Energy-resolved neutron imaging for reconstruction of strain introduced by cold working

Energy-resolved neutron transmission imaging is used to reconstruct maps of residual strains in drilled and cold-expanded holes in 5-mm and 6.4-mm-thick aluminum plates. The possibility of measuring the positions of Bragg edges in the transmission spectrum in each 55 × 55 µm2 pixel is utilized in the reconstruction of the strain distribution within the entire imaged area of the sample, all from a single measurement. Although the reconstructed strain is averaged through the sample thickness, this technique reveals strain asymmetries within the sample and thus provides information complementary to other well-established non-destructive testing methods

Energy selective neutron imaging for the characterization of polycrystalline materials

This multipart dissertation focuses on the development and evaluation of advanced methods for material testing and characterization using neutron diffraction and imaging techniques. A major focus is on exploiting diffraction contrast in energy selective neutron imaging (often referred to as Bragg edge imaging) for strain and phase mapping of crystalline materials. The dissertation also evaluates the use of neutron diffraction to study the effect of multi-axial loading, in particular the role of applying directly shear strains from the application of torsion. A portable tension-torsion-tomography loading system has been developed for in-situ measurements and integrated at major user facilities around the world. Promising applications for the Bragg edge technique are implemented at the neutron imaging facility CONRAD at the reactor source BER-II as well as at neutron time of flight instruments. Strain mapping is successfully demonstrated for all cases to yield quantifiable results, but is limited in practicality due to limitations in choice of the scattering vector (direction of probed strain tensor component) and the gauge volume selection. The use of Bragg edge imaging for crystalline phase mapping was explored and appears to be a very promising technique. The extension to three-dimensionally resolved tomography is presented for samples exhibiting the TRansfomation Induced Plasticity (TRIP) effect, while challenges with characterizing textured samples are discussed. Individual crystallites within a polycrystalline material exhibit elastic anisotropy which is significant as that can lead to stress concentrations and inhomogeneities during plastic deformation. Characterization of elastic anisotropy is important to understand the effects of texture on the macroscopic mechanical properties. Diffraction methods can do this, by probing the response of individual lattice planes to externally applied mechanical stress. Past experimental data using diffraction based methods have largely been limited to uni‑axial tensile and/or compressive loading conditions, even though shear dominates most common failure mechanisms for structural materials. Within this dissertation, experimental techniques have been established for the measurement of lattice strains under applied torsion (pure shear) and lattice specific shear moduli are reported. This is accomplished using a (traditional) neutron diffractometer instrument, in conjunction with special alignment procedures and the specifically designed axial-torsional loading system

Bragg edge tomography characterization of additively manufactured 316L steel

In this work we perform a neutron Bragg edge tomography of stainless steel 316L additive manufacturing samples, one as built via standard laser powder bed fusion, and one using the novel three-dimensional (3D) laser shock peening technique. First, we consider conventional attenuation tomography of the two samples by integrating the signal for neutron wavelengths beyond the last Bragg edge, to analyze the bulk density properties of the material. This is used to map defects, such as porosities or cracks, which yield a lower density. Second, we obtain strain maps for each of the tomography projections by tracking the wavelength of the strongest Bragg edge corresponding to the {111} lattice plane family. Algebraic reconstruction techniques are used to obtain volumetric 3D maps of the strain in the bulk of the samples. It is found that not only the volume of the sample where the shock peening treatment was carried out yields a higher bulk density, but also a deep and remarkable compressive strain region. Finally, the analysis of the Bragg edge heights as a function of the projection angle is used to describe qualitatively crystallographic texture properties of the samples.Fil: Busi, Matteo. Laboratory for Neutron Scattering and Imaging; SuizaFil: Polatidis, Efthymios. Laboratory for Neutron Scattering and Imaging; SuizaFil: Malamud, Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Kockelmann, Winfried. No especifíca;Fil: Morgano, Manuel. No especifíca;Fil: Kaestner, Anders. Laboratory for Neutron Scattering and Imaging; SuizaFil: Tremsin, Anton. University of California at Berkeley; Estados UnidosFil: Kalentics, Nikola. Ecole Polytechnique Fédérale de Lausanne; SuizaFil: Logé, Roland. Ecole Polytechnique Fédérale de Lausanne; SuizaFil: Leinenbach, Christian. No especifíca;Fil: Shinohara, Takenao. No especifíca;Fil: Strobl, Markus. Laboratory for Neutron Scattering and Imaging; Suiz

Neutron diffraction and diffraction contrast imaging for mapping the TRIP effect under load path change

The transformation induced plasticity (TRIP) effect is investigated during a load path change using a cruciform sample. The transformation properties are followed by in-situ neutron diffraction derived from the central area of the cruciform sample. Additionally, the spatial distribution of the TRIP effect triggered by stress concentrations is visualized using neutron Bragg edge imaging including, e.g., weak positions of the cruciform geometry. The results demonstrate that neutron diffraction contrast imaging offers the possibility to capture the TRIP effect in objects with complex geometries under complex stress states.Fil: Polatidis, Efthymios. Paul Scherrer Institute; SuizaFil: Morgano, Manuel. Paul Scherrer Institute; SuizaFil: Malamud, Florencia. Comision Nacional de Energia Atomica. Gerencia D/area Invest y Aplicaciones No Nucleares. Departamento Haces de Neutrones del Ra10 - Cab.; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Bacak, Michael. Paul Scherrer Institute; SuizaFil: Panzner, Tobias. Paul Scherrer Institute; Suiza. Swissneutronics; SuizaFil: Van Swygenhoven, Helena. Paul Scherrer Institute; Suiza. École Polytechnique Fédérale de Lausanne; SuizaFil: Strobl, Markus. Paul Scherrer Institute; Suiz

Single Crystal to Polycrystal Neutron Simulation

The holy grail for inspection of manufactured parts is being able to place an arbitrary part in a measurement system that generates a 3-D map of grain size, orientation, and strain within the part at 10μm resolution. Current measurement capabilities are far from this ideal and development of models, instruments and algorithms is needed to reach this ideal. Over the past two decades the technique of Bragg-edge neutron transmission along with computed tomography algorithms has materialized as a potential technique to obtain three-dimensional maps within the bulk of materials. To date, these techniques have been applied only to simplistic three-dimensional strains without consideration of texture. In this work, a new approach to modeling Neutron Bragg-edge transmission is investigated. The basic principle of the Bragg-edge transmission technique is the measurement of transmission of cold and thermal neutrons through polycrystalline materials. The spectral signatures of the transmission are based on the sample’s-crystal symmetry, and atomic parameters. The shape, position, and relative magnitude of these Bragg-edge spectral signatures contain information about grain size, grain orientation, and the average strain within the sample that is collinear with the incident beam. The focal point of this thesis is the development of a new neutron Bragg-edge transmission simulation code in which the user can define distributions for grain size, mosaic distribution per grain, grain orientations (texture), and general three-dimensional strain on the grain systems of the sample. A theoretical neutron cross-section calculation for single crystals dependent on crystallographic description of the sample, granular topology, and the strain state of the grain is applied to each crystal in the defined distribution to model the Bragg-edge effect in polycrystalline materials. The cross-section calculation is implemented using the python scripting language and the simulation tool is used to investigate the transmission spectrum of single crystals and polycrystalline materials. In order to verify the transmission spectrum,simulations spectra are compared to neutron transmission taken at the VULCAN instrument at the Spallation Neutron Source. Comparison of the simulation spectra to those found in literature are also presented