Changes In Cross-sectional Shape Research Articles

Deformation Mechanisms of Dry Textile Preforms under Mixed Compressive and Shear Loading

Elementary analyses of a standard compression test and a new test configuration for combined compression and shear illuminate the mechanisms of deformation of dry fiber bundles in fiber preforms under multi-axial loads. Such stress states are central to fiber bundle behavior in thick preforms that are being manipulated in pressurized tools. Sliding, buckling, compression, and shear deformation mechanisms are observed to operate at the single fiber bundle (tow) level. Shear includes deformation transverse to the axis of a tow (changes in cross-sectional shape) and axial shear (shear deflection of a tow segment acting as a short beam). Not all of these mechanisms have been addressed before in the extensive literature on textile deformation. The presence of multiple mechanisms complicates the quest for simple experiments that could be treated as standards for data acquisition. The need for new tests to be invented and standardized is highlighted.

Modified shear lag model for fibers and fillers with irregular cross-sectional shapes

For fibers with irregular cross sections such as ultrahigh modulus polyethylene (UHMPE) fibers and ribbon-like carbon fibers, the original shear lag model would not provide accurate calculations for interfacial shear stress because it assumes a circular fiber cross section. In this study, a modified shear lag model is proposed to calculate the interfacial shear stress that reflects the change of fiber cross-sectional shape. Microbond test on a UHMPE fiber/epoxy system was used for verification of the model. The difference between the interfacial shear strength (IFSS) calculated using the modified model and that using the original model assuming an equivalent fiber diameter was found to be as high as 15% and it linearly increased as the irregularity of the cross-sectional shape increased. When the irregularity constant exceeds 1.12, the error in IFSS involved in using the original shear lag model and an equivalent fiber diameter is greater than 10%.

Dependence of needle architecture and chemical composition on canopy light availability in three North American Pinus species with contrasting needle length.

Morphology and chemical composition of needles of shade-intolerant southern conifers (Pinus palustris Mill. (mean needle length +/- SD = 29.1 +/- 4.1 cm), P. taeda L. (12.3 +/- 2.9 cm) and P. virginiana Mill. (5.1 +/- 0.8 cm)) were studied to test the hypothesis that foliage acclimation potential to canopy light gradients is generally low for shade-intolerant species, and in particular, because of mechanical limitations, in species with longer needles. Plasticity for each needle variable was defined as the slope of the foliar characteristic versus irradiance relationship. A novel geometrical model for needle area and volume calculation was employed for the three-needled species P. palustris and P. taeda. Needle thickness (T) strongly increased, but width (W) was less variable with increasing daily integrated quantum flux density averaged over the season (Q(int)), resulting in changes in cross-sectional needle shape that were manifested in a positive relationship between the total to projected needle area ratio (A(T)/A(P)) and Q(int) in the three-needled species. In contrast, cross-sectional needle geometry was only slightly modified by irradiance in the two-needled conifer P. virginiana. Needle dry mass per unit total needle area (M(T)) was positively related to Q(int) in all species, leading to greater foliar nitrogen contents per unit area at higher irradiances. Separate examination of the components of M(T) (density (D) and the volume (V) to A(T) ratio; M(T) = DV/A(T)) indicated that the positive effect of light on M(T) resulted solely from increases in V/A(T), i.e., from increases in the thickness of foliage elements. Foliar chlorophyll content per unit mass increased with increasing Q(int), allowing an improvement in light-harvesting efficiency in low light. The variables characterizing needle material properties (D, the dry to fresh mass ratio, and needle carbon content per unit mass) were generally independent of Q(int), suggesting that needles were less stiff and had greater tip deflections under their own weight at lower irradiances because of smaller W and T. Comparisons with the literature revealed that plasticity in foliar characteristics tended to be lower in the studied shade- intolerant species than in shade-tolerant conifers, but plasticity among the investigated species was unaffected by needle length. However, we argue that, because of mechanical limitations, plastic changes in needle cross section in response to low irradiance may decrease rather than increase light-interception efficiency in long-needled species.

