Dayakar Penumadu

Mechanical properties of blended single-wall carbon nanotube composites

The improvement in mechanical properties of blended nanocomposites prepared using a low-viscosity, liquid epoxy resin and purified single-wall carbon nanotubes (SWCNTs) was evaluated. The macroscopic tensile stress–strain behavior for hybrid materials made with varying amounts of SWCNT was determined and showed little improvement in the breaking tensile strength. The corresponding variations in modulus and hardness were obtained using nanoindentation considering time effects and showed quantifiable but modest improvements. The small changes in the observed stiffness and breaking strength of carbon nanotube composites is due to the formation of bundles and their curvy morphology.

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-imaging for strain mapping under in situ tensile loading

Wavelength selective neutron radiography at a cold neutron reactor source was used to measure strain and determine (residual) stresses in a steel sample under plane stress conditions. We present a new technique that uses an energy-resolved neutron imaging system based on a double crystal monochromator and is equipped with a specially developed (in situ) biaxial load frame to perform Bragg edge based transmission imaging. The neutron imaging technique provides a viewing area of 7 cm by 7 cm with a spatial resolution on the order of ∼ 100 μm. The stress-induced shifts of the Bragg edge corresponding to the (110) lattice plane were resolved spatially for a ferritic steel alloy A36 (ASTM international) sample. Furthermore it is demonstrated that results agree with comparative data obtained using neutron diffraction and resistance based strain-gauge rosettes.

High-Resolution Neutron and X-Ray Imaging of Granular Materials

AbstractHigh spatial resolution (∼13.7 mm/pixel) neutron tomography was performed on partially water-saturated compacted silica sand specimens with two different grain morphologies (round and angular) at Helmholtz Zentrum Berlin using cold neutrons at the cold neutron radiography and tomography beam line. A specimen mixed with heavy water was imaged for contrast comparison purposes. Microfocus X-ray imaging was also performed on these specimens with slightly higher resolution (∼11.2 mm/pixel) using geometric magnification to locate the solid phase (silica particle boundaries) more precisely. Image processing was performed to remove unwanted gammas detected because of the gadox scintillator used for the high-resolution neutron imaging system. The visualization of solid, gas, and liquid phases for different grain morphologies is presented at the grain level. Using dual-modal contrast possible from simultaneous use of neutrons and X-rays, the authors introduce, for the first time, an improved ability to dist...

Revealing microstructural inhomogeneities with dark-field neutron imaging

Dark-field neutron tomography was applied to obtain three-dimensional volumetric data representing the distribution of micrometer and submicrometer sized structures in bulk samples. This is a size range that complements the range of direct spatial resolution. A phase grating setup implemented in a conventional imaging instrument enables corresponding tomographic investigations on reasonable time scales. Different samples were investigated and demonstrate the applicability of the method for the investigation of structural materials. Local variations in the small-angle scattering in BiSn, AlSi, and aluminum samples were mapped and the results are discussed with respect to the contrast formation of the method.

Mechanical properties and creep behavior of lyocell fibers by nanoindentation and nano-tensile testing

Abstract Mechanical and time-dependent mechanical properties of lyocell fibers have been investigated as a function of depth at a nano-scale level in longitudinal and transverse directions. The nanoindentation technique was applied and extended to continuous stiffness measurement. Lyo10 and Lyo13 lyocell fibers were investigated. The individual fiber properties were measured using a nano-tensile testing system to obtain reference data for mechanical properties. The hardness and elastic modulus obtained from nanoindentation test are described using two different approaches. The first uses mean values for a depth of 150–300 nm, while the second uses unloading values at the final indentation depth. There is no significant difference between modulus values inferred from nanoindentation and those obtained from single fiber tensile testing. Hardness and elastic modulus values were higher in the longitudinal direction than those in the transverse direction and Lyo13 values were higher than those for Lyo10 in both directions. The time-dependent mechanical properties were also investigated as a function of the holding time. Increasing the holding time led to an increase in indentation displacement and a decrease in hardness. Stress exponents were calculated from the linear relationship between contact stress and contact strain using a power-law creep equation.

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.

Transmission Bragg edge spectroscopy measurements at ORNL Spallation Neutron Source

Results of neutron transmission Bragg edge spectroscopic experiments performed at the SNAP beamline of the Spallation Neutron Source are presented. A high resolution neutron counting detector with a neutron sensitive microchannel plate and Timepix ASIC readout is capable of energy resolved two dimensional mapping of neutron transmission with spatial accuracy of ~55 μm, limited by the readout pixel size, and energy resolution limited by the duration of the initial neutron pulse. A two dimensional map of the Fe 110 Bragg edge position was obtained for a bent steel screw sample. Although the neutron pulse duration corresponded to ~30 mA energy resolution for 15.3 m flight path, the accuracy of the Bragg edge position in our measurements was improved by analytical fitting to a few mA level. A two dimensional strain map was calculated from measured Bragg edge values with an accuracy of ~few hundreds μistrain for 300s of data acquisition time.

Thermal Neutron Scintillator Detectors Based on Poly (2-Vinylnaphthalene) Composite Films

A series of novel 6Li-loaded plastic scintillation films have been designed and fabricated to detect thermal neutrons. Organolithium salts containing enriched 6Li were synthesized and interspersed in a series of matrices comprising a polymer doped with an antenna fluor. Thermal neutron capture by 6Li produces charged particles with kinetic energy which is sufficient to ionize and excite the polymeric matrix. This energy is collected by the antenna fluor, which produces a photonic response detectable by a photomultiplier tube. Design and optimization of these scintillation films is discussed herein. A current problem in the design and fabrication of polymeric composite materials is phase separation. The matrix is a low-dielectric aromatic polymer; hence, ionic molecules in the composite tend to phase-separate from the matrix and produce agglomerates which decrease the quantum efficiency by scattering and/or quenching the photonic response to thermal neutrons. Based on the experimental results, the importance of synthesizing polymers with high quantum yield and efficiency in energy migration and transport for making effective composite neutron scintillators is emphasized.

Neutron Scattering for Moisture Detection in Foamed Asphalt

AbstractFoamed warm-mix asphalt (WMA) has been widely accepted and used in the United States and many other countries around the world. However, several key concerns about WMA technology still need to be answered, including the major issue of moisture-induced damage. Because of the reduced production temperatures and the foaming process with water, moisture may be entrapped in pavements after compaction. The trapped moisture decreases the adhesion between asphalt binder and aggregates and the cohesion among asphalt binder, resulting in stripping and other forms of pavement distress. The neutron scattering technique provides a unique tool for the determination of the microscopic structure of asphalt and for the detection of the presence of moisture and its spatial distributions in asphalt. In particular, small-angle neutron scattering (SANS) in the wave vector transfer range from 0.003−0.5  A−1 is suitable to probe the spatial density fluctuations in the real space from 200−1  nm, which has a resolution se...