Single and multiple fiber tests for determining comparative breaking loads of wool fibers

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Introduction

For many years wool technologists have tested the breaking load and the breaking stress of wool by using individual fibers. The number of fibers that must be broken individually in order to yield mean values of significance varies mostly with the uniformity of diameter of the fibers constituting the sample. Most staples or locks of wool, even from a small area on a purebred animal of an improved breed, will contain some fibers twice as coarse as others lying almost contiguous in the same sample. Among the breeds less improved from the standpoint of wool production, a lock taken from a small spot on the shoulder of the animal, where presumably the least variation might be found, may show differences as high as 400 per cent in fiber diameter. The sheep breeder aims to produce animals with uniform fleeces in which fiber diameter is the same over all parts of the body. As yet, however, man’s effort to hasten the evolution of the sheep has not resulted in fleeces which can be called uniform if judged by standards for commodities other than wool. A staple of wool may be exceedingly uniform by comparison with a similar staple from another individual; but if its component fibers are compared in uniformity of diameter with drawn copper wire or with the fibers from a single cotton plant, the comparison is discouraging. In the field of wool production, therefore, uniformity is relative, not absolute.

As these statements indicate, though the accurate determination of the mean breaking load of one staple of wool may require x fibers, a similar determination for a less uniform staple may require 2x or 3x fibers. Obviously, too, such a determination, even on the very best wools, necessitates the use of relatively large numbers of fibers. The engineer may be able to test ten bars of a steel alloy and say with some certainty just what strains will cause rupture; the wool investigator cannot base conclusions on so small a number.

Considerations other than diameter variations must also be reckoned with in determining the breaking load of wool.