Biomechanical scaling of the hominoid mandibular symphysis.

Experimental investigation of mandibular bone strain in cercopithecine primates has established that the mandible is bent in the transverse plane during the power stroke of mastication. Additional comparative work also supports the assumption that the morphology of the mandibular symphysis is functionally linked to the biomechanics of lateral transverse bending, or "wishboning" of the mandibular corpus. There are currently no experimental data to verify that lateral transverse bending constitutes an important loading regime among hominoid primates. There are, however, allometric models from cercopithecoid primates that allow prediction of scaling patterns in hominoid mandibular dimensions that would be consistent with a mechanical environment that includes wishboning as a significant component. This study uses computed tomography (CT) scans to visualize cortical bone distribution in the anterior corpus of a sample of four genera of extant hominoids. From the cortical bone contours, area properties of the mandibular symphysis are calculated, and these variables are subjected to an allometric analysis to detect whether scaling of jaw dimensions are consistent with a wishboning loading regime. Scaling of the hominoid symphysis recalls patterns observed in cercopithecoid monkeys, which lends indirect support for the hypothesis that wishboning is an integral part of the masticatory loading environment in living apes. Inclination of the symphysis, rather than changes in cross-sectional shape or development of the superior transverse torus, represents a morphological solution for minimizing the potentially harmful effects of wishboning in the jaws of these primates.

Electromagnetic theory of an open resonator.

Particular higher-order sources give rise to electromagnetic Gaussian beams, which are linearly polarized and have their maximum in the propagation direction. For this dipolar beam the cross-sectional shape changes in the propagation direction. Nodal surfaces exist on which the tangential component of the electric field vanishes in the standing wave that is formed by the two oppositely directed dipolar, electromagnetic Gaussian beams. These surfaces are identified as the mirror shapes for an open resonator that supports this standing wave. For standing waves that have a particular cross-sectional shape at the waist the cross section of the beam near the mirror surfaces is circular. The resonant frequencies for the fundamental transverse mode of such a resonator have been determined as a function of the geometry and the axial mode number. By a perturbation technique the resonant frequency of an open resonator with spherical mirrors has been obtained. This result is valid in only the paraxial approximation. Illustrative numerical results are included.

Anisotropic creep in CMSX-4 in orientations distant from 〈001〉

Constant load creep tests have been performed on the single crystal nickel base superalloy CMSX-4 in crystallographically unstable orientations distant from the 〈001〉 pole of the standard stereographic triangle at 1023 K (750°C) and varying stress levels. Orientations distant from the 〈001〉–〈111〉 boundary had high creep rates and relatively low rupture lives, while the orientations closer to it had relatively low initial creep rates and longer rupture lives. Electron back scatter diffraction patterns, cross section shape change measurements on creep tested specimens, and optical microscopy indicate that the operating slip system is of the type {111}〈112〉 in orientations far from the 〈001〉–〈111〉 boundary. The results are explained based on the resolved shear stress acting on the operating slip system taking into consideration the rotations and changes in cross section. A theory based on the ease of generation of superlattice intrinsic stacking faults is proposed to account for the observed creep rates in various orientations. A finite element model based on the slip system approach is used to demonstrate the validity of the experimental results.

Boundary Shear in Circular Pipes Running Partially Full

The distribution of boundary shear stress in circular conduits flowing partially full, with and without a smooth flat bed simulating deposited sediments, has been examined experimentally ranging from 0.375 < F < 1.96 (F = Froude number) and 65,000 < R < 342,000 (R = hydraulic radius), using the Preston tube technique. The invert level of the flat bed and the water depth have been varied to simulate a wide range of possible flow conditions that may occur in culverts, sewers, and hydropower tunnels. The distribution of boundary shear stress around the wetted perimeter is shown to be highly sensitive to changes in cross-sectional shape. The results have been analyzed in terms of the variation of local/global shear stress versus perimetric distance, and the percentage of the total shear force acting on the wall or bed of the conduit. The %SFw results (%SFw = the percentage of the average wall shear force of the overall shear force) have been shown to agree well with Knight's empirical formula for prismatic channels. The influence of secondary flows on the distribution of boundary shear stress and the implications of this for sediment transport have also been examined.

