Horizontal Cross-section Research Articles

Effect of Segregation of Secondary Phase Particles and “S” Line on Tensile Fracture Behavior of Friction Stir-Welded 2024Al-T351 Joints

A 5-mm-thick 2024Al-T351 plate was friction stir welded (FSWed) at welding speeds of 100, 200, and 400 mm min−1 with a constant rotation rate of 800 rpm, and the microstructure and tensile fracture behavior of the joints were investigated in detail. FSW resulted in the redistribution of secondary phase particles along the recrystallized grain boundaries at the nugget zone (NZ), forming linear segregation bands consisting of secondary phase particles. The segregation bands, mainly present in the shoulder-driven zone, were believed to result from periodic material flow, with the average band spacing on the longitudinal and horizontal cross sections equal to the tool advancement per revolution. At a low welding speed of 100 mm min−1, in spite of the highest density of segregation bands, the FSWed 2024Al-T351 joint fractured along the low hardness zone (LHZ) of the heat-affected zone because of large hardness gap between NZ and LHZ. Increasing the welding speed to 200 and 400 mm min−1 reduced both the hardness gap between NZ and LHZ and the density of segregation bands. In this case, the segregation bands played a role, resulting in unusual fracture of the joints along the segregation bands. The “S” line originated from the oxide film on the initial butting surfaces and did not affect the fracture behavior of the FSWed 2024Al-T351 joints.

Magnifying endoscopy with narrow‐band imaging findings in the diagnosis of Barrett's esophageal adenocarcinoma spreading below squamous epithelium

It has been described that most cases of Barrett's esophageal adenocarcinoma in Japan are cases of Barrett's esophageal adenocarcinoma on a background of short-segment Barrett's esophagus, frequently occurring rostrad to Barrett's epithelium, adjacent to the squamous epithelium of the right wall of the esophagogastric junction. Barrett's esophageal adenocarcinoma may spread below the squamous epithelium when the tumor is situated adjacent to the squamocolumnar junction, so that it is usually difficult to diagnose its presence and extent by conventional endoscopy alone. We have noted that the spread of Barrett's esophageal adenocarcinoma below the squamous epithelium is recognizable as annular vascular formations (AVF) by magnifying endoscopy with narrow-band imaging (ME-NBI), and have verified it by 3-D stereo-reconstruction using serial sections from a specimen of the same lesion. When horizontal cross-sections of the tissue were viewed from the surface, AVF emerged at a depth of approximately 100 μm from the surface and disappeared at a depth of approximately 300 μm. Therefore, it would be presumed to be difficult to visualize the characteristic structural features by ME-NBI if the carcinomatous glandular ducts were situated deeper than approximately 300 μm underneath a thick layer of squamous epithelium. Thickness of the overlying squamous epithelium may be a limiting factor for whether or not the characteristic structural features can be detected.

Mathematical modelling of billet casting with secondary cooling zone electromagnetic stirrer

A three-dimensional unsteady coupled mathematical model has been applied to analyse the turbulent flow, temperature fields and macroscopic solidification of molten steel in billet continuous casting with electromagnetic stirring (EMS). Induction currents caused by fluid flow and the stirrer end effect have been investigated. The Lorentz force and Joule heat caused by induction currents have also been taken into account. The application of EMS in the secondary cooling zone results in significant changes in molten steel flow and temperature distribution, and the flow patterns on the horizontal cross-section agree well with the results in other references. Joule heat and Lorentz force are mainly located at the surface layer of the billet and decrease rapidly with penetration depth. The degree of superheat is reduced or eliminated rapidly when molten steel is stirred by EMS, hence is beneficial to generate more equiaxed grains. Stirring intensity is highest in the regions near the two ends of the stirrer. The Joule heat produced by induction has limited influence and can be ignored. The industrial trials showed that the EMS can effectively suppress the central shrinkage cavity and the centre C segregation to improve product quality.

