Chapter 5 177 is deformed can be very effective for inducing bending or twisting set, while having relatively little effect on extensional properties of the wire bundle. Under conditions that produce moving boundary reduction these effects may work together to produce set while minimizing the need to break disulfide bonds. It is probable that without the stress release mechanism described by Feughelman, it would be as difficult to set fibers in bending or torsion as it is to set them in extension, and permanent waving would be a less successful technology. Wortman and Kure62 also used their bending relaxation method to study the effect of cuticle on permanent set of hair. They reported that removal of the cuticle led to faster bending relaxation and greater bending set and hypothesized that the ability of the cuticle to interfere with bending set may lead to the relatively greater difficulty in setting fine hair compared to thicker hair, since fine hair will have a higher cuticle/cortex ratio.33 Figure 14 (a): The two phases, C and M, are equally distorted by bending of the fiber (b): With the weakening of phase M by S-S reduction, tension and compression strains in phase C are removed by distortion of phase M. Modified from Feughelman57 with permission of the Society of Cosmetic Chemists.