Chapter 7 231 affected by either bleaching or permanent treatments compared to wet mechanical properties. Wet yield stress and wet breaking stress were decreased by each bleaching treatment (5D). Extension to break was increased by bleach treatment for both wet and dry hair but the increase in break extension was higher for wet hair (5C). One of the parameters often measured to assess hair damage is “work to break,” the area under the entire stress/strain curve. This parameter may be confounded for low levels of damage, if break stress and post-yield slope only decrease slightly but “extension to break” increases. The increase in extension to break will have the effect of making work to break higher, possibly counteracting the decrease in break stress. This potential problem does not exist for work to 15–20% extension, since the degree of extension is fixed. Tate et al.34 investigated the effects of successive permanent wave treatments on various parameters that can be derived from tensile measurements as part of an extensive review of methods to study hair damage. Treatment was for 20 minutes in excess solution with 1M ammonium thioglycolate (ATGA) at pH 9.5, followed by rinsing and neutralization with 3% H 2 O 2 at pH 7.0. Table 1 shows their results recalculated as percentage change from no treatment. In contrast to bleaching treatment, which produced approximately equal reductions in yield and breaking stresses, the first permanent wave treatment reduced breaking stress by 30% while only reducing yield stress by 14%. This may result from a greater tendency of the reducing treatment compared with the bleaching treatment to result in broken disulfide bonds, thus weakening the fiber. The large reduction seen in work to break may be due to the high sensitivity of the post-yield slope to reducing treatment (refer back to Figure 2). It should be pointed out that the 20-minute treatment with excess 1M ATGA at pH 9.5 probably results in “over-processing” compared to a standard wave. This author’s experience with permanent waving indicates that a single treatment on virgin hair replicating permanent waving directly on the head will produce a reduction of 5–8% in stress at 20% extension.