356 JOURNAL OF COSMETIC SCIENCE Figure 5. Extension to break when dry generally gives step fracrures (S. Runsch, private communication). SEM by S. Ruersch, TRI Princeron. Crack initiation is generally along the cross section, while more complicated fractures occur by crack propagation in tbe axial direction. For example, fibers with flaws (in­ cluding a medulla) or with twists, or chemically damaged fibers, provide more compli­ cated fractures, such as multiple-step fractures, angle fractures (a type of step fracture), fibrillation, or splits. These more complicated fractures arise through axial propagation of cracks through the CMC, or tbrough flaws (including a medulla) or damaged areas, especially the CMC. DRY-STATE OR LOW-HUMIDITY FAILURE Deformation in the dry state (at low relative humidity) first produces scale edge lifting, and then with continued deformation, catastrophic failure occurs. l:3ETA-DELTA VERSUS l:3ETA-BETA FAILURE Extension cycling or extension at lower strain rates, at low RH, causes scale lifting and buckling of individual scales (see Figures 6, 7) rather than deep transverse cracks in the cuticle as occurs at high relative humidities or when wet at high strain rates. The extent of scale lifting and buckling of scales depends on the percentage extension and/or the number of cycles extended (12, 18).

ADHESION FAILURE IN HAIR 357 Figure 6. Scale lifting and buckling from extension (S. Ruetsch, private communication). SEM by S. Ruetsch, TRI Princeton. Failure of adhesion between overlapping scales in the cuticle is thought to occur at the hydrophobic outer beta layer, between the outer beta layer and the delta layer as first described by Negri et al. (7). Failure at this site occurs because of the weak hydrophobic bonding between branched hydrocarbons (18-MEA) and relatively short hydrocarbon amino-acid side chains of the hydrophobic fibrous protein layer (d), as depicted in the monolayer model of Figure 2A. This site is conducive to failure when the cuticle layers are strained at low moisture levels and allows the flow of cuticle past cuticle during fiber extension (2,12,14), during extension cycling (13), or even during bending (13,15), particularly in the dry state at relative humidities of 65% or lower. The lower the RH (the moisture content of the hair) the lower the strain at which failure of the scale structure can be initiated between the outer beta layer and the delta layer (13). If the bilayer model (Figure 3) exists, then this type of scale lifting failure would occur between the two fatty-acid bilayers, i.e., between the 18-MEA layer and the fatty acid layer of Figure 3, producing beta-beta failure. This type of failure is not consistent with the conclusions of Negri et al. (7), providing support for the monolayer model over the bilayer model. The different layers of the hair structure are interconnected and stretch dependently. In the dry state, failure often occurs in or between hydrophobic layers because hydrophilic layers are not as extensible at lower moisture levels and therefore force higher strains in the hydrophobic layers for any given stress. FIBER FRACTURE AT LOW RH Extension to break, at lower RH (primarily lower than 30-45% RH), of chemically