Jeffrey R. Bunn

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.

Mechanical characterization of an additively manufactured Inconel 718 theta-shaped specimen

Two sets of “theta”-shaped specimens were additively manufactured with Inconel 718 powders using an electron beam melting technique with two distinct scan strategies. Light optical microscopy, mechanical testing coupled with a digital image correlation (DIC) technique, finite element modeling, and neutron diffraction with in situ loading characterizations were conducted. The cross-members of the specimens were the focus. Light optical micrographs revealed that different microstructures were formed with different scan strategies. Ex situ mechanical testing revealed each build to be stable under load until ductility was observed on the cross-members before failure. The elastic moduli were determined by forming a correlation between the elastic tensile stresses determined from FEM, and the elastic strains obtained from DIC. The lattice strains were mapped with neutron diffraction during in situ elastic loading; and a good correlation between the average axial lattice strains on the cross-member and those determined from the DIC analysis was found. The spatially resolved stresses in the elastic deformation regime are derived from the lattice strains and increased with applied load, showing a consistent distribution along the cross-member.

Characterization of Crystallographic Structures Using Bragg-Edge Neutron Imaging at the Spallation Neutron Source

Over the past decade, wavelength-dependent neutron radiography, also known as Bragg-edge imaging, has been employed as a non-destructive bulk characterization method due to its sensitivity to coherent elastic neutron scattering that is associated with crystalline structures. Several analysis approaches have been developed to quantitatively determine crystalline orientation, lattice strain, and phase distribution. In this study, we report a systematic investigation of the crystal structures of metallic materials (such as selected textureless powder samples and additively manufactured (AM) Inconel 718 samples), using Bragg-edge imaging at the Oak Ridge National Laboratory (ORNL) Spallation Neutron Source (SNS). Firstly, we have implemented a phenomenological Gaussian-based fitting in a Python-based computer called iBeatles. Secondly, we have developed a model-based approach to analyze Bragg-edge transmission spectra, which allows quantitative determination of the crystallographic attributes. Moreover, neutron diffraction measurements were carried out to validate the Bragg-edge analytical methods. These results demonstrate that the microstructural complexity (in this case, texture) plays a key role in determining the crystallographic parameters (lattice constant or interplanar spacing), which implies that the Bragg-edge image analysis methods must be carefully selected based on the material structures.

Residual Stress Analysis in Girth-welded Ferritic and Austenitic Steel Pipes Using Neutron and X-Ray Diffraction

This paper is dedicated to the thorough experimental analysis of the residual stresses in the vicinity of tubular welds and the mechanisms involved in their formation. Pipes made of a ferriticpearlitic structural steel and an austenitic stainless steel are each investigated in this study. The pipes feature a similar geometry and are welded with two passes and comparable parameters. Residual strain mappings are carried out using X-ray and neutron diffraction. The combined use of both techniques permits both near-surface and through-wall analyses of the residual stresses. The findings allow for a consistent interpretation of the mechanisms accounting for the formation of the residual stress fields due to the welding process. Since the results are similar for both materials, it can be concluded that residual stresses induced by phase transformations, which can occur in the structural steel, play a minor role in this regard. Introduction Current fatigue design standards and recommendations, like the ones given by the International Institute of Welding (IIW) [1], are based on the assumption of yield strength magnitude tensile residual stresses if the actual residual stress state is unknown. This postulate reflects uncertainties about the initial residual stress state after welding, which may depend on numerous parameters, as well as about the possible relaxation of residual stresses, which occurs when the static or cyclic yield strength is exceeded locally. Therefore, research concerning both the development and the relaxation of residual stresses is needed to improve the generalized approach given in [1]. Experimental and numerical analyses of welding residual stresses hardly ever show that the conservative assumptions made in the IIW recommendations hold. In girth-welded pipes, the pipe geometry and the heat input have been identified as the governing factors for the residual stress development [2,3], apart from material parameters. Under suitable conditions, pipe wall bending can occur, leading to compressive axial residual stresses at the weld toe of girth welds, but also to tensile residual stresses at the weld root, which was shown by other authors, see e.g. [2,3], and in previous studies on ferritic-pearlitic and austenitic steel pipes using X-ray diffraction [4-6]. In this work, these will be supplemented by neutron diffraction measurements and the results obtained from the two different steels will be compared. The thorough experimental analysis of the residual stress state after welding will serve as a basis for the investigation of residual stress relaxation under loading and the validation of numerical simulations, both of which are subject of future work. Experimental work Sample preparation. Pipes of the ferritic-pearlitic structural steel S355J2H+N and of the austenitic stainless steel X6CrNiTi18-10 were used for the experiments. The yield stress of the base Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 229-234 doi: http://dx.doi.org/10.21741/9781945291173-39 230 materials is 355 MPa and 223 MPa, respectively. The tubular specimens of 200 mm length were machined on all surfaces, resulting in outer diameters of 100.5 mm and 114 mm and wall thicknesses of 7.75 mm and 7.5 mm for the structural steel and the austenitic steel, respectively. A v-shaped groove was introduced at half-length as a weld preparation. Before welding, the specimens of the structural steel were stress relieved thermally at 600 °C for 30 minutes and cooled uniformly at about 1 °C/min. Metal active gas (MAG) welding was performed in flat position using a rotary table. The filler metal, ISO 14341-A-G 4Si1 for the structural steel and ISO 14343-A-G 19 9 NbSi for the austenitic steel, was applied in two passes, which were started at the same point and were welded in the same direction. The nominal energy inputs were similar for the structural and the austenitic steel, with 8.6 kJ/cm and 9.1 kJ/cm for the root pass and 11.8 kJ/cm and 11.2 kJ/cm for the second pass, respectively. Each pass was welded at room temperature. Residual Stress Analysis. The residual stresses in the welded samples were determined using Xray diffraction (XRD) and neutron diffraction (ND) in order to analyze the stress state both at the surface and within the pipe wall. The measurements were taken at points along lines perpendicular to the welding direction at φ = 90°, where φ is the circumferential angle marking the welding direction and the start/stop location at φ = 0°. As previous work revealed virtually axisymmetric residual stress states [4-6], φ = 90° is considered to be representative. Due to the symmetry, measurements were only performed on one side of the weld centerline up to a distance of 60 mm. The coordinate x specifies the axial distance of a certain point from the weld centerline. The hoop and axial residual stresses on the surfaces of the pipes were determined by XRD. The inner surfaces were only accessible after sectioning the tubes, the released stresses being monitored by strain gauge measurements. For details concerning the XRD measurements, please

