Combined Effects Of Shear Research Articles

Combined effect of shear and nucleating agent on the multilayered structure of injection‐molded bar of isotactic polypropylene

AbstractMolten polymers are usually exposed to varying levels of shear flow and temperature gradient in most processing operations. Many studies have revealed that the crystallization and morphology are significantly affected under shear. A so‐called “skin‐core” structure is usually formed in injection‐molded semicrystalline polymers such as isotactic polypropylene (iPP) or polyethylene (PE). In addition, the presence of nucleating agent has great effect on the multilayered structure formed during injection molding. To further understand the morphological development in injection‐molded products with nucleating agent, iPP with and without dibenzylidene sorbitol (DBS) were molded via both dynamic packing injection molding (DPIM) and conventional injection molding. The structure of these injection‐molded bars was investigated layer by layer via SEM, DSC, and 2 days‐WAXD. The results indicated that the addition of DBS had similar effect on the crystal size and its distribution as shear, although the later decreased the crystal size more obviously. The combination of shear and DBS lead to the formation of smaller spherulites with more uniform size distribution in the injection‐molded bars of iPP. A high value of c‐axis orientation degree in the whole range from the skin to the area near the core center was obtained in the samples molded via DPIM with or without DBS, while in samples obtained via conventional injection molding, the orientation degree decreased gradually from the skin to the core and the decreasing trend became more obvious as the concentration of DBS increased. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Combined effect of shear and fibrous fillers on orientation-induced crystallization in discontinuous aramid fiber/isotactic polypropylene composites

The shear-induced crystallization behavior in isotactic polypropylene (iPP) composite melt containing short aramid fibers was investigated by means of WAXD (wide-angle X-ray diffraction) and SAXS (small-angle X-ray scattering) techniques using synchrotron radiation. The study was carried out in a post-shear isothermal crystallization mode at temperatures of 140–160°C. Parameters pertaining to the crystallization morphology and kinetics were analyzed, including total crystallinity, orientated crystalline and amorphous fractions, dimensions of the formed shish–kebab structure, as well as induction time and rate of crystallization. The individual contributions of shear and fibers were evaluated and the combined effect was compared. The results clearly indicated that the effect is synergistic rather than additive.

Some aspects of the effect of surface friction on flows over mountains

AbstractThe effect of surface friction on flow over mountains in stratified fluids is investigated using a two‐dimensional version of the Naval Research Laboratory's non‐hydrostatic Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). Without surface friction, flow over a mountain attains a maximum speed at the surface on the lee slope. With surface friction, the flow speed vanishes at the surface and reaches a maximum near the top of the boundary layer (BL). Corresponding to this wind distribution is a shallow layer of very large vertical wind shear beneath the jet maximum and a weaker but deeper sheared layer elsewhere. This strong shear generates maximum turbulent kinetic energy (TKE) at the jet maximum region. Meanwhile, the buoyancy effect associated with the mountain gravity wave inhibits the production of the TKE above the jet maximum region. The combined effects of shear and buoyancy lead to a very shallow BL on the lee slope. In general, surface friction reduces the amplitude of mountain gravity waves. Comparison of the flow fields with the outline of the BL height indicates that flow characteristics correspond closely to the distribution of the BL height. It is hypothesized that the reduction in mountain‐wave amplitude in the presence of surface friction is due to the reduction in the slope of the BL height as compared to the terrain height. For narrower and/or higher mountains, the BL height exhibits a secondary peak on the lee side that further modifies the wave. Simulations using the BL height as a solid mountain without surface friction confirm that characteristics of flow over the mountain are associated with the distribution of BL height rather than the terrain height field. As the non‐dimensional mountain height increases, the magnitude of the effect of surface friction decreases due to decreasing BL depth on the lee side, with the result that the BL height profile becomes closer to the terrain height field.The effect of surface friction on asymmetric mountains is also investigated. Without surface friction, the pressure drag shows distinctive reduction with a secondary peak on the downstream side, but the effect is less when the secondary peak is on the upstream side. With surface friction, all three mountains have similar pressure drag, corresponding to their similar BL height profiles. Copyright © 2003 Royal Meteorological Society

Transverse Reinforcement for Columns of RC Frames to Resist Earthquakes

An approximate formulation to determine the amount of transverse reinforcement for columns of reinforced concrete (RC) frames in seismic areas is presented. It is based on observations that suggested that the main function of transverse reinforcement is to confine the core subjected to a complex state of stresses rather than simply resist shear or improve deformability under axial compression. The combined effects of shear and axial stresses are assumed to be a function of the maximum drift ratio and they are interpreted using Coulomb’s failure criterion. A comparison between results obtained with the model developed and current design recommendations suggests that the required amount of transverse reinforcement specified in ACI 318-95 can be reduced safely for “flexible” columns (ratio of column height to effective depth ≅6) under low amplitude displacement cycles (ratio of lateral displacement to column height ≤3%) and “short” columns (ratio of column height to effective depth ≅4) under combinations of relatively low shear and axial stresses.

