26—THE “HANDLE” OF CLOTH AS A MEASURABLE QUANTITY

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Abstract
In judging the feel or “handle” of a material, use is made of such sensations as stiffness or limpness, hardness or softness, and roughness or smoothness. It is desirable to devise physical tests that analyse and reflect the sensations felt and assign numerical values to the measurements. The present paper describes tests that reflect the first groups of sensations, namely, stiffness and hardness, whilst the sensation that is experienced in stroking a material, obviously connected with frictional properties, will be dealt with in another paper. An instrument is described on which it is possible to measure the angle through which a specimen of cloth droops when a definite length is held out over an edge. By means of a mathematical formula that is fully developed in an appendix to the paper this angle is converted into a term called the bending length of the material. This quantity may be defined as the length of fabric that will bend under its own weight to a definite extent. It is strictly a measure of the draping quality of a fabric. The stiffer the material, the longer is the “bending length.” From the “bending length” and the weight of the material, a simple calculation gives a quantity called the flexural rigidity which measures the resistance to bending or the stiffness that would be appreciated by the fingers. These two quantities should be measured both along the weft and along the warp in woven fabrics, but it is unnecessary to apply the test in any diagonal direction. The stiffness of the fabric as a whole is completely governed by the warp-way and weft-way stiffnesses, and these separate quantities may be converted into a single quantity for stiffness in any direction. Another property that is sensed when a fabric is grasped is thickness, but this depends on the amount by which the material is squeezed, so that the sensation combines that of thickness and hardness. The measurement of thickness is discussed in detail, for it is not so simple as it might appear. Having standardised the method, however, it is easy to measure hardness by determining thickness under different pressures. It is often desirable to compare the stiffness of materials of different thicknesses, for example, to compare a cloth before and after a process like calendering. For this purpose the above flexural rigidity may be converted into a quantity called the bending modulus that takes account of thickness. Flexural rigidity itself is highly dependent on thickness; in fact, doubling the thickness increases the flexural rigidity eightfold. The bending modulus on the other hand, is in a sense a measure of the intrinsic stiffness of the material. Generally speaking, it reflects the degree of compactness, or of the adhesion between fibres and threads—the difference between what is described as a “full” handle and “papery.” The standard procedure for determining bending length may be varied with regard to the shape and dimensions of the test specimen for very stiff or very limp materials, and the alternative calculations are demonstrated. A different but more laborious method for measuring flexural rigidity is also described in connection with a study of the effect of humidity on the stiffness of organdie.

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