Henry J. Prask

Measurement of residual stress in plasma-sprayed metallic, ceramic and composite coatings

Abstract Residual stresses in plasma-sprayed coatings were studied by three experimental techniques: curvature measurements, neutron diffraction and X-ray diffraction. Two distinct material classes were investigated: (1) single-material coatings (molybdenum) and (2) bi-material composites (nickel+alumina and NiCrAlY+yttria-stabilized zirconia), with and without graded layers. This paper deals with the effects of coating thickness and material properties on the evolution of residual stresses as a function of composition and thickness in both homogeneous and graded coatings. Mathematical analysis of the results allowed in some cases the separation of the quenching stress and thermal stress contributions to the final residual stress, as well as the determination of the through-thickness stress profile from measurements of different thickness specimens. In the ceramic–metal composites, it was found that the quenching stress plays a dominant role in the metallic phase, whereas the stress in the ceramic phase is mostly dominated by thermal mismatch. The respective thermal expansion coefficients and mechanical properties are the most important factors determining the stress sign and magnitude. The three residual stress measurement methods employed here were found to be complementary, in that each can provide unique information about the stress state. The most noteworthy outcomes are the determination of the through-thickness stress profile in graded coatings with high spatial resolution (curvature method) and determination of stress in each phase of a composite separately (neutron diffraction).

Quenching, thermal and residual stress in plasma sprayed deposits: NiCrAlY and YSZ coatings

Residual stress is an important factor in thermally sprayed deposits which affects both processing and performance. High stress magnitudes and/or concentrations can undermine the structural integrity of sprayed parts and impair their functionality. Therefore, it is important to know the stress state, understand its generation and be able to control it. Results of experimental stress determination in plasma sprayed deposits are presented. Neutron diffraction as a non-destructive and phase-distinctive measurement method was used to determine residual stress profiles in thick NiCrAlY and yttria-stabilized zirconia (YSZ) deposits. Measurements were complemented with calculations based on experimentally determined material properties, which allowed for separation of quenching and thermal stress contributions to final residual stress. Since the application of neutron diffraction to plasma sprayed deposits is relatively novel, certain verification measurements were performed. Specimens were prepared at two different deposition temperatures, to determine the effect of temperature on the stresses and relevant deposit properties.

Calculation of Single-Crystal Elastic Constants for Cubic Crystal Symmetry from Powder Diffraction Data

In this work a method is developed that allows the computation of the single-crystal elastic constants for crystals of cubic symmetry from the diffraction elastic constants. The diffraction elastic constants can be obtained by measuring the hkl-dependent lattice strain response to an applied stress. Because of their hkl dependence they represent, partially, the anisotropic nature of the single-crystal elastic constants. The computation of the single-crystal elastic constants is carried out by a least-squares refinement which fits the calculated diffraction elastic constants to the measured ones.

Multiwafer focusing neutron monochromators and applications

Multi-wafer silicon monochromators for neutron focusing instruments have been developed at MURR. A first unit, made from commercial thin silicon [100] wafers with manual control of horizontal curvature was designed and fabricated for the MURR stress machine. It was tested on the stress machine at the HFIR reactor at ORNL. A second similar unit but with stepper motor control of curvature was installed on the NIST stress machine. Both confirmed expectations, with significant intensity gains at equal or better resolution in comparison with the monochromators they replaced. A third unit with two back-to-back assemblies of non-standard silicon wafers custom sliced obliquely from big [100] ingots has been fabricated for an upgrade of the ORNL stress machine. The phase space analysis of the neutron optics of multi-wafer assemblies has revealed exciting new possibilities for applications. The correlation between the coordinates of real space and wavevector space allows a new type of focusing, the thickness focusing. The many wafers in a packet can be made to look as a single wafer when seen from a given point of a position sensitive detector (PSD). This allows high resolutions in scattering, corresponding to a bent thin single wafer, at intensities given by the whole packet, that is comparable with pyrolytic graphite crystals. One can thus have the best of two worlds - but only in PSD instruments. A whole array of new applications becomes possible, including dispersive and non-dispersive neutron imaging at the spatial resolution of a single thin wafer. Some of these applications are discussed and demo experiments are presented.

