Chapter 7 229 Tensile measurements and hair damage: For many years tensile properties have been measured to determine the level of “damage” produced by a given treatment.20,28,30-35 The mechanical properties of wet hair are greatly affected by treatments that reduce the number of keratin bonds. Five different parameters have been used to evaluate damage by various workers. These are Young’s modulus, yield stress (typically measured at 15–20% extension), work to extend the hair by 20% (the area under the stress strain curve from 0–20%), breaking stress, and the work to break (the area under the entire stress strain curve). Not all workers have used all parameters. Prior to the availability of the automation provided by the Dia- stron it was very time consuming to run large numbers of hairs. For this reason many workers relied on the fact that mechanical properties of hair extended into the yield region but not beyond can be recovered by soaking the hair in water. Beyak et al.25 extended hair by 15% and measured the force. After recovery in water, treatments were investigated using each hair as its own control. The average change between tests for 25 untreated control hairs was only -0.33%. A five-minute “cold wave” treatment led to a 12.6% decline in the force at 20% extension. A major advantage of tensile “index” methods is that, because each hair is used as its own control, it is not Figure 4. A Dia-stron tensile tester equipped with an automated sampling device (image courtesy of Dia-stron)