Three dimensional FEM simulation for spinning of non-circular fibers

A finite element method is employed for a flow analysis of the melt spinning process of a non-circular fiber, a PET(polyethylene terephthalate) filament. The flow field is divided into two regions of die channel and spin-line. A two dimensional analysis is used for the flow within the die channel and a three dimensional analysis for the flow along the spin-line. The Newtonian fluid is assumed for the PET melt and material properties are considered to be constant except for the viscosity. Effects of gravitation, air drag force, and surface tension are neglected. Although the spin-line length is 4.5 m, only five millimeters from the spinneret are evaluated as the domain of the analysis. Isothermal and non-isothermal cases are studied for the flow within the die channel. The relationship between the mass flow rate and the pressure gradient is presented for the two cases. Three dimensional flow along the spin-line is obtained by assuming isothermal conditions. It is shown that changes in velocity and cross-sectional shape occur mostly in the region of 1mm from the die exit.

Conservation of the elastic and flexural moduli of osteopenic femoral cortical bone in experimental inflammatory arthritis in the rabbit.

Experimental inflammatory arthritis (EIA) produced by carrageenan injection provokes a rapid bone remodeling state with cortical and cancellous bone loss. The objective of this study was to determine whether changes in cortical mechanical properties and/or geometry occur in long bones, either near or remote to the site of inflammation. EIA was induced in the right tibio-femoral joint of rabbits over 56 days. The right humerus and right femur from 15 normal and 25 arthritis group animals were excized. Semi-cylindrical specimens of the medial cortical shaft were subjected to non-destructive four-point bending tests. Transverse sections at the four contact sites of the loading jig were photographed and digitized to obtain average cross-sectional area (A) and moment of inertia (I). Moment of inertia and slope of the load/deflection curve permitted calculation of modulus of elasticity (E) for each specimen. Load/time curves were also used to calculate per cent stress remaining in relaxation experiments. Per cent stress remaining, E, A, I and square root of I/A (radius of gyration) were examined for differences by bone (humerus, femur) and by treatment (N,A) using two way ANOVA. The induction of inflammatory arthritis did not significantly alter the modulus of elasticity in either the femur or humerus; however, arthritis reduced the moment of inertia from 34.54 +/- 2.88 x 10(-12) m4 to 25.06+/-1.80 x 10(-12) m4 (mean+/-SEM, p < 0.05). This was observed in the femur (near the arthritic joint), but not in the humerus (remote from arthritic joint). Analysis of area and ratio I/A demonstrated that this geometric effect of treatment was due to reduced area without gross cross-sectional shape changes. Per cent stress remaining in the femur (but not in the humerus) was higher in the arthritis specimens than in the normal specimens (N: 80.86 < 0.97%; A: 83.25 +/- 0.71%, p < 0.05). Thus, in this arthritis model, the principal mechanical or geometric effect on cortical bone was reduction of the cross-sectional area and moment of inertia. The viscoelastic relaxation response of bone was also altered, perhaps due to loss of water or collagen degradation.

Modeling generalized cylinders via Fourier morphing

Generalized cylinders provide a compact representation for modeling many components of natural objects as well as a great variety of human-made industrial parts. This paper presents a new approach to modeling generalized cylinders based on cross-sectional curves defined using Fourier descriptors. This modeling is based on contour interpolation and is implemented using a subdivision technique. The definition of generalized cylinders uses a three-dimensional trajectory which provides an adequate control for the smoothness of bend with a small number of parameters and includes the orientation of each cross-section (i.e, the local coordinate system) in the interpolation framework. Fourier representations of cross-sectional curves are obtained from contours in digital images, and corresponding points are identified by considering angular and arc-length parametrizations. Changes in cross-section shape through the trajectory are performed using Fourier morphing. The technique proposed provides a comprehensive definition that allows the modeling of a wide variety of shapes, while maintaining a compact characterization to facilitate the description of shapes and displays.

