Sliding stresses measurements

Konrad was the first to address the issue of pulsed piston transport using the properties of the solids as they slide through the pipe in a plug-like motion. The friction generated in such systems often can be likened to bin and hopper flow and design, requiring shear stress measurements such as carried out by the Jenike shear stress unit. The final expression using the Konrad approach can be written for horizontal flow as... 

Measurements of the sliding stress, t, and the debond energy, T, have been obtained by a variety of approaches (Table 1.1). The most direct involve displacement measurements. These are conducted in two ways (1) fiber push-through7push-in, by using a small-diameter indentor 33 38,39 and (2) tensile loading in the presence of matrix cracks,5,44,45 Indirect methods for obtaining r... 

Stress measurements at high temperature were performed with a similar equipment as used in drying experiments. The clamped strip-top layer combination is placed in a small tubular furnace positioned horizontally by a suitable sliding arrangement [28]. A small hole is provided at the bottom of the furnace for the laser beam. The exit at the bottom of the furnace must be placed close to the detector and so must be provided with a radiation screen and be cooled adequately. 

Tensile stress is used to characterize cohesiveness between particles, or in a certain powder cake, coating resistance in an encapsulated powder. Shear stress refers to the stress component tangential to the plane on which forces act and is mainly used to determine frictional properties (e.g., angle of internal friction) between particles under a pressure load. Furthermore, because individual particles predominantly slide across each other in a shearing action during flow, shear stress measurement allows determination of flow properties. 

If new paper is torn the fibers slide apart without breaking. This can be sho wn with pictures from a scanning electron microscope of the angles of cracks of aged and non-aged paper after a strain-to-stress measurement (Fig. 13.5). The fibers of the new paper only slid apart whereas in the aged paper they broke. This means that the fiber-fiber bonds in new paper and the individual fibers in aged paper are the mechanically weaker points. 

A sliding plate rheometer (simple shear) can be used to study the response of polymeric Hquids to extension-like deformations involving larger strains and strain rates than can be employed in most uniaxial extensional measurements (56,200—204). The technique requires knowledge of both shear stress and the first normal stress difference, N- (7), but has considerable potential for characteri2ing extensional behavior under conditions closely related to those in industrial processes. 

In this chapter the physical properties of resins related to processing will be described. The chapter and Appendix A4 include physical properties for many resins. These properties include bulk density and compaction, lateral stress ratio, stress at a sliding interface, melting flux, heat capacity, thermal conductivity, and melt density. Some of these properties are easy to measure by many laboratories while others such as the melting flux and stress at a sliding interface can be measured in only a few places using highly specialized equipment. 

Here, V is the sliding velocity in cm/s, P is the pressure in MPa, and T is the interface temperature in degrees Celsius. The constants in the equation have units and these units have been omitted for clarity. At temperatures less than about 110 °C, the stress at the interface is due to a frictional force mechanism. At temperatures higher than 110 °C, the forces are from a viscous mechanism. The friction coefficients provided by Eq. 5.31 have the proper trends. Coefficients derived from the equation, however, need to be used with care because of its empirical nature and the difficulties in measuring frictional data. 

The stress at the interface was measured as a function of temperature and sliding velocity for the resin using the equipment shown in Fig. 4.11, and the data are shown in Fig. 12.33. The stress curve had two maximums the first peak was at the Tg of the resin at 150 °C, and the second peak occurred at a temperature of about 240 °C. In order to maximize solids conveying while maintaining a viable process, the optimal forwarding forces would occur at a barrel surface temperature near 240 °C, and the retarding forces at the screw surfaces would be minimized at temperatures less than about 120 °C. In order to maintain the high rate of this line and the inside barrel wall at a temperature near 240 °C, the first zone of the extruder needed to be maintained at a temperature of 310 °C. 

PRESSURE. If a body of fluid is at rest, the forces are in equilibrium or the fluid is in static equilibrium. The types of force that may aci on a body are shear or tangential force, tensile force, and compressive force. Fluids move continuously under the action of shear or tangential forces. Thus, a fluid at rest is free in each part from shear forces one fluid layer does not slide relative to an adjacent layer. Fluids can be subjected to a compressive stress, which is commonly called pressure. The term may be defined as force per unit area. The pressure units may be dynes per square centimeter, pounds per square foot, torr. mega-Pascals, etc. Atmospheric pressure is the force acting upon a unit area due to the weight of the atmosphere. Gage pressure is the difference between the pressure of the fluid measured (at some point) and atmospheric pressure. Absolute pressure, which can be measured by a mercury barometer, is the sum of gage pressure plus atmospheric pressure. 

The results presented show that three levels have to be distinguished when investigating attrition processes. The first one is the stress mode as derived from the process function which is essential to know if the attrition process is to be simulated successfully in a simple experimental setup. The second point is the material reaction to this stress mode, i.e. the material function which varies depending on material properties like storage and loss modulus as measured by DMA. Finally, the microscopic attrition mechanisms (see [18] for impact and [19,20] for sliding friction) describing the formation of attrition on a microscopic scale constitute the bottom level. 

Figure 7.7. Total, elastic, and viscous stress-strain curves for uncrosslinked self-assembled type I collagen fibers.Total (open squares), elastic (filled diamonds), and viscous (filled squares) stress-strain curves for self-assembled uncrosslinked collagen fibers obtained from incremental stress-strain measurements at a strain rate of 10%/min. The fibers were tested immediately after manufacture and were not aged at room temperature. Error bars represent one standard deviation of the mean value for total and viscous stress components. Standard deviations for the elastic stress components are similar to those shown for the total stress but are omitted to present a clearer plot. The straight line for the elastic stress-strain curve closely overlaps the line for the viscous stress-strain curve. Note that the viscous stress-strain curve is above the elastic curve suggesting that viscous sliding is the predominant energy absorbing mechanism for uncrosslinked collagen fibers.

In Eqn. (12), rd represents the dynamic interfacial shear stress, which may differ from that which would be measured from fiber push-out experiments, which are typically conducted at low sliding velocities. Equation (12) holds for partial sliding along the interface. When the minimum applied stress is equal to zero, the area of the hysteresis loop can also be calculated as the integral from zero to (Tmax of the difference between the strain paths for loading and unloading (Eqns. (3) and (4)) ... 

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