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© 1984 Society of Cosmetic Chemists
Journal of the Society of Cosmetic Chemists, Vol. 35, No. 1, 21-43


Mechanical and fractographic behavior of Negroid hair
Y. K. Kamath , Sidney B. Hornby , H.-D. Weigmann

ABSTRACT

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.

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© 1984 Society of Cosmetic Chemists
Journal of the Society of Cosmetic Chemists