Elastic stretching

In false twist texturing (FTT), shown schematically in Figure 7, a device twists yam upstream of its location as the threadline passes across a heater, and the yam untwists downstream from the device and is wound up. If the yam is not heated downstream from the twist device, it has bulk and high elasticity (stretch). If the yam is heated downstream from the twist device, it has bulk, but much less stretch. FTT machines initially used two steps to sequentially draw and texture. Later machines combined those steps to simultaneously draw and texture (92). Initial machines used pin spindles as the false twist device. Texturing speeds were about 150 m/min and slowly increased to about 300 m/min. As POY quaUty and stmctural stabiUty improved, new... 

A similar method, ASTM D3107, has been developed for measuring stretch and stretch recovery of woven fabrics made in whole or in part from stretch yams. The term stretch yams refers to thermoplastic filament or spun yams having a high degree of potential elastic stretch and rapid recovery. These yams are characterized by a high degree of yam cud. 

Take-Ups. A take-up is requited on a belt conveyor to ensure the proper belt tension at the drive pulley and along the conveyor, as weU as to ensure the proper troughing contour between idlers. A take-up is also needed to compensate for changes in belt length caused by elastic stretch during start-up, and any elongation characteristics of the belt that occur over a period of time. 

Manually adjusted screw or ratchet take-ups that adjust the position of the tail pulley to control belt tension can be used on relatively short, light duty conveyors. Automatic take-ups are used on conveyors over about 25 to 30 m long. The most common is the weighted automatic gravity take-up (see Fig. la). Other types of automatic take-ups have hydrauHc or pneumatic powered devices to adjust a snub pulley position and maintain a constant belt tension. The requited take-up movement varies according to the characteristics of the belt constmction and the belt length. Typically, take-up movements for pHed belts are 2% to 3% of the center distance between head and tail pulley, and about 0.5% for steel cable belts. The take-up movements requited for soHd woven belts are usually shorter because of the lower elastic stretch. Take-up requirements for a particular situation should be confirmed by the belt manufacturer. 

Should the pressure become excessive either by heating the fluid or by additional flow into the vessel, the pressure in the material used to constmet will cause the metal to first elastically stretch. Further increase in pressure will exceed the elastic limit of the wall material and it will yield, i.e., take a permanent deformation. 

A larger upward force F is then applied to the drillstring. This causes the free portion of the drillstring to elastically stretch by an amount L (ft). The stretch (or elastic displacement) is measured by the movement of the original reference mark. The magnitude of F is limited by the yield, or elastic limit of the pipe steel. 

Since in the elastic region (0 In L/0 In Ot.p is always positive, the force-temperature coefficient at constant length and pressure must be of opposite sign to PL. For rubbers, at some extensions the isometric inversion [(0 In f/6T), P = 0] must occur since pL of the isotropic sample is always positive. For solids, such measurements correspond to the determination of the coefficient (5 of an elastically stretched sample which, however, does not differ from the usual coefficient of thermal expansion. 

Fig. 22. Dependence of the linear coefficient of thermal expansion of LDPE on the angle to the drawing direction 7>. Values of [ were obtained from the heat of elastic stretching.

Fig. 29 Self-assembly of hairy-rod molecules. From left to right the interfacial tension becomes relatively more important compared to the elastic stretching of the side chains [225]...

Under the action of the craze traction cr, the craze thickens a bit further by the elastic stretch of the craze fibrils. If E is the effective averaged craze lentil modulus in the fibril direction, the final shape h(x) of the craze lentil under full traction becomes... 

The melting point falls as the binding forces holding chains together decrease, and higher elastic stretch is an indication of random molecular arrangement. The ethylene glycol condensation polymers formed with... 

It can be shown that the fracture toughness of a composite due to elastic stretching of a partially debonded reinforcing phase at the crack tip with no interfacial friction is given by ... 

Either use a hot air blower (for paint stripping), or put the cup on a layer of aluminium foil in an oven at 120 °C and note the gradual shape reversion to a nearly-flat sheet (Fig. 1.16b). The thermoforming process involved the elastic stretching of a sheet of polymer melt, and this orientation was frozen into the cup when it cooled. On reheating, the plastic attempts to return to its original shape. 

Raschel Warp knit raschel fabrics are produced in a similar way as tricot fabrics except for their stitch motions. The motion results in elastic yams being put into raschel construction as lay-in-yams versus knit-in yams on tricot constractions as shown in Figure 3.7. This lay-in stmcture leads to the raschel fabrics with elastic stretch in the length direction. Other elastic bar movements are possible to increase stretch and modulus in width. Raschel machines can have up to 64 yam guide bars to produce fabric patterns as extensive as jacquard lace. Elastomeric fabrics made by raschel showed good elastic stretch, recovery, and modulus in both length and width directions as elastic yam lap increases. 

Elastic stretch and recovery of elastomeric fabric is tested using ASTM D 4964-96 (2012) method. Fabric slippage will be more for elastic fabrics, which will affect the end results. This can be controlled by using band clamps at both the edges. Kielty et al. (2002) recommended that the stretch properties are better tested by deformation of a sample clamped all around, as in a burst tester. 

At this stage, we neglect the radial gradients in the polymer density distribution within the corona and in the elastic stretching of the A and B chains. In other words, we implement a boxlike model wherein the average concentration of monomer units inside the corona is given by ... 

The most extensive and definitive experiments on craze microstructure in homopolymers were carried out by Kramer and co-workers (Kramer 1983 Kramer and Berger 1990). In these experiments crazes were formed in thin, electron-transparent cast films of polymer deposited on copper TEM grids and firmly bonded to them by solvent vapors. When such films were subsequently stretched slightly, crazes were formed in the stretched films, permitting in-situ study of their microstructure and its mechanical response in the elastically stretched state, under tension, eliminating any possibility of relaxation in craze matter that might have occurred in unloaded fibrils. 

The return mapping techniques in inelastic solutions are a natural consequence of splitting the total strain into elastic and inelastic strains. Let tensor uy, an incremental field to describe the deformation, and its gradient, Vt/,y, show the deformation rate. The solution is implemented by the following steps. Step 1 introduces a loading condition such as F = (/, 4- V ,t)Fj." where ly is the unity second-rank tensor and the superscripts n and n 4-1 represent, respectively, the previous and current load steps. In step 2 the material is elastically stretched... 

One has to take into accoimt the non-eqnilibriiun sitnation of high-elastic deformations, that is its relaxation behavior (approach towards equilibrium in the course of time). This is so because deformation is never purely elastic. Stretching of chains (in the network) is accompanied by slippage of chains or creeping of the sample. As a result, deformation is a superposition of elastic (reversible) deformation and flow (irreversible deformation). 

Cylindrical Indenters. The elastic stretching u produced by a rigid planar indenter in a semi-infinite plate was deduced by Streicher [15] ... 

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