opening on the mechanical response as well as tensile instability. The MTS constitutive model was successfully implemented to the tensile regime of loading. # 2003 Elsevier Science B.V. All rights reserved.

Evaluation of the collapsing thick-walled cylinder technique for shear-band spacing

Abstract: The thick-walled cylinder (TWC) technique was successfully used to investigate the shear-band patterning in AISI 304 stainless steel. Several factors that may influence the shear-band distribution and spacing in the TWC configuration were examined. The role of machining, annealing, and shrink fitting, as well as the variation of the shear-band distribution along the longitudinal axis of the cylindrical specimen were evaluated. Experimental results indicate that the machined surface at the internal boundary of the cylindrical specimen, where shear bands initiate, provides a strain-hardened layer that significantly changes the condition for their initiation. Specimens with such a layer have a higher density of bands with a smaller spacing, in comparison with those without a work-hardened layer. The nature of contact interface in the cylindrical specimen assembly, either causing a clearance that changes the initial loading conditions or introducing a pre-strained layer with shrink-fitting technique, does not influence the spacing of shear bands, but does affect the evolution and development of multiple shear bands at the initial stage. The distribution of shear bands along the cylinder has a constant spacing but the maximum lengths of bands are sensitive to the position. The collapse process of the cylindrical specimen was simulated by using the RAVEN hydrocode. The deformation, temperature, and velocity histories during the cylinder collapse were calculated. The calculated results are in good agreement with the previous experimental data.

High-pressure, high-strain-rate lattice response of shocked materials

Abstract: Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the published Hugoniot Elastic Limit (HEL) for these materials. In situ x-ray diffraction has been used to directly measure the response of the shocked lattice during shock loading. Static film and x-ray streak cameras recorded x rays diffracted from lattice planes both parallel and perpendicular to the shock direction. In addition, experiments were conducted using a wide-angle detector to record x rays diffracted from multiple lattice planes simultaneously. These data showed uniaxial compression of Si (100) along the shock direction and three-dimensional compression of Cu (100). In the case of the Si diffraction, there was a multiple wave structure observed. This is evaluated to determine whether there is a phase transition occurring on the time scale of the experiments, or the HEL is much higher than previously reported. Results of the measurements are presented.

Multiple film plane diagnostic for shocked lattice measurements (invited)

Abstract: Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the Hugoniot elastic limit. In these experiments, static film and x-ray streak cameras recorded x rays diffracted from lattice planes both parallel and perpendicular to the shock direction. These data showed uniaxial compression of Si(100) along the shock direction and three-dimensional compression of Cu(100). In the case of the Si diffraction, there was a multiple wave structure observed, which may be due to a one-dimensional phase transition or a time variation in the shock pressure. A new film-based detector has been developed for these in situ dynamic diffraction experiments. This large-angle detector consists of three film cassettes that are positioned to record x rays diffracted from a shocked crystal anywhere within a full π steradian. It records x rays that are diffracted from multiple lattice planes both parallel and at oblique angles with respect to the shock direction. It is a time-integrating measurement, but time-resolved data may be recorded using a short duration laser pulse to create the diffraction source x rays. This new instrument has been fielded at the OMEGA and Janus lasers to study single-crystal materials shock compressed by direct laser irradiation. In these experiments, a multiple wave structure was observed on many different lattice planes in Si. These data provide information on the structure under compression.

Laser shock compression of copper monocrystals: Mechanisms for dislocation and void generation

Abstract: Copper with two orientations ([001] and [134]) was subjected to high intensity laser (energy levels of 40-300 J; energy densities of 15-70 MJ/m 2 and durations below 10 ns). The defects created are characterized by transmission electron microscopy. An orientation-dependent threshold stress for twinning is observed. The results are rationalized in terms of a criterion in which slip and twinning are considered as competing mechanisms. A constitutive description is applied to the two orientations, incorporating both slip and twinning. The predictions are in agreement with experiments. The threshold stress for twinning in the [001] orientation is 20-40 GPa, whereas the one for the [134] orientation is 40-60 GPa. The threshold stress is calculated, considering the effect of shock heating. The constitutive description provides a rationale for the experimental results; the calculated thresholds are 18 GPa for [001] and 25 GPa for [134]. A mechanism for void generation and growth based on the emission of geometrically necessary dislocations is proposed and analytically formulated.

Nano and microstructural design of advanced materials : a commemorative volume on Professor G. Thomas' seventieth birthday

Abstract: The importance of the nanoscale effects has been recognized in materials research for over fifty years, but it is only recently that advanced characterization and fabrication methods are enabling scientists to build structures atom-by-atom or molecule-by molecule. The understanding and control of the nanostructure has been, to a large extent, made possible by new atomistic analysis and characterization methods pioneered by transmission electron microscopy. "Nano and Microstructural Design of Advanced Materials" focuses on the effective use of such advanced analysis and characterization techniques in the design of materials. It teaches effective use of advanced analysis and characterization methods at an atomistic level. It contains many supporting examples of materials in which such design concepts have been successfully applied.

On plastic void growth in strong ductile materials

Abstract: A b s t r a c t An elastoplastic analysis of spherical and cylidrical void growth under uniform tension is presented. The stress dependence on the void size is derived for an ideally plastic and an incompressible elastic{ linearly hardening material, accurate to flrst order in the ratio of the plastic hardening rate and the elastic modulus. The critical stress is calculated at which an unstable cavitation takes place for selected material properties. The implications to spalling strength of strong ductile materials are discussed, with a particular referral to nanocrystalline materials processed by severe plastic deformation and characterized by exceedingly high values of the hardening rate. Relationship to earlier work on the subject is also given.

The onset of twinning in plastic deformation and martensitic transformations

Abstract: A quantitative constitutive description for the criterion postulated by Thomas [ 1. , 2. , 3. ] for the morphology of martensitic transformations is presented. Thomas observed that the temperature and strain-rate sensitivities of slip are much higher than those for twinning, rendering twinning a favored deformation mechanism at low temperatures and high strain rates. Constitutive relationships for slip and twinning are presented and applied to the martensitic transformation in steels: the lath to plate morphology change that is observed with increasing carbon content is successfully predicted by calculations incorporating the two modes of deformation. The Hall-Petch coefficient, for the inclusion of grain size effects is two times larger for twinning than slip. A simple calculation of the strain rates during martensitic transformation is also provided. For FCC metals, the constitutive description for the slip-twinning transition incorporates the effects of material (stacking-fault energy, grain size, composition) as well as external (temperature, strain rate) parameters successfully. It can also applied to the shock compression regime, where the shock front thickness (and, consequently, strain rate) is related to the peak pressure by the Swegle-Grady relationship. Predictions are compared to seminal shock loading work by Johari and Thomas [4] and Nolder and Thomas [5] demonstrating that there is a threshold pressure for twinning in copper and nickel.