On the Relationship Between Ultrasonic and Micro-Structural Properties of Imperfect Interfaces in Layered Solids

The interaction of ultrasonic waves with interfaces formed by two non-conforming, rough surfaces in contact has been the subject of numerous investigations [1–10]. The motivations behind these studies have been various: from the assessment of the real area of contact between two rough surfaces [1], to the modeling of crack closure near the tip of a fatigue crack [4]; from the identification of the nature of interfacial imperfections in kissing and partial bonds [6], to the generation of ultrasonic waves [8]. In most of these studies, the characterization of the interfacial properties has been attempted by studying the reflection of longitudinal and shears waves at normal incidence. Only recently, the problem concerning the interaction of ultrasonic waves with realistic complex systems such as that formed by two neighboring imperfect interfaces has been addressed. Lavrentyev and Rokhlin [9, 10] used ultrasonic spectroscopy to evaluate the interfacial conditions from the spectra of longitudinal and shear waves reflected normally from the interfaces

Interaction of ultrasound with imperfectly contacting interfaces

The need to characterize imperfectly contacting interfaces is encountered in a wide variety of scientific and engineering problems, as illustrated in Fig. 1. Parts (a)–(c) illustrate the stages of diffusion bonding, [1,2] in which the condition of the interface evolves from one of isolated contacts at the initial stages of bonding through one containing distributed micropores, which exist during intermediate stages, to a state in which the material is fully bonded but in which there may be some near-interface microstructural variations. The ability to determine the degree to which bonding has passed through these conditions is presently needed in NDE of products after manufacturing and could be extended to process control if appropriate in-situ sensors were available. Part (d) illustrates the partial contact that can occur during fatigue crack growth [3]. Ideally, one might think of the surfaces of the fatigue crack as being free of stress. However, plastic deformation of ligaments during the failure process, motion of oxide debris, and shearing of the two faces of the crack can all lead isolated regions of contact along the crack face. These contacts are important in the fracture process since they can influence the loads which act on the tip of the crack during fatigue. From the NDE perspective, they can influence the strength of an ultrasonic signal which might be scattered by the crack. Part (e) illustrates a classical problem in tribology. Knowledge of the true area of contact between two surfaces is an essential ingredient in relating macroscopic variables such as applied force to the microscopic force and deformation distributions that exist at the interface [4]. Finally, as shown in part (f), interface design is an important aspect of the development of advanced engineering materials such as composites. It is often desirable to control the mechanical behavior of the interface is such a way that the overall response of the material is optimized, and the presence of pores, precipitates or other impurities may play an important role in controlling this interface behavior. Nondestructive characterization of such interfacial conditions is important in both ensuring the quality of material after fabrication and in assessing the degree to which service induced damage has occurred

The interaction of ultrasound with a partially contacting solid-solid interface in the low frequency regime

When real engineering surfaces touch, contact occurs between the asperities of the surface roughness. For this reason the true area of contact between components can be significantly less than the apparent contact area and the stresses at the asperities are considerably higher than the average (nominal) contact pressure. Measurement of the degree of contact between solids is important in a number of applications such as the design of contacting elements (e.g. gears and bearings) [1] and the detection of ‘kissing’ bonds [2]

Dynamic anoxic ferruginous conditions during the end-Permian mass extinction and recovery

The end-Permian mass extinction, ~252 million years ago, is notable for a complex recovery period of ~5 Myr. Widespread euxinic (anoxic and sulfidic) oceanic conditions have been proposed as both extinction mechanism and explanation for the protracted recovery period, yet the vertical distribution of anoxia in the water column and its temporal dynamics through this time period are poorly constrained. Here we utilize Fe–S–C systematics integrated with palaeontological observations to reconstruct a complete ocean redox history for the Late Permian to Early Triassic, using multiple sections across a shelf-to-basin transect on the Arabian Margin (Neo-Tethyan Ocean). In contrast to elsewhere, we show that anoxic non-sulfidic (ferruginous), rather than euxinic, conditions were prevalent in the Neo-Tethys. The Arabian Margin record demonstrates the repeated expansion of ferruginous conditions with the distal slope being the focus of anoxia at these times, as well as short-lived episodes of oxia that supported diverse biota

Low back pain in 17 year olds has substantial impact and represents an important public health disorder: a cross-sectional study