Anatomic Study for the Horizontal Cut of the Sagittal Split Ramus Osteotomy

The purpose of this study was to determine the posterior extension of trabecular bone and the height at which both the lateral and medial cortical bone layers of the mandibular ramus merge, using a reference point on the anterior margin of the mandibular ramus. A total of 80 adult human mandibles were studied. Starting at reference point X, we marked a line running from the anterior margin to the posterior margin of the mandibular ramus on the medial aspect of the study samples. On 2 vertical sections of the right ramus, the level at which both the medial and lateral cortical bone layers merged was determined-H1 and H2, respectively. On the horizontal cross section of the left mandible at point X, the posterior extension of trabecular bone was determined. The median values of H1 and H2 were 10.45 mm and 4.52 mm, respectively. The median value of H2 was inversely proportional to the width of the ramus. The mean extension of trabecular bone posterior to point Z (where the upper margin of the lingula merges into the continuous medial aspect of the ramus) was 17.17 mm, bearing no correlation with the ramus width. Point X is a safe and easy-to-find anatomic reference to establish the level of a sagittal split ramus osteotomy (SSRO) horizontal cut. The wider the mandibular ramus, the closer to the mandibular foramen the SSRO horizontal cut should be performed. An SSRO horizontal cut should not be performed farther than 4 mm above line X. A safe posterior limit for the SSRO horizontal cut could not be determined.

Non-isothermal model experiments and numerical simulations for directional solidification of multicrystalline silicon in a traveling magnetic field

A new experimental setup containing a GaInSn melt with a square horizontal cross section of 10×10cm2 and a variable melt height up to 10cm has been developed. The melt is positioned in the center of a coil system generating a traveling magnetic field (TMF). Using a cooling system at the bottom and a heating system at the top of the melt, a vertical temperature difference up to approximately 50K can be applied to the melt, imitating the thermal conditions during the directional solidification of multicrystalline silicon. Direct measurements of the time-dependent velocity and the temperature profiles were performed using ultrasonic Doppler velocimetry and thermocouples, respectively. Complementary three-dimensional (3D) numerical simulations of the model experiments were used to validate the numerical tools and to gain a deeper insight into the characteristics of TMF flows in square melts. The classical toroidal flow structure known from isothermal cylindrical melts is shown to obtain a large horizontal central vortex at a small height of the square melt, whereas a distinct 3D asymmetry appears at a large height. A vertical temperature gradient tends to suppress the vertical melt motion and leads to new complex horizontal flow structures.

Palm-Print Classification by Global Features

Three-dimensional (3-D) palm print has proved to be a significant biometrics for personal authentication. Three-dimensional palm prints are harder to counterfeit than 2-D palm prints and more robust to variations in illumination and serious scrabbling on the palm surface. Previous work on 3-D palm-print recognition has concentrated on local features such as texture and lines. In this paper, we propose three novel global features of 3-D palm prints which describe shape information and can be used for coarse matching and indexing to improve the efficiency of palm-print recognition, particularly in very large databases. The three proposed shape features are maximum depth of palm center, horizontal cross-sectional area of different levels, and radial line length from the centroid to the boundary of 3-D palm-print horizontal cross section of different levels. We treat these features as a column vector and use orthogonal linear discriminant analysis to reduce their dimensionality. We then adopt two schemes: 1) coarse-level matching and 2) ranking support vector machine to improve the efficiency of palm-print recognition. We conducted a series of 3-D palm-print recognition experiments using an established 3-D palm-print database, and the results demonstrate that the proposed method can greatly reduce penetration rates.

