Robin Woracek

3D mapping of crystallographic phase distribution using energy-selective neutron tomography.

Nondestructive 3D mapping of crystallographic phases is introduced providing distribution of phase fractions within the bulk (centimeter range) of samples with micrometer-scale resolution. The novel neutron tomography based technique overcomes critical limitations of existing techniques and offers a wide range of potential applications. It is demonstrated for steel samples exhibiting phase transformation after being subjected to tensile and torsional deformation.

Neutron Bragg-edge mapping of weld seams

Abstract Cold neutrons have a wavelength that is in the same range as the lattice spacings of most polycrystalline metallic materials. Imaging of such materials with monochromatic cold neutrons of different wavelengths provides a unique contrast due to coherent Bragg scattering. Additionally, the spectral positions of the Bragg edges can be mapped for each point of an image by using the transmission data of corresponding wavelength scans. We present investigations of welded components with such energy-selective neutron radiography around specific Bragg-edges in the transmission spectrum. Features in the local microstructure of the weld have been visualized.

CONRAD-2: the new neutron imaging instrument at the Helmholtz-Zentrum Berlin

The construction of the new neutron imaging instrument at the BER-2 research reactor of the Helmholtz-Zentrum Berlin has greatly increased the potential of the facility. The redesign of the facility included improvements of the neutron extraction and transportation systems, more effective shielding, and innovative instrumentation. The cold neutron flux at the neutron guide exit was increased by more than one order of magnitude, which allowed for an implementation of methods that require monochromatic or polarized beams, thus enabling the exploitation of nonconventional contrast mechanisms such as phase, diffraction and magnetic contrasts. The improved instrument design also facilitates the development of high-resolution neutron tomography by providing an increased beam intensity at the sample position.

Detection of water with high sensitivity to study polymer electrolyte fuel cell membranes using cold neutrons at high spatial resolution

Thermal neutron imaging is a powerful non-invasive diagnostic technique to study water management within a proton exchange membrane (PEM) fuel cell. To isolate the hydration behavior of membrane, significant increase in detecting thin water films is needed. A humidity cell is developed to study hydration of a PEM using cold neutron imaging. Spatial and temporal changes of PEM water uptake are quantified. Water film as small as 1 μm thick and corresponding to a volume containing 10 ng of liquid water at high spatial resolution is detected. This represents an order of magnitude improvement in water detection efficiency achievable at thermal neutron imaging facilities.

Method to determine hkl strains and shear moduli under torsion using neutron diffraction

An experimental method, using in-situ neutron diffraction for the measurement of shear strain, based on (hkl) lattice spacing changes under torsional loading, is described. This method provides the ability to probe the response of crystallographic planes to application of shear stress, inside the bulk of samples that are subjected to torsion. To demonstrate the method, shear moduli corresponding to bcc (211), (200), and (110) were experimentally determined for a solid cylinder of ferritic alloy 12L14 under elastic loading. Results indicate that the elastic constants determined under torsional shear show a different degree of anisotropy than those obtained from tensile loading.

Small Angle Scattering in Neutron Imaging—A Review

Conventional neutron imaging utilizes the beam attenuation caused by scattering and absorption through the materials constituting an object in order to investigate its macroscopic inner structure. Small angle scattering has basically no impact on such images under the geometrical conditions applied. Nevertheless, in recent years different experimental methods have been developed in neutron imaging, which enable to not only generate contrast based on neutrons scattered to very small angles, but to map and quantify small angle scattering with the spatial resolution of neutron imaging. This enables neutron imaging to access length scales which are not directly resolved in real space and to investigate bulk structures and processes spanning multiple length scales from centimeters to tens of nanometers.