<p>Abstract</p> <p>Background</p> <p>Prevalence of low back pain (LBP) rises rapidly during adolescence, reaching adult levels by the age of 18. It has been suggested that adolescent LBP is benign with minimal impact, despite limited evidence.</p> <p>Methods</p> <p>The aim of this study was to investigate the impact of LBP and the influence of chronicity, gender and presence of other spinal pain comorbidities at age 17. Subjects (n = 1283) were categorised according to experiencing current and chronic LBP, gender and presence of other areas of spinal pain. LBP impact was ascertained via questions regarding seeking professional assistance, using medication, missing school/work, limited normal or recreational physical activity and health related quality of life (HRQOL).</p> <p>Results</p> <p>12.3% of participants reported current but not chronic LBP, while 19.9% reported current chronic LBP. LBP was more commonly reported by females than males. Other spinal pain comorbidities were common in the LBP groups. Impact was greater in subjects with chronic LBP, in females and in those with other spinal pain comorbidities.</p> <p>Conclusion</p> <p>LBP, and particularly chronic LBP, has a significant negative impact at 17 years. It is commonly associated with care seeking, medication use, school absenteeism, and reduced HRQOL. These findings support that adolescent LBP is an important public health issue that requires attention.</p

Motor control or graded activity exercises for chronic low back pain? A randomised controlled trial

Background: Chronic low back pain remains a major health problem in Australia and around the world. Unfortunately the majority of treatments for this condition produce small effects because not all patients respond to each treatment. It appears that only 25-50% of patients respond to exercise. The two most popular types of exercise for low back pain are graded activity and motor control exercises. At present however, there are no guidelines to help clinicians select the best treatment for a patient. As a result, time and money are wasted on treatments which ultimately fail to help the patient

Shamanistic Shakespeare: Korea's Colonization of Hamlet

"Shamanistic Shakespeare: Korea's Colonization of Hamlet" offers a timely reminder about the dangers of imposing a reformulated national myth on international Shakespeare productions. Focusing on a London performance of Korea’s Yohangza Theatre Company’s shamanized Hamlet, this case study invites far broader consideration of the readability of global Shakespeares, and the cultural competence required by Western audiences to appreciate their political, historical, and local complexity. The Korean theatre industry’s colonizing of Hamlet is traced to a Seoul-based nationalist intellectual agenda to reinvent Korea’s mythic identity after a century of cultural oppression. The chapter demonstrates how this concretizing of shamanic symbolism, packaged for Westernized theatregoing consumption, has created a supposedly authentic Koreanized Shakespeare genre that is confusing to local and global audiences alike

Biodiversity of the Deep-Sea Continental Margin Bordering the Gulf of Maine (NW Atlantic): Relationships among Sub-Regions and to Shelf Systems

Background: In contrast to the well-studied continental shelf region of the Gulf of Maine, fundamental questions regarding the diversity, distribution, and abundance of species living in deep-sea habitats along the adjacent continental margin remain unanswered. Lack of such knowledge precludes a greater understanding of the Gulf of Maine ecosystem and limits development of alternatives for conservation and management. Methodology/Principal Findings: We use data from the published literature, unpublished studies, museum records and online sources, to: (1) assess the current state of knowledge of species diversity in the deep-sea habitats adjacent to the Gulf of Maine (39–43uN, 63–71uW, 150–3000 m depth); (2) compare patterns of taxonomic diversity and distribution of megafaunal and macrofaunal species among six distinct sub-regions and to the continental shelf; and (3) estimate the amount of unknown diversity in the region. Known diversity for the deep-sea region is 1,671 species; most are narrowly distributed and known to occur within only one sub-region. The number of species varies by sub-region and is directly related to sampling effort occurring within each. Fishes, corals, decapod crustaceans, molluscs, and echinoderms are relatively well known, while most other taxonomic groups are poorly known. Taxonomic diversity decreases with increasing distance from the continental shelf and with changes in benthic topography. Low similarity in faunal composition suggests the deep-sea region harbours faunal communities distinct from those of the continental shelf. Non-parametric estimators of species richness suggest a minimum of 50% of the deep-sea species inventory remains to be discovered. Conclusions/Significance: The current state of knowledge of biodiversity in this deep-sea region is rudimentary. Our ability to answer questions is hampered by a lack of sufficient data for many taxonomic groups, which is constrained by sampling biases, life-history characteristics of target species, and the lack of trained taxonomists