Elongation fiber

Crimp. The tow is usually relaxed at this point. Relaxation is essential because it gready reduces the tendency for fibrillation and increases the dimensional stabiUty of the fiber. Relaxation also increases fiber elongation and improves dye diffusion rates. This relaxation can be done in-line on Superba equipment or in batches in an autoclave. Generally saturated steam is used because the moisture reduces the process temperatures required. Fiber shrinkage during relaxation ranges from 10 to 40% depending on the temperature used, the polymer composition used for the fiber, and the amount of prior orientation and relaxation. The amount of relaxation is also tailored to the intended apphcation of the fiber product. 

Fiber Elongation, % Modulus, GPa " Strength, GPa Energy to break, /kg... 

The optical character of chalcedony is distinct from that expected for the normally uniaxial mineral, quartz, and signals the fibrous nature of a particular sample. The direction of fiber elongation is often parallel to the [1120] crystallographic direction of the quartz structure (Fig. 2.19A), but other fiber directions have also been determined within a single sample (Frondel, 1985). The presence of helically twisted fibers are suspected from the variations in extinction and birefringence noted along the fiber length (Fig. 2.19C). More detailed information on the optical or other physical and chemical properties of quartz and its many varieties can be found in volume 3 of Palache et al. (1962) and in Frondel (1985). 

The elastic properties discussed so far relate to stresses applied at relatively low rates. When forces are applied at rapid rates, then dynamic moduli are obtained. The energy relationships and the orders of magnitude of the data are much different [570]. Because of the experimental difficulties, only little work at rapid rates has been carried out with cotton fiber compared to that done with testing at low rates of application of stress. In contrast, cotton also responds to zero rate of loading, i.e., the application of a constant stress. Under this condition the fiber exhibits creep that is measured by determining fiber elongation at various intervals of time after the load has been applied. Creep is time-dependent and may be reversible upon removal of the load. However, even a low load applied to a fiber for a long period of time will cause the fiber to break. 

Figure 3. Integrated intensities plotted as a function of percent PPTA fiber elongation A - carbonyl band at 1656 cm, (Y-scale to the far left) ...

Under a constant large load, instantaneously applied, animal fibers elongate rapidly by about 30 % in cold water and then continue to creep for a prolonged period (Speakman, 1926). In the post-yield region, after... 

Figure 9.14 a) Schematic cross section of viscometer, b) Fiber elongation method. 

Letourneau, P.C. (1978) Chemotactic response of nerve fiber elongation to nerve growth factor. Dev. Biol. 66 183-196. 

It is recommended for bend rafios over 5% or fiber elongation greater than 40% that the coils be heat treated after forming. The bend ratio can be computed as follows ... 

The softening point is more properly termed the Littleton softening point, after the specific test used to define this reference point. The viscosity of 10 Pa s does not represent the deformation temperature for all objects. This particular reference point is defined in terms of a well-specified test involving a fiber -0.7 mm in diameter, with a length of 24 cm. The softening point is defined as the temperature at which this fiber elongates at a rate of 1 mm min when the top 10 cm of the fiber is heated at a rate of 5 K min. In fact, if the density of the fiber is significantly different from that of a typical soda-lime-silica composition, the viscosity will not be exactly 10 Pa s at this temperature. 

Once an object is formed, the internal stresses which result from cooling are usually reduced by annealing. The annealing point (cited in various sources as either 10 or 10 " Pa s), which is also determined using a fiber elongation test, is defined as the temperature where the stress is substantially relieved in a few minutes. The strain point (1013.5 ig defined as the temperature where stress is substantially relieved in several hours. The strain point is determined by extrapolation of data from annealing point studies. Other tests are also used for these two reference points, with slightly different results. 

Fiber elongation measurements are based on the rate of elongation, dL/dt, where L is the fiber length, of a fiber of cross-sectional area, A, which is suspended vertically in a furnace. The elongation rate is determined by the viscosity of the melt and the applied stress, F/A, where F is the force applied to the fiber. The viscosity is then given by the expression ... 

Since the area of the fiber is continually decreasing as the fiber elongates, a correction for the changing fiber area must be applied throughout the measurement. The large surface to volume ratio of the fiber also frequently results in compositional changes at the fiber surface, by either reaction with atmospheric gases or by evaporation of melt components. 

The Kissinger method can also be used to determine the activation energy for viscous flow, by replacing the exothermic peak maximum temperature with the glass transformation temperature in equation 12.8. This method is particularly useful for glasses where it is difficult to form the sample needed for application of the beam-bending or fiber elongation methods discussed in Chapter 6. 

The determination of viscosity from the couple exerted on concentric cylinders is used in many viscometers. For silicate glasses, a wide range of techniques are used for measuring viscosity. For low viscosities (1 Pa s to 1 MPa s), a rotating spindle technique, based on the above principle, is often used. For higher viscosities, techniques based on fiber elongation, compression of a cylinder and be im-bending are utilized. 

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