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A new procedure for the determination of the smallest Representative Volume Element (RVE) of granular media is proposed in the present investigation. The procedure is based on the simulation of consolidated granular assemblies using the Discrete Element Method (DEM). The existence of a lower limit for the dimensions of specimens used in the high-strain rate experiments on granular materials as a function of their consolidation state is demonstrated. The repeatability of the experimental results presented demonstrates the validity of the proposed method for the determination of the RVE. The results obtained show clearly the influence of chemical/physical composition, grain shape, initial consolidation state and type of confinement on the measured mechanical response.

This paper describes non-destructive three-dimensional (3D) mapping of crystallographic phase distributions within the bulk (centimeter range) of samples with micrometer-scale resolution. The technique leverages diffraction contrast due to Bragg scattering and the large penetration power of neutrons through high-atomic-number-element-based structural materials. Our tomographic approach overcomes critical limitations of existing techniques by allowing spatially resolved phase mapping in bulk samples and offers a wide range of potential applications. The technique is demonstrated for (metastable 304L stainless) steel samples that exhibit strain-induced martensitic phase transformation after being subjected to tensile and torsional deformation. The distribution of phase fractions within the 3D reconstructed volumes was verified at select locations using neutron diffraction as well as EBSD, and results agree with transformation kinetics theory.

Our evaluation of a high-resolution digital imaging detector capable of providing millisecond time resolution and high sensitivity for neutrons is presented. This detector is a modified version of the high-resolution gamma-ray imager developed previously at Radiation Monitoring Devices, Inc. (RMD, Inc.), and consists of an Electron Multiplying Charge Coupled Device (EMCCD) attached to a neutron-sensitive scintillator via a fiberoptic taper. By virtue of its internal gain, the EMCCD permits high speed readout without introducing additional noise, thereby enabling high frame rate operation with an enhanced signal-to-noise ratio (SNR). Detector sensitivity is enhanced through the use of a back-thinned EMCCD, which provides high quantum efficiency over a typical emission range for many neutron-sensitive scintillators. Preliminary evaluations conducted at the Liquids Reflectometer beam port of the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL) demonstrate that this new detector has the sensitivity to detect individual neutrons and the acquisition speed to perform energy-selective imaging with a temporal resolution of milliseconds. While substantial improvements in timing and imaging performance are planned, this prototype detector has already generated the first ever images of the SNS Liquids Reflectometer beam profile and was also used to demonstrate a technique for obtaining Bragg edge transmission imaging using energy-selective neutrons. The preliminary data, along with the detector design, evaluation, and planned developments are discussed in this paper.