Mechanical Properties of Hair 228 Tensile testing methodology: In order to determine mechanical properties such as the elastic modulus of an object, the cross- sectional dimension must be known to a high degree of accuracy. Determining the cross-sectional dimensions of a hair can be challenging. Hair is a thin fiber that it is not necessarily uniform in cross section. While Asian and Caucasian hairs are generally elliptical in shape, significant variations from ellipticity can occur in some hairs.24 The high elliptical ratio and non-uniform shape of African hair compounds the problem.19 Laser micrometers can be used for this purpose. Several measurements should be made along the fiber and the fiber must be rotated to be sure to measure the major and minor axes of the ellipse. The most accurate method of determining the cross-sectional area of a hair is probably to section it and measure the diameters directly from the micrographs but this is not very practical for large sample sizes. Prior to the introduction of the Dia-Stron line of miniature tensile testers (Dia-Stron, Broomall, PA), stress/strain measurements on hair were mostly made using Instron tensile testers (Instron, Norwood, MA).25-28 The Dia-Stron is more compact and portable than most Instron models and the instrument can be equipped with an automated sample head29 to allow running of up to 100 hairs in a single test (Figure 4). It can be interfaced with a laser micrometer system to automatically measure the dimensions of the hair fibers, and the Dia-stron software can automatically record the stress/strain curve and report various parameters to the operator, including elastic gradient (the slope of the force/extension curve in the linear region), work of extension, and breaking load. For these reasons the Dia-Stron has become the most commonly used instrument for measuring the tensile properties of hair. Variations in relative humidity (RH) can be a significant source of error when making dry measurements of elastic modulus or yield region properties, and precise RH control is critical. The implications of the variation of modulus with RH to the structure of hair will be discussed. As discussed, the post-yield slope is generally much less sensitive to RH.