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© 1999 Society of Cosmetic Chemists
Journal of the Society of Cosmetic Chemists, Vol. 50, No. 2, 69-77


Plastic yielding and fracture of human hair cuticles by cyclical torsion stresses
Manuel Gamez-Garcia

ABSTRACT

Synopsis

Plastic yielding in the form of crazing and shear bands was found to occur in human hair cuticles subjected to cyclical torsion stresses. This type of damage appeared in the form of helicoidal longitudinal strips around the main axis of the hair fiber following sections of maximum shear stress during twisting. The hair regions with shear bands and crazing were approximately 30 microns wide and 2 to 3 millimeters long and gave the appearance that the hair had partially lost its cuticles. SEM analysis revealed, however, that the cuticular material was still there, and that rather the damaged cuticular regions had lost their structure and boundaries because they were filled with microvoids, microcracks, and sometimes very narrow long vertical cracks. This type of plastic deformation was found to be a mechanism for dissipating mechanical energy in cuticles in response to the torsional shear stresses that are expected to attain a maximum value at the hair surface. Shear band and craze formation was found to be very sensitive to the moisture content in hair, and at high relative humidities it did not occur at all. Analysis of hair from a panel of 100 individuals showed that shear band and craze formation is also frequently found in hair collected from panelists who employed only conventional grooming practices. The effect of shear bands and crazes on split end formation is also discussed.

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© 1999 Society of Cosmetic Chemists
Journal of the Society of Cosmetic Chemists