Fibers strength

The first hurdle that phosphate fibers were required to overcome was a tensile strength deficiency, and then a justified harassment by concerned management desiring to know if the fibers had any value for anything, anywhere. The first approach was to calculate the theoretical strength of phosphate fibers while making 

The measured values for phosphate fibers compare very favorably with other fiber systems of common use. It is interesting that sodium calcium polyphosphate fibers have a higher tensile strength than calcium polyphosphate fibers. Fiber diameter could account for this difference, or that anions of calcium polyphosphate are more linearly packed in the crystals. Kevlar is a fiber that has been extensively used in nonasbestos brakes for automobiles and it performs well. 

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For nosetip materials 3-directional-reinforced (3D) carbon preforms are formed using small cell sizes for uniform ablation and small pore size. Figure 5 shows typical unit cell dimensions for two of the most common 3D nosetip materials. Carbon-carbon woven preforms have been made with a variety of cell dimensions for different appHcations (27—33). Fibers common to these composites include rayon, polyacrylonitrile, and pitch precursor carbon fibers. Strength of these fibers ranges from 1 to 5 GPa (145,000—725,000 psi) and modulus ranges from 300 to 800 GPa. 

An additional issue in fiber strength is that of fatigue (22), which can produce delayed failure of a fiber. Fatigue is thought to be caused by a surface reaction of fiber and OH causing the growth of subcritical flaws to the point where fracture occurs. 

Chemical Properties. The hydrolysis of PET is acid- or base-catalyzed and is highly temperature dependent and relatively rapid at polymer melt temperatures. Treatment for several weeks in 70°C water results in no significant fiber strength loss. However, at 100°C, approximately 20% of the PET tenacity is lost in one week and about 60% is lost in three weeks (47). In general, the hydrolysis and chemical resistance of copolyester materials is less than that for PET and depends on both the type and amount of comonomer. 

Fine adjusting and optimization of each step of this process is stiU underway, and a PVA fiber having a single fiber strength as high as 2 N/tex (21 gf/dtex), which is close to that of aramid fiber, has been reported (18). 

Deterioration. The causes of degradation phenomena in textiles (155—158, 164) are many and include pollution, bleaches, acids, alkaUes, and, of course, wear. The single most important effect, however, is that of photodegradation. Both ceUulosic and proteinaceous fibers are highly photosensitive. The natural sensitivity of the fibers are enhanced by impurities, remainders of finishing processes, and mordants for dyes. Depolymerization and oxidation lead to decreased fiber strength and to embrittlement. 

Prior to deposition on a moving belt or screen, the molten polymer threads from a spinnerette must be attenuated to orient the molecular chains of the fibers in order to increase fiber strength and decrease extendibiUty. This is accompHshed by hauling the plastic fibers off immediately after they have exited the spinnerette. In practice this is done by accelerating the fibers either mechanically (18) or pneumatically (17,19,20). In most processes, the fibers are pneumatically accelerated in multiple filament bundles however, other arrangements have been described wherein a linearly aligned row(s) of individual filaments is pneumatically accelerated (21,22). 

Melt-spun fiber is produced from PMP at 280°C and is drawn around three times in air at 95°C its fiber strength is 0.18—0.26 N/tex (2—3 g/den), its elongation is around 30%. Melt-spun hoUow fibers are also manufactured. PMP has one of the highest permeabiHties for gases, and many of its appHcations capitalize on this property. 

Empirical attempts have been made to relate strip and grab test results, particularly for cotton fabrics, so that if one strength is known, the other can be calculated. The relationship is complex, depending on fiber strength and modulus, yam size and crimp, yam-to-yam friction, fabric cover factor, weave, weight, and other factors (19). 

The reinforcing capacity of asbestos fibers in a cement matrix constitutes another key criteria for the evaluation of asbestos fibers. This property is assessed by preparing samples of asbestos —cement composites which, after a standard curing period, are tested for flexural resistance. The measured mpture modub are converted into a parameter referred to as the fiber strength unit (FSU) (34). 

Fig. 6. Each of carbonization temperature on PAN-based carbon fiber strength and modulus (31). To convert GPa to psi, multiply by 145,000.

Figure 7 Dependence of fiber strength on the content of cellulose and on the angle of fibrils [91].

In [148] it was shown that the strength of the composite depends on the fiber strength at the so-called ineffective length, so that only a few percent of the total amount of the ineffective fiber lengths in the composite become broken before the specimen fails completely. The authors of [148] proposed to express the relationship between the average fiber strength [Pg.19]

However, in composites, fiber ruptures result in development of mainline cracks even at lower stresses. It is therefore necessary to know the so-called attainment coefficient in order to predict the mechanical properties of composites. The fiber strength in a PCM [Pg.20]

The fiber strength is determined for a particular cramped length (for example, for the 10 mm base) and the specimen is tested under tension (bending) at two P values. One then solves the set of simultaneous equations ... 

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