Modular coils and finite-β operation of a quasi-axially symmetric tokamak

Quasi-axially symmetric tokamaks (QA tokamaks) are an extension of the conventional tokamak concept. In these devices the magnetic field strength is independent of the generalized toroidal magnetic co-ordinate even though the cross-sectional shape changes. An optimized plasma equilibrium belonging to the class of QA tokamaks has been proposed by Nührenberg. It features the small aspect ratio of a tokamak while allowing part of the rotational transform to be generated by the external field. In this article, two particular aspects of the viability of QA tokamaks are explored, namely the feasibility of modular coils and the possibility of maintaining quasi-axial symmetry in the free-boundary equilibria obtained with the coils found. A set of easily feasible modular coils for the configuration is presented. It was designed using the extended version of the NESCOIL code (Merkel, P., Nucl. Fusion 27 (1987) 867). Using this coil system, free-boundary calculations of the plasma equilibrium were carried out using the NEMEC code (Hirshman, S.P., Van Rij, W.I., Merkel, P., Comput. Phys. Commun. 43 (1986) 143). It is observed that the effects of finite β and net toroidal plasma current can be compensated for with good precision by applying a vertical magnetic field and by separately adjusting the currents of the modular coils. A set of fully three dimensional (3-D) auxiliary coils is proposed to exert control on the rotational transform in the plasma. Deterioration of the quasi-axial symmetry induced by the auxiliary coils can be avoided by adequate adjustment of the currents in the primary coils. Finally, the neoclassical transport properties of the configuration are examined. It is observed that optimization with respect to confinement of the alpha particles can be maintained at operation with finite toroidal current if the aforementioned corrective measures are used. In this case, the neoclassical behaviour is shown to be very similar to that of a conventional tokamak.

The Simulation of the Geometry of Two-component Yarns. Part I: The Mechanics of Strand Compression: Simulating Yarn Cross-section Shape

The geometry of two-components yarns is investigated, and a new approach is presented which addresses some of the limitations of other models The model takes account of the compressional deformation of the individual strands caused by transverse forces generated during the twisting process. The change in the cross-section takes account of the mechanical properties of the strands and the observed changes in cross-section shape caused by lateral compression. The outcome is a methodology and predictive equations for modelling the compressed shape of strands in two-component yarns; the model has been successfully used in CAD systems to simulate yarn appearance.

綿クレープ設計における撚糸および織物構造の影響 第２報 収縮率，しぼ数およびしぼ形状に及ぼす製織条件の影響

Effects of weaving condition on creping of cotton crepe fabrics such as shrinkage, the number of ribs and surface appearance are studied experimentally using fabrics made by sample weaving .The number of ribs on the crepe fabrics are related to the shrinkage by a quadratic function in the range of this experiment.The shrinkage and the number of ribs increase with increasing the twist number of hard twist yarn of weft .The shrinkage decreases with increasing weft density and the surface appearance becomes more irregular.The number of ribs decreases with decreasing the weft density in the lower range of the density, but increase in thehi gher range.This reason is that the complicated ribs becomes like one ribs by the change of cross-sectional shape along the weft direction with decreasing the weft density .Both shrinkage and the number of ribs become greater with decreasing tex count of warp and/or weft.For controlling the cover factor of the crepe fabrics, the yarn count causes greater influence on the shrinkage, as compared with the effect of weft yarn density .

