Ultrasonic scattering from spherically orthotropic shells

Concerns over the detectability of embrittlement in high strength alloys has led to studying a simple anisotropic shell model [1] for grain boundaries decorated by precipitates, or otherwise enriched by segregated inhomogenieties. In this model the shell is presumed to be “spherically orthotropic,” having five independent elastic constants and symmetry about the origin of a spherical coordinate system. This structure is analogous to transversely isotropic materials in a Cartesian coordinate system. By studying ultrasonic scattering from such shells (embedded in an isotropic host, and surrounding an isotropic core), we hope to learn whether their presence could be detected, and differentiated from scattering due to the inherent anisotropy of single metal crystals [2,3]

Precision Ultrasonic Thickness Measurements of Thin Steel Disks

The accurate in-situ measurement of part dimensions during fabrication is of much interest to the manufacturing industry, especially for untended manufacturing. The goal of this work is to apply non-contacting ultrasonic techniques to the precise thickness measurement, during machining, of metal parts of rotation having a nominal wall thickness of 1.5 mm. The desired accuracy is ±.0025 mm at all points on the approximately 200 mm diameter steel shells, where part access is restricted to one side at a time for the measurement. In a feasibility study, dimensional information using eddy current techniques was overwhelmed by conductivity variations in the 304-stainless steel samples [1]. The approach here is to precisely measure ultrasonic echo transit times, and calulate part dimensions, knowing the material sound speed. To that end, feasibility results on flat disk specimens possessing a wide range of grain sizes representative of the shell’s variable metallurgy are reported here. Factors affecting ultrasonic dimensional precision including grain size, texture, sample temperature and surface roughness are discussed, with an emphasis on precision limitations due to finite grain sizes in thin parts. Both longitudinal (10 to 30 MHz) and shear (3 MHz) wave measurements were made, the latter using electromagnetic acoustic transducers (EMATS).</p

Ultrasonic Sensing of Porous Granular Media

Emerging high temperature materials such as intermetallic alloys and composites are intrinsically brittle and cannot be either processed or machined by conventional methods. Near net shape processing (of rapidly solidified powders and plasma sprayed foils) using hot isostatic or vacuum hot pressing has recently emerged as a promising method for overcoming these problems. Interestingly, these consolidation processes determine both the component’s final shape and its mechanical properties (which depend on relative density, grain size, etc.). Thus a need has emerged for the control of mechanical properties (1,2)

Issues in the High Resolution Acoustoelastic Measurement of Stress

The acoustoelastic measurement of stress is a topic with a rich history and the basic principles are well known [1]. In summary, one takes advantage of various nonlinearities which govern the elastic response of a solid, including but not limited to anharmonicities in interatomic forces, which lead to a stress dependence of the ultrasonic velocity. The basic idea, then is to precisely measure the velocity and to infer stress from a relation of the form V=Vo+Kσ where V is the measured velocity in the presence of a stress σ, Vo is the value that would have been observed in the absence of that stress, and K is known as the acoustoelastic constant.</p

Theoretical Study of High Frequency Ultrasonic Wave Attenuation in Polycrystalline Materials

Three different regimes for scattering of ultrasonic waves in poly-crystalline materials exist, depending on the ratio of the mean grain size to the wavelength: (i) the low frequency (Rayleigh) region with scattering-induced attenuation proportional to the fourth power of the frequency and to the cube of the mean grain diameter, (ii) the medium frequency (stochastic) region with scattering proportional to the square of the frequency and to the mean grain diameter, and (iii) the high-frequency (geometric) region with scattering independent of frequency

