j. Soc. Cosmet. Chem., 35, 21-43 (January/February 1984) Mechanical and fractographic behavior of Negroid hair Y. K. KAMATH, SIDNEY B. HORNBY, and H.-D. WEIGMANN, Textile Research Institute, P.O. Box 625, Princeton, NJ 08540. Received August 26, 1983. Synopsis The fracture behavior of Negroid hair was studied to clarify the causes of fiber breakage at low levels of extension. Visual observation and ellipticity measurements reveal frequent twists, with random reversals in direction and pronounced flattening which can lead to stress concentrations during tensile deformation. Simultaneous measurements of the effect of tensile load on extension and on axial angle of untwisting of specimens with a single twist indicate that failure at low extensions is due to the initiation of cracks at numerous flaws near the twists, which relieves torsional stresses in these regions. Extension at failure is higher in wet fibers, probably because plasticization relaxes these stresses. Scanning electron microscopy of fracture ends reveals a predominance of step fractures, indicating a large number of flaws, and a large proportion of fibrillated ends, reflecting poor cohesion between cortical cells. Fatiguing via a method devised to simulate the impact loading occurring during hair grooming appears to accentuate existing fiber damage and/or to reduce intercellular cohesion in the cortex, since fibrillated fracture ends predominate among fibers that fracture during fatiguing. The large number of premature failures in surviving fibers suggests that new damage may be initiated at the highest fatiguing loads and may also occur during combing and picking. INTRODUCTION There are three general types of human hair which can be associated with the three major groups of the human race, i.e., Caucasian, Negroid, and Oriental. Among these three types Negroid hair is distinctive because of a high frequency of kinks along the fiber axis. This unique fiber configuration leads to extensive entanglements which present difficulties in combing and other grooming procedures. As a result, grooming practices have been developed that can cause considerable damage to the hair. One such practice is hot combing with a metal comb, during which the hair is stretched at relatively high temperatures. This can cause partial straightening and possible damage to the surface of the fiber. Extensive loss of cuticle is frequently observed in hair of Negroid origin which has been subjected to this type of treatment, resulting in poor mechanical and fractographic behavior. Repeated combing or picking of a hair assembly is equivalent to subjecting the fibers to cyclic tensile loading, or tensile fatigue, which can have damaging effects on fiber structure. Because of the twisting of the Negroid hair fiber over its longitudinal axis, such tensile loading will also develop torsional stresses, thus subjecting the fiber to 21

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.