Study of dry oxidation of triangle-shaped silicon nanostructure

Silicon nanostructures along [011] direction with upside down triangle cross sections on the top of the sawtooth structure with (111) facets are prepared by using the lithography technique, reactive ion etching, and anisotropic wet chemical etching. These triangle-shaped silicon nanostructures are thermally oxidized in dry oxygen over a range of temperature from 850 to 1000 °C, which is characterized by scanning electron microscopy. The self-limiting oxidation phenomenon observed in silicon nanostructures is discussed. Cross-sectional shape change of the silicon nanostructure under different oxidation temperatures is demonstrated. A silicon quantum wire is successfully fabricated by two-step thermal oxidation of the silicon nanostructures.

Sensitivity analysis and optimization of truss/beam components of arbitrary cross-section—I. Axial stresses

In this paper, a general method for detailed design and optimization of the design parameters of an arbitrary cross-section of a truss/beam design component is proposed. The method draws upon already existing computer aided design (CAD) parametric capabilities to parametrize a two-dimensional closed contour. Any complex composite cross-section made of two-dimensional closed contours can be assigned to a design component. The method proceeds by computing the centroid, the cross-sectional area, the moments of inertia referred to the arbitrary origin and to the centroid. The principal angle of inertia is used to redefine the orientation of the coordinate reference system for finite element performance analysis. During the design optimization process the cross-section shape changes according to the design parametrization defined during the formulation of the design problem. The change in shape causes change in the orientation of the principal axes, causing in turn critical stresses to occur at new vertex locations. The continuum method of design sensitivity analysis is used to compute design sensitivity information with respect to the cross-sectional design parameters. The method is shown to be well suited for integration with an existing CAD and FEM code.

Morphology, mechanics, and locomotion: the relation between the notochord and swimming motions in sturgeon

To examine the relation between morphology and performance, notochordal morphology was correlated with notochordal mechanics and with steady swimming motions in white sturgeon, Acipenser transmontanus. In a still-water tank, motions of four sturgeon varied with changes in swimming speed and axial position along the body. For a 1..34 m sturgeon, slow and fast swimming modes were distinguished, with speeds at the fast mode more than two times those at the slow mode without changes in tailbeat frequency. This increase in speed is correlated with an increase in the body's maximal midline curvature (m−1), suggesting a role for curvature-related mechanical properties of the notochord. Maximal midline curvature also varied with axial position, and surprisingly was uncorrelated with axial changes in the notochord's cross-sectional shape - as measured by height, width, inner diameter, and lateral thickness of the sheaths. On the other hand, maximal midline curvature was negatively correlated with the axial changes in the notochord's angular stiffness (N m rad−1) and change in internal pressure (% change from baseline of 58.6 kPa), both of which were measured during in vitro bending tests. In vivo curvature and in vitro angular stiffness were then used to estimate the bending moments (Nm) in the notochord during swimming. In the precaudal notochord, the axial pattern of maximal stiffness moments was congruent with the pattern of maximal notochordal curvature in the precaudal region, but in the caudal notochord maximal angular stiffness was located craniad to maximal curvature. One interpretation of this pattern is that the precaudal notochord resists bending moments generated by the muscles and that the caudal notochord resists bending moments generated by hydrodynamic forces acting on the tail.

Asymmetric creep deformation of a single crystal superalloy

The anisotropic creep behaviour of single crystal superalloys leads to changes in the orientation and shape of specimens of arbitrary initial orientations during high temperature deformation. A model of anisotropic creep has been applied to a creep database for the single crystal superalloy SRR99 tested in uniaxial tension on 〈011〉 and 〈111〉 orientations in order to evaluate the model parameters. These have been used to predict the changes in specimen cross-sectional shape resulting from creep deformation of more complex orientations. Experimental measurements have been made of the cross-sectional dimensions of initially cylindrical specimens as a function of local creep strain. The agreement between experimental measurement and model prediction is normally very good. The biggest disagreement, for the 〈211〉 specimen, indicates that macroscopic cube slip is dominant rather than mixed cube and octahedral deformation as indicated by the model.