Influence of Columnar Microstructure on Ultrasonic Backscattering

Most structural materials are polycrystalline, that is, they are composed of numerous discrete grains, each having a regular, crystalline atomic structure. The elastic properties of the grains are anisotropic and their crystallographic axes are differently oriented. When an ultrasonic wave propagates through such a polycrystalline aggregate, it is scattered at the grain boundaries. The fraction of sound energy thus removed from the main beam is responsible for important phenomenons like attenuation and dispersion of the main beam, and background “noise” associated with a given ultrasonic inspection system. The amount of sound energy removed from the main beam depends on the size, shape, and orientation distributions of the grains. If the grains are equiaxed and randomly oriented, propagation direction of the ultrasonic wave has no effect upon the magnitude of the scattered energy. Such is not the case when an acoustic wave travels through materials like centrifugally cast stainless steel and austenitic stainless steel welds, which are used extensively in nuclear power plants. The microstructures of these stainless steels vary from randomly oriented, equiaxed grains to highly oriented, columnar grains.1,2 Since the backscattered signals tend to mask the signals from small and subtle defects, the estimation of probability of detection of such defects requires quantitative description of these signals. Consequently, an effort has been undertaken in this research to quantify the backscattered signals from microstructures with favored grain orientation and grain elongation

Observation and Interpretation of Microstructurally Induced Fluctuations of Back-Surface Signals and Ultrasonic Attenuation in Titanium Alloys

During ultrasonic inspection for flaws in engineering materials, it is important to understand the interactions between the inspecting beam and the microstructure in which flaws are embedded. It has been found that in certain materials such interactions can have dramatic effects on the characteristics of the beam as it propagates to and from a flaw and consequently can have deleterious effects on both flaw characterization and the probability of detection. It is well known that, the microstructure can backscatter energy, creating noise which can mask small flaws. In addition, a flaw signal can be attenuated by the removal of energy from the beam by absorption and scattering. Considerable progress has been made towards developing a theoretical understanding of these phenomena. For example, backscattered grain noise has been successfully modeled by Han and Thompson [1] for duplex microstructures that commonly occur in Ti-17 and Ti-6A1-4V alloys used in the rotating components of aircraft engines. In addition, attenuation has been modeled for randomly oriented, equiaxed, cubic microstructures [2], for textured, equiaxed, cubic, stainless-steel [3], and also for elongated textured microstructures [4]

The role of recombination in the emergence of a complex and dynamic HIV epidemic

<p>Abstract</p> <p>Background</p> <p>Inter-subtype recombinants dominate the HIV epidemics in three geographical regions. To better understand the role of HIV recombinants in shaping the current HIV epidemic, we here present the results of a large-scale subtyping analysis of 9435 HIV-1 sequences that involve subtypes A, B, C, G, F and the epidemiologically important recombinants derived from three continents.</p> <p>Results</p> <p>The circulating recombinant form CRF02_AG, common in West Central Africa, appears to result from recombination events that occurred early in the divergence between subtypes A and G, followed by additional recent recombination events that contribute to the breakpoint pattern defining the current recombinant lineage. This finding also corrects a recent claim that G is a recombinant and a descendant of CRF02, which was suggested to be a pure subtype. The BC and BF recombinants in China and South America, respectively, are derived from recent recombination between contemporary parental lineages. Shared breakpoints in South America BF recombinants indicate that the HIV-1 epidemics in Argentina and Brazil are not independent. Therefore, the contemporary HIV-1 epidemic has recombinant lineages of both ancient and more recent origins.</p> <p>Conclusions</p> <p>Taken together, we show that these recombinant lineages, which are highly prevalent in the current HIV epidemic, are a mixture of ancient and recent recombination. The HIV pandemic is moving towards having increasing complexity and higher prevalence of recombinant forms, sometimes existing as "families" of related forms. We find that the classification of some CRF designations need to be revised as a consequence of (1) an estimated > 5% error in the original subtype assignments deposited in the Los Alamos sequence database; (2) an increasing number of CRFs are defined while they do not readily fit into groupings for molecular epidemiology and vaccine design; and (3) a dynamic HIV epidemic context.</p

Corrigendum: An ethnically relevant consensus Korean reference genome is a step towards personal reference genomes.

This corrects the article DOI: 10.1038/ncomms13637