BEHAVIOR OF NEGROID HAIR 23 TORSIONAL EFFECTS IN TENSILE LOADING Because of the twisted configuration of the fiber, tensile loading can lead to torsional untwisting. This untwisting was measured by following the rotation of a horizontal bar (• lg) mounted on the lower tab of a tensile test specimen which was loaded manually with weights in small increments. The arrangement is shown in Figure 1. The position of the bar under its own weight was taken as the reference point, and the deflections from this position were measured 60 s after each load increment counterclockwise rotation is expressed as positive. In an improved version of this method, measurements were made on a fiber specimen (1-2-mm-long) having a single twist isolated with the help of a microscope. Extension measurements as a function of loading were made simultaneously with the measurement of untwisting the position of the bar under a load of 5 g was taken as the reference point (0ø). TENSILE FATIGUE Tensile fatigue measurements were carried out on an instrument developed at TRI and shown schematically in Figure 2. It consists of a stationary upper disc on which 50 Stretched Relaxed fiber fiber We .... -, - -' ght Figure 2. Tensile fatigue testing apparatus (schematic). fibers (• 50-mm-long) are mounted, each loaded with a suitable weight. The lower disc is moved up and down at a selected frequency by an electric motor. The stroke length (•25 mm) is adjusted so that the weights mounted on the fibers clear the lower (movable) disc at its lowest position, thereby straightening and impact loading the fiber. At the highest position, the weight rests on the disc and the fiber assumes its natural unstressed configuration. This flexing/deformation cycle is intended to simulate the deformation experienced by the fibers when subjected to picking or combing. The frequency of the cycling was chosen at 1.13 cycles/s (68 cycles/min), and the runs lasted for 9.72 ks, so that the surviving fibers experienced •11,000 cycles of loading.

24 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Experiments were done with loads of 10, 20, 30, and 40 g. Fibers surviving this fatigue test were subjected to tensile fracture on an Instron © tensile tester, and the extension at break was determined. In the course of this work it was observed that some fibers had a tendency to assume a permanently twisted configuration produced by a unidirectional twisting movement of the weight as it lifted from the lower disc. This twisting action was prevented by inserting restraining clamps between the weights. MICROSCOPIC TECHNIQUES Throughout this work, optical and scanning electron microscopic (SEM) techniques have been used extensively to examine various aspects of fiber structure. Fiber cross sections were prepared by embedding the fibers in an epoxy resin matrix and using a microtome with a glass knife. Fractured ends of fibers after various mechanical tests were sputtered with gold prior to SEM examination. Figure 3. Negroid hair fibers showing regions of twist. 300x.