Functional magnetic resonance imaging of horizontal rectus muscles in esotropia

Monkey neurophysiology suggests that changes in neural drive rather than extraocular muscle structure underlie sensory-induced strabismus. If this is true, then extraocular muscle structure should be normal. We used magnetic resonance imaging to measure horizontal rectus muscle size and contractility to determine whether muscle structure is a factor in human concomitant esotropia. High-resolution, quasicoronal plane magnetic resonance imaging was performed in target-controlled central gaze, abduction, and adduction in 13 orthotropic controls (mean age, 38±19 years) and 12 adults (mean age, 52±16 years) who had concomitant esotropia averaging 28(Δ)±18(Δ) at distance. Thyroid ophthalmopathy was excluded. Horizontal rectus muscle cross sections were determined in 6 contiguous, 2-mm-thick midorbital image planes. Contractility was computed in each plane as the difference in cross section from contraction to relaxation. Medial rectus muscle cross sections in multiple planes averaged up to 39% larger in esotropic patients than in controls (P<0.005), whereas lateral rectus muscle cross sections in esotropia were up to 28% larger but only significantly larger in one plane (P<0.02). Medial rectus contractility was increased by up to 60% in esotropic patients (P<0.005), whereas lateral rectus contractility in esotropia was slightly but not significantly supernormal. Medial rectus muscle size is supernormal and lateral rectus muscle size is not subnormal in concomitant esotropia. This finding indicates that human concomitant esotropia is associated with peripheral muscular abnormality.

Visualization of transport pathways for organic compounds in undisturbed soil monoliths

Small amounts of organic compounds are often found at much larger depths than expected from their organic carbon normalized distribution coefficient (Koc). In this study the food dye Brilliant Blue (BB) was used to visualize the (preferential) transport pathways in undisturbed soil monoliths taken from an Orthic Luvisol and from a Eutric Cambisol. After spiking the monoliths with 1.0MBq of either 14C-labeled benazolin (leacher) or 14C-labeled benzo[a]pyrene (BaP, nonleacher), they were intermittently irrigated under free-draining conditions twice weekly with 4mm of rain water for 17weeks. BB (4gL−1) was added to the rain water during the last 4weeks. After irrigation, the monoliths were sliced and photographs were taken from horizontal cross-sections. Soil was dried and incinerated to measure total 14C-activity. The mean travel depth for benazolin (6–7cm) was larger than for BaP (2cm), which is in line with their Koc values, although BaP was much more mobile than expected from the Koc value. BB patterns, arbitrarily classified into five intensity classes, showed that large parts of the monolith were bypassed below the completely stained upper two to five centimeter. Furthermore, BB patterns indicated the locations of strongly sorbing compounds in the deeper soil layers and are therefore a helpful tool to selectively sample the soil for these compounds. A continuously stained root channel below the 20-cm depth contained substantial amounts of BaP, indicating that soil structure cannot be neglected to assess the fate of nonleachers in undisturbed soils.

Length-Scale Similarity of Turbulent Organized Structures over Surfaces with Different Roughness Types

We examine the similarity of turbulent organized structures over smooth and very rough wall flows. Turbulent flow fields in horizontal cross-sections were measured using particle image velocimetry, and the characteristics of turbulent organized structures over four types of surfaces were investigated. Measurements were conducted at several measurement heights across the internal boundary layer. The length and width of turbulence structures were quantified using a two-point correlation method. We selected two thresholds of two-point correlation coefficients to consider both large-scale and small-scale structures; the validity of these choices was examined through the analyses using proper orthogonal decomposition. For large-scale structures, the length and aspect ratios (streamwise length/spanwise width) of structures were highly correlated with the velocity gradient for each measurement height and boundary-layer thickness. This relationship was also examined in the results of previous studies, and the scaling of the aspect ratio with the non-dimensional velocity gradient again showed the importance of the velocity gradient, with slight differences found between smooth and rough surfaces. In contrast, the small-scale structures exhibited weak dependency on the velocity gradient and boundary-layer thickness. Instantaneous snapshots of turbulent organized structures at the same shear level also displayed differences in small-scale structures, but the structures of the organized motions resembled each other, as in the results of the two-point correlation method.