On the combined effects of vertical shear and zonal electric field patterns on nonlinear equatorial spread F evolution

In order to understand observations of modulated irregularities in a localized region on the bottomside of the equatorial ionosphere without the presence of vertically rising large‐scale plume events, an investigation is carried out by means of a nonlinear numerical simulation model using different background ionospheric conditions. The investigation revealed that the development of long wavelength (200 km) perturbations can only be confined to the bottomside of the ionosphere in a localized region with the combined effects of shear in the zonal plasma drift and a nighttime westward electric field. The variabilities in the occurrence characteristic are shown to depend on the variabilities in the prereversal enhancement in the zonal electric field. Further, the resurgence of a plume event at a later time, as seen by the Jicamarca radar, is explained on the basis of confined (preseeded) structures subject to a fresh instability source. First evidence is presented that a source of fresh instability may actually be a wind and/or a temperature disturbance launched hours earlier by a small substorm.

Protein denaturation by combined effect of shear and air-liquid interface

The effect of shear alone on the aggregation of recombinant human growth hormone (rhGH) and recombinant human deoxyribonuclease (rhDNase) has been found to be insignificant. This study focused on the synergetic effect of shear and gas-liquid interface on these two model proteins. Two shearing systems, the concentric-cylinder shear device (CCSD) and the rotor/stator homogenizer, were used to generate high shear (> 10(6)) in aqueous solutions in the presence of air. High shear in the presence of an air-liquid interface had no major effect on rhDNase but caused rhGH to form noncovalent aggregates. rhGH aggregation was induced by the air-liquid interface and was found to increase with increasing protein concentration and the air-liquid interfacial area. The aggregation was irreversible and exhibited a first-order kinetics with respect to the protein concentration and air-liquid interfacial area. Shear and shear rate enhanced the interaction because of its continuous generation of new air-liquid interfaces. In the presence of a surfactant, aggregation could be delayed or prevented depending upon the type and the concentration of the surfactant. The effect of air-liquid interface on proteins at low shear was examined using a nitrogen bubbling method. We found that foaming is very detrimental to rhGH even though the shear involved is low. The use of anti-foaming materials could prevent rhGH aggregation during bubbling. The superior stability exhibited by rhDNase may be linked to the higher surface tension and lower foaming tendency of its aqueous solution. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 503-512, 1997.

Design for punching shear in concrete: critical review of Canadian Standard CSA-A23.3-94: Discussion

The authors review the provisions in CSA-A23.3-94 for punching shear in concrete slabncolumn connections. They identify two items that are missing from this standard: (i )a value of αs = 2 for corner columns when determining the fac- tored shear stress resistance for the concrete and (ii) an upper limit for the stirrup spacing of s ≤ 0.5d when stirrups are used for shear reinforcement. The writers agree that these two omit- ted items should have been included. The authors also indicate they believe that the provisions for slab shear reinforcement when such reinforcement is re- quired have been changed from the previous edition of the Standard in a manner that can lead to unsafe design and refer to three tests that they claim substantiate this belief. These two items are discussed separately. The authors assert that CAN3-A23.3-M84 limits the shear stress due to the combined effect of Vf and (γvMf)x and y to 0.2λφc√ fcon a critical section outside the shear-reinforced zone. In the opinion of the writers, this is highly debatable. Clearly, this is the interpretation provided in Chapter 10 of Concrete Design Handbook (CPCA 1985) where, for an edge column, the shear moment interaction is calculated on a sec- tion outside the shear reinforcement. It is of interest to note that a similar example (18.4) can be found in Notes on ACI 318-89 (PCA 1990). This is relevant, since both CAN3- A23.3-M84 and ACI 318-89 use almost identical language regarding combined shear and moment transfer at slabncol- umn connections. In the PCA document, however, only the average shear stress due to Vf is considered at the critical section outside the shear-reinforced zone. In the opinion of the writers, neither CAN3-A23.3-M84 nor CAN3-A23.3-M94 re- quires the combined effects of shear and moment to be con- sidered at any section other than that located d/2 from a support, concentrated load, or change in slab thickness. Ideally, this discussion should depend not on semantics but on a consensus understanding of the behaviour of slabncolumn connections. The writers do not believe that there is as yet such a consensus. At best, the procedure in the Standard provides a reasonable metric of the severity of loading on a connection. It has been calibrated for a critical section d/2 from the column face. To extrapolate the procedure to sections located at greater distances from the column places inordinate faith in the fun- damental soundness of the procedure and ignores profound changes in the behaviour of the slab as distance from the col- umn support increases. In analyzing the test results, the authors do not appear to have considered all of the provisions that may govern the strength of a connection. Clause 13 of A23.3-94 gives three possible limits on the strength of a shear-reinforced interior slabncolumn connection. The first of these is a straight-line shearnmoment interaction calculated for the critical section located d/2 from the column face. The end points of the inter- action are given by the shear capacity of the connection with no unbalanced moment and the maximum unbalanced moment capacity with no net vertical shear. The second limit is on the magnitude of the average vertical shear outside the shear-re- inforced zone, called Vcode by the authors. Finally, Clause 13.11.2 requires that flexural reinforcement sufficient to resist γfM be placed within a band c + 3h. This provision places a flexural upper limit, Mflex, on the magnitude of the unbalanced