Residual stress measurements at the NIST reactor

Abstract A neutron-diffraction residual-stress measurement program has been in place for several years at the National Institute of Standards and Technology (NIST). Very recently a new, dedicated, state-of-the-art diffractometer has been installed at the NIST reactor. With it, the residual stress fields in a variety of engineering specimens have been determined. These include slices of railroad rails, weldments in steel plates and plastically bent steel pipes. Features of the new instrument and various experimental results will be discussed.

Residual stresses in girth welds of carbon steel pipes: neutron diffraction analysis

Abstract In the oil and gas industry, residual stresses in welded components, such as risers and oil offloading lines, play a key role in determining their fatigue and fracture performance. The focus of this work is on measuring the residual stresses in girth welds of large diameter (508 mm, 20 in. outside diameter) thick walled (25·4 mm, 1 in. wall thickness) pipes. Residual stresses were measured using neutrons on two girth welds on X-65 grade line pipes. One was a carbon steel weld with fully ferritic weld metal, and the second was an Inconel 625 dissimilar weld with fully austenitic weld metal. It was observed that while there were subtle differences, the overall magnitude and distribution of axial and hoop residual stresses were comparable in the two girth welds. These data are consistent with available previous measurements and computer models from the literature.

A monolithic polycapillary focusing optic for polychromatic neutron diffraction applications

We have conducted measurements at five different thermal neutron wavelengths to determine the transmission characteristics of a tapered monolithic focusing lens with a focal length of 100 mm, suitable for time-of-flight diffraction. Both the width of the focused beam and the intensity gain of the optic increase as a function of wavelength. We have performed similar measurements on a polychromatic beam on a pulsed neutron source, where the results are subject to background from short wavelength neutrons. The use of a beryllium filter shows the increased effective gain for the longer wavelengths at the expense of an increased focused beam width by a factor of 2.

Measurement and Calculation of Elastic Properties in Low Carbon Steel Sheet

Low carbon steel (usually in sheet form) has found a wide range of applications in industry due to its high formability. The inner and outer panels of a car body are good examples of such an implementation. While low carbon steel has been used in this application for many decades, a reliable predictive capability of the forming process and “springback” has still not been achieved. NIST has been involved in addressing this and other formability problems for several years. In this paper, texture produced by the in-plane straining and its relationship to springback is reported. Low carbon steel sheet was examined in the as-received condition and after balanced biaxial straining to 25%. This was performed using the Marciniak in-plane stretching test. Both experimental measurements and numerical calculations have been utilized to evaluate anisotropy and evolution of the elastic properties during forming. We employ several techniques for elastic property measurements (dynamic mechanical analysis, static four point bending, mechanical resonance frequency measurements), and several calculation schemes (orientation distribution function averaging, finite element analysis) which are based on texture measurements (neutron diffraction, electron back scattering diffraction). The following objectives are pursued: a) To test a range of different experimental techniques for elastic property measurements in sheet metals; b) To validate numerical calculation methods of the elastic properties by experiments; c) To evaluate elastic property changes (and texture development) during biaxial straining. On the basis of the investigation, recommendations are made for the evaluation of elastic properties in textured sheet metal.

Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions

Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright

Residual-stress determination by means of neuron diffraction

Neutron diffraction is becoming an increasingly important tool for the determination of triaxial residual stress distributions for materials science and engineering applications. Indeed, virtually all of the major neutron centers in the world have, or are planning, programs in this area. In this paper the basic principles of the technique will be described, along with advantages and limitations relative to other nondestructive residual stress measurement methods. Examples of the application of the technique to sub-surface residual stress determination in metals and composites will be presented, along with some near-surface results. The significant features of new neutron instruments for residual stress study will be summarized.