Mechanical Properties of Hair 240 of permanent waves (discussed further in Chapter 5 on the changing the shape of hair). The balanced fiber method has the disadvantage of requiring a relatively long fiber but is simple to use compared to some other methods. The bending strength of hair can be measured by a three-point beam deflection method. The fiber is supported at both ends and the force required to produce a small deflection in the middle is measured. Zuidema et al. used this approach to study bending stress relaxation at a function of relative humidity.54 Three-point beam deflection has also been used to measure the bending stiffness of beard hairs.55 Another approach is the cantilever beam method reported by Yasuda et al.56 In cantilever beam measurements, the hair is fixed at one end and deflected at the other. Bending measurements may be very useful for study of effects on the cuticle or for studies of hair setting (again, the reader is referred to Chapter 5). However, they are Figure 10. Illustration of the balanced fiber method of Scott and Robbins modified from Ref. 53 with permission of the Society of Cosmetic Chemists Figure 11. Bending stiffness versus Hookean slope from Scott and Robbins reprinted with permission of the Society of Cosmetic Chemists