Spray-coating of rhDNase on lactose: Effect of system design, operational parameters and protein formulation

The feasibility of spray-coating fine lactose powders with recombinant human deoxyribonuclease (rhDNase) using a bench-top fluid-bed processor (STREA-1) was studied. The effect of operating parameters, system design and protein formulation on coating performance was evaluated and compared with a laboratory-scale, Würster processor (GPCG-1), as reported previously (Maa and Hsu, 1996a). Protein denaturation occurred during spray-coating in both processors, though to a lesser degree in STREA-1 than in GPCG-1. The cause of protein denaturation during coating was determined to be thermally induced and most likely occurred during drying. The combined effect of shear and heat on protein aggregation during atomization was found to be insignificant. GPCG-1 outperformed STREA-1 in terms of particle agglomeration and product yield. Particle agglomeration in the latter could be reduced by increasing the atomizing pressure and decreasing the liquid feed rate. Overall, this report demonstrates that it is feasible to use the bench-top fluid-bed processor for protein spray-coating, but the application on fine carriers (< 100 µm) is limited.

Combined effect of shear and large deformation on stress intensity factors

AbstractAn attempt has been made to study the influence of large deformation on the stress intensity factor in a cracked plate subjected to bending including shear deformation. It is assumed that singular terms for stress resultants and strains in the case of large deformation have the same angular distribution and order of singularity as in the case of a linear problem. With this in view the small deformation singular element has been used at the crack tip region surrounded by large deformation plate bending elements. The finite element analysis, based on total Lagrangian formulation combined with the modified Newton–Raphson technique, has been used to get numerical results. Several examples connected with large deformation of cracked plates subjected to bending are studied. Using the above technique stress intensity factors for linear and non‐linear cases have been compared.

Equatorial Kelvin waves embedded in mean flow, with application to the deep jets

Stationary equatorial Kelvin wave solutions are forced below the thermocline by a vertical velocity representing large‐scale convergence or divergence patterns associated with the upper ocean circulation, in the presence of a mean westward flow U mimicking the Equatorial Intermediate Current. For constant U, solutions exhibit a dominant vertical scale as given by McCreary and Lukas (1986), but in the presence of vertical mean shear, considerable changes in amplitude and vertical scale of the waves with depth occur. The combined effects of shear and linear friction on both the oscillatory and decay vertical scales of the wave flows are examined for typical shear profiles. In general, the faster the mean flow is at the top, the deeper energy is able to propagate before being dissipated. Shallow critical layers (where U = 0) may provide a barrier to the penetration of wave energy to great depths. The relevance of these ideas to the dynamics of the equatorial deep jets is discussed.

Bending tests of large diameter fabricated steel cylinders

Columns, stacks, conveyor galleries, and other similar civil engineering structures are often constructed in the form of an unstiffened tube or as a discretely stiffened tube that behaves in an essentially unstiffened manner. Tubes of this type are used extensively in the materials-handling operations of industrial plants. These are fabricated circular steel cylinders, typically 2.4-4 m in diameter, with radius-to-thickness ratios in the range of 150–400. They support and enclose conveyor systems, and they customarily span distances up to about 50 m. The task of the designer is similar to that faced with plate girders: the bending capacity, shear capacity, and the combined effects of shear and bending all must be examined. The results of a test program established to examine the flexural strength of large diameter fabricated steel tubes is reported herein. Although the number of tests is not great, the program is distinguished by the fact that the tests were done using relatively large specimens made in a way similar to that which would be employed in full-size tubes. These tests are believed to represent the best information currently available, and the results should be useful to both designers and other researchers who are approaching the problem using analytical tools. Key words: buckling, conveyors, pipes, steel, structural engineering, tubes.