Modeling vertical exchange of heat, salt, and other dissolved substances in the Cariaco Basin

A simple 1.5-dimensional model of vertical exchange for heat, salt, and other dissolved substances has been developed for the Cariaco Basin. The model parameters are derived based on the temperature and salinity data collected monthly at a deepwater station in the eastern part of the basin from 1995 through 2007 during the CARIACO time series program. The model describes the processes of turbulent (eddy) diffusion, which includes the integrated effect of diffusive exchange mechanisms acting in the basin, and of vertical advection, which arises following injection of dense water into deep layers following an inflow from the Caribbean Sea. The model takes into account the changes in the horizontal cross-section area of the basin with respect to depth.Temporal variability is an important feature of the hydrography of the Cariaco Basin. To assess profiles of the vertical eddy diffusion coefficient and vertical advection velocity, we examined a time series of CTD profiles (potential temperature and salinity). Two distinct time intervals were identified as the result of this examination. During the first period, the thermohaline structure of the basin was apparently influenced by one or more inflows. The second period, in contrast, showed little or no influence of an inflow. The data from the second period, where no inflows were observed, were incorporated into corresponding transfer equations to assess the profile of the vertical eddy diffusion coefficient, k(z). Then, the result of this assessment was used with the data from the first period to estimate the profile of the vertical advection velocity, W(z), for a time when the effects of an inflow were evident. For that case, the transfer equations include the terms describing the effect of the inflow.Analysis of the vertical profile of the turbulent diffusion coefficient suggests that, in the upper stratified part of the water column, the diffusion mechanism is mostly associated with transient mixing events, which occur due to shear instability in the field of low frequency internal waves. We speculate that below 400m bottom friction over the sloping bottom and geothermal heat flux play the decisive role in the vertical exchange. Analysis of the W(z) profiles reveals two layers dominated by the entrainment of the ambient fluid into the down flow of dense water from the Caribbean Sea, and two layers where this down flow breaks down through the formation of isopycnal intrusions.

Rainwater content estimated using polarimetric radar parameters in the Heihe River Basin

The rainwater content of cold and arid regions has strong spatial and temporal heterogeneity. Representing rainwater content at high resolution can help us understand the characteristics of inland river basin water cycles and improve the prediction accuracy of hydrological models. Data were used from the Watershed Allied Telemetry Experimental Research (WATER) project of the Heihe River Basin, which is the second largest inland river basin in the arid regions of northwest China. We used raindrop size distributions to improve the rain water content estimation of meteorological radar and to obtain accurate rain water content data in this area. Subsequently, four estimation methods applied in the polarimetric radar were tested. The results of a non-linear regression method show that M(KDP, ZH, ZDR) has the highest accuracy for measuring rain water content. Finally, the formula for measuring the spatial rain water content was applied to a polarimetric radar with an X-band (714XDP). The influence of raindrop size distribution (DSD) on the formula M(KDP, ZH, ZDR) is lowest sensitivity, and it can be explained as follows. On the one hand, the horizontal and vertical front reflection cross sections of the radar are different, so KDP is proportional to the 3rd power of the raindrop diameter. On the other hand, the rear cross section of the radar is proportional to the sixth power of the raindrop diameter. The rainfall's spatial water content M is proportional to the 3rd power of the raindrop diameter, so the influence of the drop size distribution (DSD) on KDP is much smaller than that of ZH.

INTEGRATION OF TERRESTRIAL LASER SCANNING POINTS AND 2D FLOOR PLANS BASED ON MAXIMUM SEQUENTIAL SIMILARITY

Abstract. This paper presents a Maximum Sequential Similarity Reasoning (MSSR) algorithm based method for co-registration of 3D TLS data and 2D floor plans. The co-registration consists of two tasks: estimating a transformation between the two datasets and finding the vertical locations of windows and doors. The method first extracts TLS line sequences and floor plan line sequences from a series of horizontal cross-section bands of the TLS points and floor plans respectively. Then each line sequence is further decomposed into column vectors defined by using local transformation invariant information between two neighbouring line segments. Based on a normalized cross-correlation based similarity score function, the proposed MSSR algorithm is then used to iteratively estimate the vertical and horizontal locations of each floor plan by finding the longest matched consecutive column vectors between floor plan line sequences and TLS line sequences. A group matching algorithm is applied to simultaneously determine final matching results across floor plans and estimate the transformation parameters between floor plans and TLS points. With real datasets, the proposed method demonstrates its ability to deal with occlusions and multiple matching problems. It also shows the potential to detect conflict between floor plan and as-built, which makes it a promising method that can find many applications in many industrial fields.

