22 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS both tensile and torsional fatigue. Generally Negroid hair has a lower extension to break than Caucasian hair. It is also a common observation that Negroid hair, when subjected to mechanical handling, breaks into half-wave-length sections. This observa- tion suggests that the regions of twist along the fiber axis are particularly prone to damage, so that failure occurs at relatively low levels of extension. The purpose of the study reported in this paper was to gain a better understanding of the cause of such premature failure based on an investigation of the fracture behavior of Negroid hair. As part of this investigation, torsional effects during tensile loading of the fiber have been studied as well as the effects of fatiguing fibers at low load levels. EXPERIMENTAL HAIR SAMPLE The hair used in this work was obtained from a 31-year-old male from the United States and was specified not to have been exposed to thermal or chemical treatments. The sample was washed with a 12.5% aqueous solution of sodium lauryl sulfate, rinsed exhaustively with distilled water, dried at room temperature overnight, and conditioned at 65% RH and 21øC. TENSILE MECHANICAL PROPERTIES These properties were determined on the Instron tensile tester using 50-mm-long specimens mounted on tabs. Both vibroscopic and microscopic methods were used to determine the cross-sectional area of the fiber. Measurements in distilled water ("wet" condition) were carried out after soaking the specimens for 3.6 ks. //••,•,•-• Light II -,---- Fiber • 3-•'", •-"' Indicc•tor bc•r ]---•Weight Figure 1. Arrangement to study fiber untwisting during tensile loading.

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.