Lawrence Hull

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.

Damage in low alloy steel produced by sweeping, interacting detonation waves

Detonation waves that sweep along the surface of a metal plate induce reduced pressure and enhanced shear, relative to the same detonation at normal incidence. Detonation waves at intermediate obliquity impress intermediate combined stress states. Release waves from the free surfaces may enter into play and contribute to the damage. Initiation of explosive at discrete points produces strong pressure, density, and velocity gradients in the gaseous explosive products in areas where the waves collide, are impressed in an adjacent metal, causing similar stress gradients within the metal that often leading to intense damage. In this work, we investigate damage generated in AISI 4130 steel by the combined effects of oblique drive and interacting detonation waves. The experimental data consist of multipoint velocimetry points probing the free surface in regions loaded by interacting detonation waves and regions between the interactions. Metallography on recovered plate records the plastic flow and damage correlated with the velocimetry data. Spall is indicated in most regions, but not some, and the alpha-epsilon stress-induced phase transformation appears in most regions, but not all.

A comparison of techniques for extracting transverse speed from photon Doppler velocimetry signal content

We recently demonstrated that a single optical probe is capable of simultaneously measuring a surfaces velocity along the beam axis and its speed transverse to the beam axis. Doppler shifts in the measured data are related to axial motion, while intensity fluctuations, induced by speckle dynamics, are related to transverse motion. While it is readily apparent that speckle dynamics manifest themselves in the measured data, the ability to extract transverse speed from a particular (speckle-induced) signal feature is feature-dependent. In this paper, we relate a signals coherence, variance, and frequency content to surface dynamics, in an effort to determine the suitability of each of these features for calculating transverse motion (classification release number: LA-UR 13-26315).

Shot H3837: Darht's first dual-axis explosive experiment

Test H3837 was the first explosive shot performed in front of both flash x-ray axes at the Los Alamos Dual Axis Radiographic Hydrodynamic Test (DARHT) facility. Executed in November 2009, the shot was an explosively-driven metal flyer plate in a series of experiments designed to explore equation-of-state properties of shocked materials. Imaging the initial shock wave traveling through the flyer plate, DARHT Axis II captured the range of motion from the shock front emergence in the flyer to breakout at the free surface; the Axis I pulse provided a perpendicular perspective of the shot at a time coinciding with the third pulse of Axis II.

Surface shear strains induced by quasi-steady sweeping detonation waves

Sweeping wave experiments create conditions of greater shear than corresponding onedimensional motion experiments, and are of current interest for material damage characterization. Sweeping waves are also important with regards to the spectrum of applications of explosives driving metals. The intensity of the shear developed in a sweeping wave experiment may be monitored using crossed beams of Photon Doppler Velocimetry (PDV). During the time the material is traversing the volume defined by the crossed beams, the interferometer is measuring the velocity of the same mass element (approximately) from two directions. It is known that PDV measures the velocity component that lies along the beam direction, so that with crossed beams, two independent directions are simultaneously measured and therefore the vector velocity (both magnitude and direction) are captured. The vector velocity is readily related to the strain rates on the surface (after removing the rigid rotation rates), and the equations are integrat...

Ti Hemi boombox experiment

Previous deformation experiments in which IR imaging was used pointed to a correlation in between IR signature in areas where heat was expected to be. The surface is not uniform during deformation experiments which cause cracks in the image in areas with increased temperature. To measure temperature under dynamic conditions, simultaneous reflectivity and radiance measurement under events of interest is needed. To measure a temperature measurement, a Reflectance measurement taken by framing camera at the edge of the camera sensitivity (700nm). Allows relative measurement at this wavelength. At reasonable temperatures the spectral radiance should peak near 2u and be 3-4 orders of magnitude higher than at 700 nm.

Velocity spectra from explosively driven powders and balls

The capability to measure velocity distributions using Photon Doppler Velocimetry (PDV) has given rise to much data that were not measurable with previous velocimetry techniques. In our PDV measurements on explosively driven metals, we have often seen a single velocity disappear in a wide distribution of velocities. We have attributed this to HE gases, metal pieces, or a mix emerging from cracks in the metal after it fails. However, we are unaware of any experiments that demonstrate this interpretation. We have applied X-rays, cameras and PDV to explosively driven powders, balls and brass rings and found PDV spectra similar to what we observed in our experiments in which the metal fails. We present these spectra to help workers interpret their velocity spectra.