In-Situ Microstructure Evolution Under Stress Using a Large-Chamber SEM

A large-chamber scanning electron microscope (LC-SEM by VisiTec) shown in Fig. 1a was used to investigate samples in-situ using a custom developed tensile testing system. The LC-SEM used in this research is equipped with the following: secondary electron detector, backscattered electron detector (4 quadrants), electron backscatter diffraction (EBSD), energy dispersive x-ray spectrometry (EDS), and variable pressure mode. A vacuum-suitable mechanical testing system, developed by the authors (see Fig. 1b), with an axial force capacity of 90kN was fabricated to perform unique in-situ studies on metallic and polymeric composite samples. The testing system uses custom developed LabView based data acquisition and control software for performing both stress and strain controlled tests. In-situ SEM and TEM investigations are not novel concepts [1-2], but a key feature of this system is the ability of testing larger specimens having geometry and dimensions similar to those used in traditional mechanical testing laboratories. The LC-SEM eliminates the need for using artificially small specimens, reducing unwanted size effects associated with applied deformation on the microstructure. Deformation mechanisms, such as twinning and slip, as well as other microscopic features, such as pores, impurities, and cracks were examined in-situ under tensile stress at desired magnifications ranging from 50X to 20,000X. Fig. 2a shows the stress/strain curve of an austenitic stainless steel specimen for which SEM images were obtained at various target deformation locations as noted on the graph. This specimen was cold rolled, 40mm in length and had 1mm cross sectional area, and its surface was mechanically polished. Originally the sample was flat with no slip bands, but when stress was applied the grains were distorted and elongated. This grain elongation is seen in the SEM images in Fig. 2b, and a marker is placed to follow the same grain through subsequent deformation. Prior to rupture, necking occurred and grain boundaries were hard to distinguish because the grains were heavily distorted and had developed significant slip planes. Figure 3 shows the EDS to investigate impurities in a crack region. Fig. 4 shows a continuous fiber polymeric composite specimen made of vinyl ester resin and carbon fiber (CF/VE) before and after fracture. A typical fracture surface for a specimen with 45 degree fiber orientation caused by fracture resulting from interfacial debonding in the carbon fiber/vinyl ester can be seen. The cracks tend to run along the matrix between the fibers which indicate a brittle microstructure for the composite specimen. Ongoing research includes the use of EBSD technique to evaluate the grain level deformation as a function of external stress.

Damage evolution in VARTM-based carbon fiber vinyl ester marine composites and sea water effects

Carbon fiber-reinforced vinyl ester composite facings are being considered by the US Navy for sandwich structures with either low-density polyvinyl chloride foam or balsa wood as the core materials manufactured using Vacuum-Assisted Resin Transfer Molding process. In this paper, experimental techniques are developed to evaluate the variation of surface strain spatially using three-dimensional Digital Image Correlation technique and internal damage evolution using radiation-based tomography. Both X-ray and neutron radiation was utilized to evaluate the microstructure of composite laminate facings in three dimensions at high resolution. Since naval structures are exposed to harsh sea environment, samples were soaked in simulated sea water at 40℃ for long duration, well beyond saturation stage of Fickian diffusion ascertained using weight gain measurements. Results for a series of mechanical tests on fiber dominated samples of [0/90]2S and matrix dominated samples of [±45]2S orientation are reported. Variable specimen sizes (12.5 mm wide by 100 mm long, 25 mm by 200 mm, and 25 mm by 300 mm, all with an average uniform thickness of 2.8 mm) for laboratory tests are utilized in order to evaluate the applicability of results for large ship structures. Different carbon fiber-reinforced vinyl ester failure mechanisms of matrix crack interactions, such as matrix dominated transverse tension, tension along fibers, and fiber delamination were observed. Digital Image Correlation technique was useful to track the existence of localized damage for both fiber and matrix dominated carbon fiber-reinforced vinyl ester laminates. High resolution X-ray and neutron tomography under in-situ mechanical loading were used to investigate interior microstructure evolution and damage at chosen stress levels. The new techniques presented in this paper can enable a comparison between interior specimen damage and surface damage (readily observable using Digital Image Correlation). Use of different modality of radiation (neutrons versus X-rays) was very useful to probe damage resulting from moisture diffused in the matrix resin and fiber interface/interphase and shows a lot of promise for studying the failure modes and damage evolution in polymeric composites and sandwich structures.