VOXEL-BASED APPROACH FOR ESTIMATING URBAN TREE VOLUME FROM TERRESTRIAL LASER SCANNING DATA

Abstract. The importance of single trees and the determination of related parameters has been recognized in recent years, e.g. for forest inventories or management. For urban areas an increasing interest in the data acquisition of trees can be observed concerning aspects like urban climate, CO2 balance, and environmental protection. Urban trees differ significantly from natural systems with regard to the site conditions (e.g. technogenic soils, contaminants, lower groundwater level, regular disturbance), climate (increased temperature, reduced humidity) and species composition and arrangement (habitus and health status) and therefore allometric relations cannot be transferred from natural sites to urban areas. To overcome this problem an extended approach was developed for a fast and non-destructive extraction of branch volume, DBH (diameter at breast height) and height of single trees from point clouds of terrestrial laser scanning (TLS). For data acquisition, the trees were scanned with highest scan resolution from several (up to five) positions located around the tree. The resulting point clouds (20 to 60 million points) are analysed with an algorithm based on voxel (volume elements) structure, leading to an appropriate data reduction. In a first step, two kinds of noise reduction are carried out: the elimination of isolated voxels as well as voxels with marginal point density. To obtain correct volume estimates, the voxels inside the stem and branches (interior voxels) where voxels contain no laser points must be regarded. For this filling process, an easy and robust approach was developed based on a layer-wise (horizontal layers of the voxel structure) intersection of four orthogonal viewing directions. However, this procedure also generates several erroneous "phantom" voxels, which have to be eliminated. For this purpose the previous approach was extended by a special region growing algorithm. In a final step the volume is determined layer-wise based on the extracted branch areas Ai of this horizontal cross-section multiplied by the thickness of the voxel layer. A significant improvement of this method could be obtained by a reasonable determination of the threshold for excluding sparsely filled voxels for noise reduction which can be defined based on the function change of filled voxels. Field measurements were used to validate this method. For a quality assessment nine deciduous trees were selected for control and were scanned before felling and weighing. The results are in good accordance to the control trees within a range of only -5.1% to +14.3%. The determined DBH values show only minor deviations, while the heights of trees are systematically underestimated, mainly due to field measurements. Possible error sources including gaps in surface voxels, influence of thin twigs and others are discussed in detail and several improvements of this approach are suggested. The advantages of the algorithm are the robustness and simple structure as well as the quality of the results obtained. The drawbacks are the high effort both in data acquisition and analysis, even if a remarkable data reduction can be obtained by the voxel structure.

3-D analysis of grain selection process

It is known that the grain selection plays an important role in the manufacturing process for turbine blades. There are some analytical or numerical models to treat the grain selection. However, the detailed mechanism of grain selection in 3-D is still uncertain. Therefore, an experimental research work using Al-Cu alloy has been carried out in order to understand the grain selection in 3-D.A mold made by Al2O3 was heated to 600 °C ( = liquids temperature of the alloy) and was set on a water-colded copper chill plate. Molten Al-20 wt%Cu alloy was cast into the mold and unidirectional solidified ingot was prepared. The size of ingot was approximately ϕ25×65H mm. To obtain the thermal history, 4 thermocouples were placed in the mold. It is confirmed that the alloy solidified unidirectionally from bottom to top. Solidified structure on a longitudinal cross section was observed and unidirectional solidification up to 40 mm was ensured. EBSD analysis has been performed on horizontal cross section at an interval of ca.200 μm. These observations were carried out 7-5 mm from the bottom surface. Crystallographic orientation of primary Al phase and size of solidified grains were characterized. A large solidified grain, the crystallographic orientation of which is approximately <101> along heat flow direction, is observed near the lowest cross section. The area of <101> grain decreased as solidification proceeded. On the other hand, it is found that the area of <001> grain increased.

