Tensile work recovery

Tensile work recovery can be considered a dynamic-loss modulus obtained at very low cycling rates. A fiber Is stressed to 3 g/den. and Immediately allowed to recover. The work recovered exhibits a marked reduction at the glass transition temperature (Figure 7). 

For a long time, the fiber industry had been aware of PTT having desirable properties for fiber applications. In a 1971 patent [3], Fiber Industries, Inc. found PTT fiber to have a lower modulus, better bending and work recoveries than PET, and was therefore more suitable than PET for making fiberfill and carpets. Ward et al. [4] compared the mechanical properties of the three polyester fibers, and found PTT indeed had a better tensile elastic recovery and a lower modulus than both PET and PBT. These two properties are very desirable and are valued... 

The tensile strain recovery and the work recovery % are also calculated. The procedure for crimped specimens is much the same, except that the test specimen is first pretensioned to take out the crimp and the extension shown on the recording chart eliminated by rezeroing the recorder. The location of the cross-head is locked in the memory at this point, so that on recovery the cross-head does not reenter the crimp range. 

When a specimen is stretched less than the breaking elongation and allowed to relax by removal of the strain according to a predetermined procedure. ( = tensile recovery curve, work recovery curve, and strain recovery curve.)... 

In addition to dimensional recoveiy, work recoveiy often is studied for tensile deformation. While stretching a fiber, the total work done is either stored in chemical bonds or is lost, typically in the form of heat. The work stored in the chemical bonds is recoverable, but the work lost is not. In Figure 15.23, the area under the stretch curve is the total work per unit volume done during stretching. The area under the recovery curve is the work per unit volume returned during recovery. Work recovery can then be defined as ... 

For tensile creep, TJ would be the tensile viscosity. When the viscosity is high (e.g., when working at relatively low temperatures or with very high-molecular-weight polymers) it can be difficult to determine tl-x accurately, so creep recovery measurements are made. Here the load is released after a given creep time and the strain is followed as the specimen shrinks back toward its new equilibrium dimensions. 

More recently, Robbins and Crawford [8] provided additional evidence that the cortex and not the cuticle is responsible for the tensile properties of human hair and that severe damage can occur in the cuticle that cannot be detected by tensile property evaluation. This work involved selective oxidation of the cuticle with m-diperisophthalic acid. This treatment produces extensive cuticular damage that is detectable microscopically, but this damage could not be detected by either wet or dry tensile property evaluation. These results are consistent with the fact that wet extension of hair fibers to 30% can damage the cuticle [9] yet on relaxation in water, tensile recovery occurs producing virtually identical elongation-recovery curves in a before and after type of evaluation. This basic procedure is commonly used throughout the industry. 

Fiber Tenacity (g/den) Breaking extension (%) work factor Elastic recovery 2% elongation (%) Stiffness (g/tex) Toughness (g/tex) Work to rupture (mN/tex) biitial modulus (N/tex) Ratio tensile modulus over shear modulus Breaking twist angle (a°)... 

In the original paper [20], the authors reported work on the uniaxial tension of plasticized poly(vinyl chloride), sulphur vulcanizates of butyl rubber and polyisobutylene. Very successful predictions were made at extension ratios of up to 5. Zapas and Craft [21] applied their formulation to multistep stress relaxation and creep and recovery of both plasticized poly(vinyl chloride) and pol3dsobuty-lene. McKenna and Zapas applied a modified form of the model to the torsional deformation of poly(methyl methacrylate) [22]. McKenna and Zapas [23] have used the model in analysis of the tensile behaviour of carbon-black-fllled butyl rubbers. 

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