Shuh-Rong Chen

On the modeling of the Taylor cylinder impact test for orthotropic textured materials: experiments and simulations

Abstract Taylor impact tests using specimens cut from a rolled plate of tantalum were conducted. The tantalum was experimentally characterized in terms of flow stress and crystallographic texture. A piece-wise yield surface was interrogated from an ODF corresponding to this texture assuming two slip system modes, in conjunction with an elastic stiffness tensor computed from the same ODF and single crystal elastic properties. This constitutive information was used in EPIC-95 3D simulations of a Taylor impact test, and good agreement was realized between the calculational results and the experimental post-test geometries in terms of major and minor side profiles and impact-interface footprints.

Predicting material strength, damage, and fracture The synergy between experiment and modeling

ConclusionsThe Taylor cylinder impact test, the plane-strain tensile test, and explosively driven hemisphere test represent readily conducted experiments that probe the deformation, damage evolution, and fracture behavior of materials. Because these tests are very sensitive to large gradients of stress, strain, strain rate, and shock loading, we are using them to evaluate and validate the correctness of our mechanical models that are implemented and destined to be implemented into large-scale 3-D simulation codes.Robust models that capture the physics of high-rate material response are required for developing predictive capability for highly dynamic events. The increased effort to link experiments and modeling within the computational mechanics community and the increased emphasis on code verification and validation within the Los Alamos National Laboratory defense programs are accelerating this development. These efforts are already receiving recognition through the recent establishment of verification and validation committees within various technical societies.

Characterization of Tri-lab Tantalum Plate.

This report provides a detailed characterization Tri-lab Tantalum (Ta) plate jointly purchased from HCStark Inc. by Sandia, Los Alamos and Lawrence Livermore National Laboratories. Data in this report was compiled from series of material and properties characterization experiments carried out at Sandia (SNL) and Los Alamos (LANL) Laboratories through a leveraged effort funded by the C2 campaign. Results include microstructure characterization detailing the crystallographic texture of the material and an increase in grain size near the end of the rolled plate. Mechanical properties evaluations include, compression cylinder, sub-scale tension specimen, micohardness and instrumented indentation testing. The plate was found to have vastly superior uniformity when compare with previously characterized wrought Ta material. Small but measurable variations in microstructure and properties were noted at the end, and at the top and bottom edges of the plate.

Rayleigh-Taylor strength experiments of the pressure-induced α→ε→α′ phase transition in iron

We present here the first dynamic Rayleigh-Taylor (RT) strength measurement of a material undergoing solid-solid phase transition. Iron is quasi-isentropically driven across the pressure-induced bcc (α-Fe) → hcp (e-Fe) phase transition and the dynamic strength of the α, e and reverted α′ phases have been determined via proton radiography of the resulting Rayleigh-Taylor unstable interface between the iron target and high-explosive products. Simultaneous velocimetry measurements of the iron free surface yield the phase transition dynamics and, in conjunction with detailed hydrodynamic simulations, allow for determination of the strength of the distinct phases of iron. Forward analysis of the experiment via hydrodynamic simulations reveals significant strength enhancement of the dynamically-generated e-Fe and reverted α′-Fe, compareable in magnitude to the strength of austenitic stainless steels.

Advanced plasticity models applied to recent shock data on beryllium

Plate impact experiments were performed on vacuum hot-pressed S-200F Beryllium. This hexagonal close-packed (HCP) metal shows significant plasticity effects. To examine the validity of plasticity models in the shock regime, the experiments were modeled using a Lagrangian hydrocode. Two constitutive strength (plasticity) models, the Preston-Tonks-Wallace (PTW) and Mechanical Threshold Stress (MTS) models, were calibrated using the same set of quasi-static and Hopkinson bar data taken at temperatures from 77K to 873K and strain rates from 0.001/sec to 4300/sec. In spite of being calibrated on the same data, the two models give noticeably different results when compared with the measured wave profiles. The reasons for the differences are explored and discussed.