Residual stress evaluation of components produced via direct metal laser sintering

Direct metal laser sintering is an additive manufacturing process which is capable of fabricating three-dimensional components using a laser energy source and metal powder particles. Despite the numerous benefits offered by this technology, the process maturity is low with respect to traditional subtractive manufacturing methods. Relationships between key processing parameters and final part properties are generally lacking and require further development. In this study, residual stresses were evaluated as a function of key process variables. The variables evaluated included laser scan strategy and build plate preheat temperature. Residual stresses were measured experimentally via neutron diffraction and computationally via finite element analysis. Good agreement was shown between the experimental and computational results. Results showed variations in the residual stress profile as a function of laser scan strategy. Compressive stresses were dominant along the build height (z) direction, and tensile stresses were dominant in the x and y directions. Build plate preheating was shown to be an effective method for alleviating residual stress due to the reduction in thermal gradient.

Current capabilities of the residual stress diffractometer at the high flux isotope reactor

The engineering diffractometer 2nd Generation Neutron Residual Stress Facility (NRSF2) at the Oak Ridge National Laboratorys High Flux Isotope Reactor was built specifically for the mapping of residual strains. NRSF2 is optimized to investigate a wide range of engineering materials by providing the user a selection of monochromatic neutron wavelengths to maintain the selected Bragg reflection near 2θ = 90°, which is the optimal scattering geometry for strain mapping. Details of the instrument configuration and operation are presented, and considerations for experimental planning are also discussed. Selected examples of recent residual stress work completed with NRSF2 are presented to highlight capabilities.

Residual stress determination of direct metal laser sintered (DMLS) inconel specimens and parts

ORNL collaborated with Honeywell Aerospace to determine the residual stresses within parts manufactured by Honeywell. The project demonstrated the feasibility of neutron residual stress measurements in complex parts providing one method of feedback to change Honeywells direct metal laser sintering (DMLS) process. The improved understanding of residual stress related to the DLMS process will expedite the implementation of DLMS at Honeywell Aerospace. Background Honeywell Aerospace is creating components using DMLS, an additive manufacturing process, because of the savings in time and cost relative to casting processes. Because the process is relatively new, testing, modeling and validation are required to assure this process can produce components that meet the design specifications.

Path length dependent neutron diffraction peak shifts observed during residual strain measurements in U–8 wt% Mo castings

This study reports an angular diffraction peak shift that scales linearly with the neutron beam path length traveled through a diffracting sample. This shift was observed in the context of mapping the residual stress state of a large U–8 wt% Mo casting, as well as during complementary measurements on a smaller casting of the same material. If uncorrected, this peak shift implies a non-physical level of residual stress. A hypothesis for the origin of this shift is presented, based upon non-ideal focusing of the neutron monochromator in combination with changes to the wavelength distribution reaching the detector due to factors such as attenuation. The magnitude of the shift is observed to vary linearly with the width of the diffraction peak reaching the detector. Consideration of this shift will be important for strain measurements requiring long path lengths through samples with significant attenuation. This effect can probably be reduced by selecting smaller voxel slit widths.

Tensile Residual Stress Mitigation Using Low Temperature Phase Transformation Filler Wire in Welded Armor Plates

Hydrogen induced cracking (HIC) has been a persistent issue in welding of high-strength steels. Mitigating residual stresses is one of the most efficient ways to control HIC. The current study develops a proactive in-process weld residual stress mitigation technique, which manipulates the thermal expansion and contraction sequence in the weldments during welding process. When the steel weld is cooled after welding, martensitic transformation will occur at a temperature below 400 C. Volume expansion in the weld due to the martensitic transformation will reduce tensile stresses in the weld and heat affected zone and in some cases produce compressive residual stresses in the weld. Based on this concept, a customized filler wire which undergoes a martensitic phase transformation during cooling was developed. The new filler wire shows significant improvement in terms of reducing the tendency of HIC in high strength steels. Bulk residual stress mapping using neutron diffraction revealed reduced tensile and compressive residual stresses in the welds made by the new filler wire.