Nylon fatigue failure

Fig. 5.8 Typical fatigue failure of a nylon 6,6 fiber is shown by SEM (A and B) of a fiber tested at 0-55% of nominal breaking load, with a lifetime of 1.3 X 10 cycles at 50 Hz. Fiber failure is initiated at the surface resulting in a tail on one side (A) and long furrow on the other side (B).

Figure 13.48 Relationship between the stress intensity range AK, corresponding to an arbitrary value ofdc/dN 7.6 x 10 m/cycle, and the maximum stress intensity factor range AKmax observed at failure for a group of polymers. The polymers are ( ) cross-linked polystyrene, (2) PMMA, (3) PVC, (4) LDPE, (5) polystyrene, (6) polysulfone, (7) high-impact polystyrene, (8) ABS resin, (9) chlorinated polyether, (10) poly(phenylene oxide), (11) nylon 6, (12) polycarbonate, (13) nylon 6 6 and (14) poly(vinylidene fluoride). (Reproduced from Manson, I.A. and Hertzberg, R.W. (1973) Fatigue Failure in Polymers. CRC Crit. Rev. Macromol. Sci., 1, 433. Copyright (1973) Taylor and Francis.)...

Figure 19.7. Fatigue resistance of acetal resin compared with nylon 66 and with polyethylene. Measured as the cycles to failure at a given applied stress. (Du Pont trade literature)...

Fatigue tests on unnotched samples have demonstrated that the fatigue strength (the stress corresponding to failure at a given number of cycles) is reduced by as much as 30% when nylon-6,6 is equilibrated at 50% rh (152) (see Fig. 34). In marked contrast, it has been found that fatigue growth rates in nylon-6,6 exhibit a pronoimced minimum at an absorbed moisture content of 2.6 wt% water, and... 

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... 

Figure 11.25 Fatigue lifetimes versus stress level for nylon 6, poly(methyl methacrylate), and polytetrafluoroethylene (46). This plot is known as an S-N plot, tor stress and number of cycles to failure.

Impact fatigue tests have also been conducted on some polymers namely Nylon 66, HIPS acrylonitrile butadiene styrene (ABS), and rubber-toughened Nylon. The impact energy was 98% of the crack initiation energy. Specimens withstood 100-200 cycles of loading before failure. 

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