Modelling high-temperature creep of anisotropic material

Cast single-crystal superalloys are being used as turbine blades in modern aeroengines with 〈001〉 axis oriented along the direction of principal stress. This allows use of higher operating temperature with associated improvement in efficiency. Although 〈001〉 is the natural direction of crystal growth in these alloys it is often not possible to attain perfect alignment. Consequently, besides time-dependent deformation this would induce a change in cross-sectional shape and crystal orientation of the blade as a results of service exposure. Therefore in order to exploit their full potential it is necessary to develop a predictive system which could model effect of crystallographic anisotropy on hightemperature creep deformation of such materials. The paper reviews the recent developments in this direction and suggests that modelling not only helps in identifying the dominant mechanisms of deformation but also suggests simple experiments to verify them.

Comparison of intravascular ultrasound and angiography in the assessment of myocardial bridging.

In autopsy, myocardial bridging is a common finding. With coronary angiography, a systolic compression, mainly of the left anterior descending coronary artery, is observed in 1% to 3% of the patients. Controversy exists concerning the functional importance of this finding. To obtain a functional insight into the myocardial bridging, intravascular ultrasound and intracoronary Doppler were performed. Intracoronary ultrasound and Doppler were performed in 14 patients with angiographic evidence of systolic vessel compression ("milking effect") in the left anterior descending coronary artery. The 4.8F, 20-MHz ultrasound catheter could not be advanced through the entire myocardial bridge segment in 6 of the 14 patients studied because the lumen was < 1.6 mm. In these patients, only the proximal parts of the bridge segment were scanned. The changes in cross-sectional shape during the cardiac cycle were determined for both the normal proximal segment and the bridge segment by use of a semiautomatic computer program. Intracoronary Doppler (20 MHz) was performed in 7 patients with a 3F catheter. A highly characteristic systolic eccentric or concentric compression with delayed relaxation in diastole of the myocardial bridging segment was clearly visualized in all patients. The cross-sectional lumen area variation was 40 +/- 25% in the bridging segments and 9 +/- 7% in the normal segments (P < .01). No atherosclerotic lesions were detected in the bridge or the distal segment in the 8 patients in whom the IVUS catheter was successfully advanced through the entire myocardial bridge. However, atherosclerotic plaques were found in the segments proximal to the bridge in 12 of 14 patients (86%). The resting mean flow velocity was 6.4 +/- 1.2 cm/s; the maximal mean flow velocity after intracoronary administration of 10 mg papaverine was 14.1 +/- 3.4 cm/s. The coronary flow velocity reserve was 2.2 +/- 0.7. A highly characteristic pattern showing a prominent peak in coronary velocity in early diastole was observed in 86% of patients, and this pattern was enhanced after injection of intracoronary papaverine. Intravascular ultrasound demonstrated a characteristic systolic compression of the bridge segments. The delayed compression release may explain the characteristic sharp early diastolic peak in coronary flow velocity found with intracoronary Doppler in vessels with myocardial bridging. Reduced coronary flow reserve may be related to this phenomenon, possibly explaining signs of ischemia detected in some of the patients, but may alternatively be a result of the presence of atherosclerosis in the segment proximal to the bridge in these patients.

Morphological Evolution with Layer Thickness in single crystal CeO2(110)/Si(100)

AbstractSurface morphology evolution of epitaxially grown CeO2(110) layers on Si(100) substrates is studied using atomic force microscopy (AFM) and reflection high energy electron diffraction (RHEED). The surface has a faceted structure; a stripe-appearance and triangular-shape in plan- and cross-sectional views, respectively. AFM measurements clarify that as the layer thickness increases, the cross-sectional shape changes from a gable roof shape toward trapezoidal, which is consistent with RHEED analyses. The width of the facet monotonically increases with the layer thickness, while its height saturates at ∼5 nm above 600 nm in thickness, which means that the surface approaches smooth morphology. Ion channeling analyses indicate that the thicker the layer, the better the crystalline quality at the surface.