Fatigue in Textile Fibers1

Abstract

The practical advantages of a knowledge of the appropriate lifetime distribution in fatigue research are pointed out. The logarithmic lifetimes of two sets of polyester specimens, one fatigued over a 47-day period at a constant force amplitude (CFA) of 11.5 g, the other over a shorter period at a CFA of 13.4 g, were judged to conform better to the Weilbull third-asymptotic distribution than to the normal distribution, on the basis of visual inspection. A decline of about a decade and a half in the average logarithmic lifetime resulted from the higher level of loading.

Measurements of the deviations from visually fitted straight lines of observed logarithmic lifetimes of the polyester and a polyamide sample, plotted on Weibull and on normal probability coordinates, shows that the observed results conform better to the former distribution. This finding is confirmed when the analysis is applied to the deviations from a line located by the squares method.

A series of tests, employing the same polyester, was undertaken to assess the feasibility of attempting to load specimens with force-amplitudes of improved uniformity on a gram-per-denier basis. The technique adopted was to match care fully the slightly scattered linear densities of fiber specimens with the constants of the individual springs on the machine. While, in the final test, the spread in the loading (g/den) had been reduced to 5%, no clearly detectable improvement was found in the uniformity of logarithmic lifetimes—the ultimate objective. The conclusion is that structural dif ferences among specimens so far over-ride small nonuniformity in loading as to make it unprofitable to give time-con suming attention to the latter.

For fibers of the polyester sample fatigued at four successively increasing force-amplitudes, the lifetime data showed moderately good conformance to the cumultive-damage principle. Values of 0.79 and 0.75 for the sums of damage ratios were found, where simple theory calls, for unity; though a newer hypothesis predicts deviations.

A plot of the results of several tests on the polyester sample shows a general, substantial decline in logarithmic life times with increases in the force-amplitude.

The lifetimes of specimens of the aromatic polyamide sample were found to conform to a Weibull distribution and to have an average value and range comparing favorably with those of the polyester sample under relatively the same conditions.