Properties of dust-plasma structures formed in a glow discharge above the lower wall of the discharge chamber

The properties are studied of dusty plasma structures formed in a glow discharge in a dust trap above the lower wall of the side branch of the discharge tube, near the turn of the discharge channel. The dust structure is three-dimensional with a characteristic size of up to 3 cm and contains about 30000 dust grains. Depending on the experimental conditions, dust-acoustic, dissipative, and charge-gradient instabilities can develop in such a structure. When using highly polydisperse dust grains of arbitrary shape, the effect of selection of dust grains by the plasma with respect to their mean size and shape was discovered. This effect was studied quantitatively in two gases by using the method of gathering and extraction of the dust grains levitating in the trap. The morphology of the dust structures was determined from the pair correlation functions of the horizontal cross sections containing long-range order peaks and elements of a hexagonal lattice. Stratification of a uniform structure accompanied by convective rotation caused by the grain charge gradient was observed. Applications of the dusty plasma created in this type of device are discussed.

Comparison of Objective Supercell Identification Techniques Using an Idealized Cloud Model

AbstractThe accuracy, reliability, and skill of several objective supercell identification methods are evaluated using 113 simulations from an idealized cloud model with 1-km horizontal grid spacing. Horizontal cross sections of vorticity and radar reflectivity at both mid- and low levels were analyzed for the presence of a supercell, every 5 min of simulation time, to develop a “truth” database. Supercells were identified using well-known characteristics such as hook echoes, inflow notches, bounded weak-echo regions (BWERs), and the presence of significant vertical vorticity.The three objective supercell identification techniques compared were the Pearson correlation (PC) using an analysis window centered on the midlevel storm updraft; a modified Pearson correlation (MPC), which calculates the PC at every point in the horizontal using a small 3 km × 3 km analysis window; and updraft helicity (UH). Results show that the UH method integrated from 2 to 5 km AGL, and using a threshold value of 180 m2 s−2, was equally as accurate as the MPC technique—averaged from 2 to 5 km AGL and using a minimum updraft threshold of 7 m s−1 with a detection threshold of 0.3—in discriminating between supercells and nonsupercells for 1-km horizontal grid spacing simulations. At courser resolutions, the UH technique performed best, while the MPC technique produced the largest threat scores for higher-resolution simulations. In addition, requiring that the supercell detection thresholds last at least 20 min reduced the number of false alarms.

Shape recognition with point clouds in rebars

ing point clouds of reinforcing bars fr om raw point clouds The method of abstracting point clouds of reinforci ng bars Point clouds of reinforcing bars have a complicated shape. Therefore the point clouds of reinforcing bars cannot be distinguished directly from raw point clouds by using the mathematical methods such as least-square method. Hence, the authors have developed a method of abstracting point clouds of reinforcing bars from raw point clouds. The method of abstracting point clouds of reinforci ng bars in three dimensional spaces Point clouds of reinforcing bars are not a primitive shape such as plane, sphere and cylinder. Although point clouds of a primitive shape don’t include point clouds of reinforcing bars, point clouds of non-primitive shape include more much point clouds of reinforcing bars than raw point clouds data. Therefore, if point clouds of a primitive shape are removed from raw data, point clouds including reinforcing bars remain. Fig.9 Point clouds put into voxels and then replacing planes using least-square method Fig.10 Point clouds of sphere replacing a sphere model using least-square method Raw point clouds are put into voxels which represent a value on a regular grid in a three dimensional space (Figure.9). Next, the point clouds in a voxel are fitted into a sphere and plane by the least-squares method. If the sum of the squares of the errors is less than tolerance, the point clouds are fitted into a sphere (or a plane) model and then the point clouds are removed from raw data (Figure 9 and 10). Fig.11 An example of the flow diagram dealt with shape recognition in three-dimensional spaces. Remaining point clouds, which include point clouds of reinforcing bars, go on to next steps for abstracting point clouds on reinforcing bars in a cross section. Steps of abstracting point clouds on reinforcing bar in a cross section The method of abstracting point clouds on reinforcing bars in a cross section are developed by the authors consisted of the following three steps: (a) The point clouds are sliced in horizontal 2D cross sections (Figure.12) (b) In a cross section, point clouds is grouped by taking account of the continuity between pixel blocks (Figure.13 and 14) (c) The system recognizes the shape in each cluster of point clouds and then distinguishes point clouds on reinforcing bars (Figure.15) Plane