Conjugate Frame for Shear and Flexure

A method is described for extending the conjugate beam concept to the solution of the combined effect of shear and flexure in frames and nonprismatic elements. A consistent sign convention is adopted. The method distinguishes between, and evaluates, the slope and cross‐sectional plane rotation. These two different rotations usually occur under the influence of shear. In structural analysis these rotations have different considerations: the rotational stiffness and rotational flexibility, of an element, are associated with the cross‐sectional plane rotation but deflection of the element is related to the slope of the deflection curve. The method provides an efficient tool for frame analysis, especially when evaluation of displacements is required at several locations in the structure.

Flow, mixing and residence time distribution of maize starch within a twin-screw extruder with a longitudinally-split barrel

A longitudinally-split barrel was used to study the internal modifications of maize starch during extrusion-cooking in a Creusot-Loire BC-72 twin-screw extruder. After steady state conditions were obtained, the extruder barrel was dismantled and samples were taken at points along the screws in order to study changes in starch granular structure and macromolecular levels in the different phase transition zones. Mass transport is the predominant process in the extruder before the reverse flight section where comminution is the major event. In this reverse flight zone, starch granules are progressively sheared and melted. Shear along the barrel surface causes rapid cooking, with the formation of a glassy phase. The results show that the combined effect of shear, together with heat and pressure, are mainly responsible for starch modifications. The residence time distribution is a function of extruder size.

ULTRASTRUCTURAL CHANGES OCCURRING DURING THERMOPLASTIC EXTRUSION OF SOYBEAN GRITS

ABSTRACTChanges occurring in the ultrastructure of soybean grits during thermoplastic or cooking extrusion were studied with the scanning electron microscope. The extruded product was obtained with a Wenger X‐5 laboratory extruder under one set of conditions (feel moisture content: 25%; screw speed: 800 rpm; max barrel temp: 145° C; temp at die: 120°C). At steady state, the extruder barrel was disassembled and samples taken from seven preselected areas along the screw. Comminution and mixing predominate first until cooking temperatures are reached in the last turns of the screw. In this zone, completely disrupted cells are arranged into strands that orient into thin fibrils at the die. Pressure release upon exit of the extrudate from the unit causes radial flow with the formation of new, more extended surfaces. Only the combined effect of shear, heat and pressure release is shown to be responsibte for texturization.

Spectral characteristics of surface‐layer turbulence

AbstractThe behaviour of spectra and cospectra of turbulence in the surface layer is described within the framework of similarity theory using wind and temperature fluctuation data obtained in the 1968 AFCRL Kansas experiments. With appropriate normalization, the spectra and cospectra are each reduced to a family of curves which spread out according to z/L at low frequencies but converge to a single universal curve in the inertial subrange. The paper compares these results with data obtained by other investigators over both land and water.Spectral constants for velocity and temperature are determined and the variability in the recent estimates of the constants is discussed. The high‐frequency behaviour is consistent with local isotropy. In the inertial subrange, where the spectra fall as n−5/3, the cospectra fall faster: uω and ωθ as n−7/3, and uθ, on the average, as n−5/2. The 4/3 ratio between the transverse and longitudinal spectral levels is observed at wavelengths of the order of the height above ground under unstable conditions and at wavelengths of the order of L/10 under stable conditions. This lower isotropic limit is shown to be governed by the combined effects of shear and buoyancy on small‐scale eddies.

New Development in the Theory of the Metal-Cutting Process: Part II—The Theory of Chip Formation

Introduction of the concept of ploughing into the metal-cutting process lead to the abandoning of the assumption of collinearity of the resultant force on tool face and on the shear plane. With this understanding the tool face force is found to produce a bending effect causing bending stresses in the shear zone. Study of the chip formation mechanism when varying cutting speed showed that increased bending action reduces the shear angle and vice versa. A set-up for the development of an analytical model of the chip formation process based on the combined effect of shear and bending stresses in the shear zone has been given. Application of the gained insight to the design of the cutting tool for maximum tool life by controlling of the chip-tool contact was suggested. Brief introduction to the study of cyclic events in chip formation and their relation to the tool life is presented.