Detection of Closed Internal Fatigue Cracks

This paper reviews some recent work on the detection and sizing of closed internal fatigue cracks by ultrasonic techniques. Major emphasis is put on the diffraction of shear waves at the crack tip. Both fully open as well as partially closed cracks were considered. The effect of crack closure stress on back- scattered (pulse-echo) shear waves was studied with the aid of an A1 compact tension specimen. Noticeable changes with crack closure stress were documented for the structure of both the time- domain and frequency-domain representations. The techniques acquired with this specimen were applied to the study of a 50 μm radius semi-circular crack internal to a diffusion bonded Ti-alloy plate. Improved signal processing techniques were employed to detect the crack and to distinguish it from an artificial surface crack. The probability of detection, assumed to be proportional to the signal-to-noise ratio, was measured as a function of crack interrogation angle and crack closure stress to provide data on optimum probability for detection and sizing. Vigorous research efforts on good models for closed cracks in specific materials and environments are needed to refine the techniques of detection probability.</p

Scattering Of Obliquely Incident Rayleigh Waves by a Surface-Breaking Crack

Recent results on reflection, transmission and scattering of obliquely incident Rayleigh surface waves by an infinitely long surface-breaking crack are reviewed. Sets of crack-opening displacements for infinitely long cracks with various depths are used to construct approximate crack-opening fields for the scattering of a Rayleigh wave by a surface-breaking crack of large length-to-depth ratio. The scattered surface-wave fields for the finite-length crack are subsequently obtained by the use of a representation integral over an appropriate Green’s function and the approximate crack-opening displacements. Polar diagrams are presented for the amplitude of the scattered surface-wave field

Degradation of haloaromatic compounds

An ever increasing number of halogenated organic compounds has been produced by industry in the last few decades. These compounds are employed as biocides, for synthetic polymers, as solvents, and as synthetic intermediates. Production figures are often incomplete, and total production has frequently to be extrapolated from estimates for individual countries. Compounds of this type as a rule are highly persistent against biodegradation and belong, as "recalcitrant" chemicals, to the class of so-called xenobiotics. This term is used to characterise chemical substances which have no or limited structural analogy to natural compounds for which degradation pathways have evolved over billions of years. Xenobiotics frequently have some common features. e.g. high octanol/water partitioning coefficients and low water solubility which makes for a high accumulation ratio in the biosphere (bioaccumulation potential). Recalcitrant compounds therefore are found accumulated in mammals, especially in fat tissue, animal milk supplies and also in human milk. Highly sophisticated analytical techniques have been developed for the detection of organochlorines at the trace and ultratrace level

Issues in bilingual aphasia: An introduction

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Mucous granule exocytosis and CFTR expression in gallbladder epithelium

A mechanistic model of mucous granule exocytosis by columnar epithelial cells must take into account the unique physical-chemical properties of mucin glycoproteins and the resultant mucus gel. In particular, any model must explain the intracellular packaging and the kinetics of release of these large, heavily charged species. We studied mucous granule exocytosis in gallbladder epithelium, a model system for mucus secretion by columnar epithelial cells. Mucous granules released mucus by merocrine exocytosis in mouse gallbladder epithelium when examined by transmission electron microscopy. Spherules of secreted mucus larger than intracellular granules were noted on scanning electron microscopy. Electron probe microanalysis demonstrated increased calcium concentrations within mucous granules. Immunofluorescence microscopic studies revealed intracellular colocalization of mucins and the cystic fibrosis transmembrane conductance regulator (CFTR). Confocal laser immunofluorescence microscopy confirmed colocalization. These observations suggest that calcium in mucous secretory granules provides cationic shielding to keep mucus tightly packed. The data also suggests CFTR chloride channels are present in granule membranes. These observations support a model in which influx of chloride ions into the granule disrupts cationic shielding, leading to rapid swelling, exocytosis and hydration of mucus. Such a model explains the physical-chemical mechanisms involved in mucous granule exocytosis.link_to_subscribed_fulltex

Fetal and neonatal intestines: Allografts with high resistance to ischemia and reperfusion injury

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Social sustainability, sustainable development and housing development: the experience of Hong Kong

In the experimental research of ultrasonic detection and characterization of cracks, laboratory specimens are indispensible. For surface-breaking cracks, the usual method for metals is fatigue growth and for brittle materials an indentor may be used to initiate the crack. The fabrication of interior crack specimen is generally more difficult. Diffusion bonding has been used in producing specimens containing simulated interior cracks and thin cracks have also been simulated by including yttria power between the diffusion bond surfaces.1 We report here a technique for inducing interior fracture cracks in transparent plastic resin with focused laser energy. Such specimens are useful for ultrasonic characterization of crack parameters such as size and orientation. This technique has the following advantages: (i) It is quite easy to implement; (ii) The induced damages are usually flat circular cracks which closely approximate the “penny-shaped crack” — the subject of many experimental and theoretical studies for ultrasonic scattering;2 and (iii) This method provides a variety of crack size and orientation for ultrasonic sizing experiments and the results may be compared with direct optical microscopic measurements

Uncertain Data Mining: An Example in Self-Organizing Maps

Laser-based ultrasound (LBU) has a number of distinct advantages compared with conventional contact piezoelectric transducer systems. The LBU technique is noncontacting and the laser beams conform to the part surface so that couplant nonuniformities and the need for maintaining normality to, and fixed distance from, the part are eliminated. The LBU technique thus potentially allows rapid, and close to 100%, inspections of complexly contoured surfaces. Various laser techniques used to generate and receive ultrasonic waves have been reviewed by several authors [1–2]. While the generation and detection procedures employed by LBU are distinctly different from systems employing piezoelectric transducers, once ultrasound is generated by a laser, it propagates into a material and interacts with defects in precisely the same way. Thus, laser-based ultrasound can provide inspection resolution comparable to that which can be obtained using commercially available ultrasonic squirter systems, which require water coupling to the part