Effects of existing piles on lateral resistance of new piles

Cyclic lateral-loading centrifuge tests were conducted on a new pile located at the center of 2×2 existing piles, to evaluate the effects of the existing piles on the new pile's lateral resistance when the piles of an old building are left derelict. Furthermore, two-dimensional FEM analyses of the horizontal cross sections of the pile–soil systems were carried out. The centrifuge test results show that the lateral resistance of a new pile, located among existing piles, shows slight increases. The existing piles increase the horizontal subgrade reaction of the upper part of the new pile, but slightly decrease that of the lower part of the new pile. Numerical analyses reveal that the key to whether an existing pile increases the horizontal subgrade reaction of a new pile or not, is the soil displacement at the location of the existing pile. An existing pile increases the horizontal subgrade reaction of a new pile when the existing pile displacement is less than the soil displacement caused by the new pile. Conversely, an existing pile decreases the horizontal subgrade reaction of a new pile when the existing pile displacement is greater than the soil displacement caused by the new pile.

Dehydration-induced shrinkage of dentin as a potential cause of vertical root fractures

Vertical root fractures (VRF) of endodontically treated teeth constitute a severe clinical condition frequently requiring removal of the affected tooth. Numerous attempts have been made to find the cause for VRF without reaching definitive conclusions. As changes in moisture content have been reported to appear as a consequence of root canal therapy, it is the goal of this paper to evaluate associated volume changes as a possible cause for VRF.Considering disk shaped horizontal cross sections of endodontically treated teeth with a moisture content of dentin decreasing from the root surface towards the root canal, both relative circumferential and relative radial stresses resulting from volume changes of dentin were calculated.It could be shown that the presence of a root canal itself increases radial and circumferential stresses acting on root dentin by a factor of two. Reduction in moisture content of dentin at the wall of the root canal results in shrinkage of the tooth structure and tensile stress. On the outer surface of the root, compressive stresses occur. Thus, VRF would start at the canal wall and propagate to the root surface.The theory presented appears to be consistent with previous reports on stress development as a consequence of dehydration of dentin and finite element analysis on root fractures. It may be concluded that dehydration of dentin induces cracks at the walls of a root canal which subsequently grow as a result of cyclic loading or traumatic overload.

A spiral wave front beacon for underwater navigation: Transducer prototypes and testing

Transducers for acoustic beacons which can produce outgoing signals with wave fronts whose horizontal cross sections are circular or spiral are studied experimentally. A remote hydrophone is used to determine its aspect relative to the transducers by comparing the phase of the circular signal to the phase of the spiral signal. The transducers for a "physical-spiral" beacon are made by forming a strip of 1-3 piezocomposite transducer material around either a circular or spiral backing. A "phased-spiral" beacon is made from an array of transducer elements which can be driven either in phase or staggered out of phase so as to produce signals with either a circular or spiral wave front. Measurements are made to study outgoing signals and their usefulness in determining aspect angle. Vertical beam width is also examined and phase corrections applied when the hydrophone is out of the horizontal plane of the beacon. While numerical simulations indicate that the discontinuity in the physical-spiral beacon introduces errors into the measured phase, damping observed at the ends of the piezocomposite material is a more significant source of error. This damping is also reflected in laser Doppler vibrometer measurements of the transducer's surface velocity.