Amorphous fatigue strength

The amorphous orientation is considered a very important parameter of the microstructure of the fiber. It has a quantitative and qualitative effect on the fiber de-formability when mechanical forces are involved. It significantly influences the fatigue strength and sorptive properties (water, dyes), as well as transport phenomena inside the fiber (migration of electric charge carriers, diffusion of liquid). The importance of the amorphous phase makes its quantification essential. Indirect and direct methods currently are used for the quantitative assessment of the amorphous phase. 

Another major reason for the failure of plastic products is dynamic fatigue. However, being a semi-crystalline material, PP offers better fatigue resistance than amorphous thermoplastics. It should be noted that dynamic fatigue strength of PP is lower than that of engineering plastics. 

Confirmation of the general fatigue superiority of semicrystalline polymers is implicit in studies of a-N behavior at frequencies low enough to minimize temperature rises (169). While the ratios of the endurance limit to tensile strength for typical amorphous polymers (polystyrene, PMMA, and cellulose acetate) are approximately 0.2, values for nylon-6,6, acetal resin, and polyterafluoroethylene are 0.3, 0.5, and 0.5, respectively. At a low frequency, polyethylene (Mw > 50,000) exhibits no failure after 5 x 10 cycles, even at the relatively high alternating stress... 

Lin and coworkers [41] also investigated the static tensile strength and fatigue behavior of long glass-reinforced semicrystalline polyannide and amorphous polycarbonate composites. The static tensile measurement at various tanperatures and tension-tension fatigue loading tests at various levels of stress amplitudes were studied. 

Crystallinity is an important consideration in injection molding because of the impact of a part s crystalline eontent on its properties and appearance. Most polymers are semicrystalline, meaning that there is a relatively large volume increase as the polymer melts and becomes entirely amorphous. After a part has been molded, the rate at which it is cooled (the speed of cooling the part) will determine the extent of its recrystallization. For example, flex fatigue life and transparency increase when crystallinity decreases. Mechanical strength and various moduli increase at higher crystallinity. 

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