hair internal structure strength
The rather complex mechanical properties of hair are a direct consequence of its composite structure. Changes in these properties are reflections of an alteration in structure. These may be permanent changes, such as those that result from chemical treatments that deplete strength-supporting bonds or temporary changes, such as those occurring due to changing levels of plasticization that result from the moisture content of hair being dictated by the relative humidity.
We use the industry standard method for performing such experiments using a Dia-Stron Mini Tensile Tester (MTT) interfaced with a laser micrometer system to automatically measure the dimensions of the hair fibers before they are extended to break point. Tensile testing can also be performed on wet hair.
The resulting stress-strain curves from performing constant rate extension experiments provide a variety of parameters that in turn provide information about different portions of the hair structure.
Dia-Stron Mini Tensile Tester (MTT)
Example Stress-Strain Curve
Tensile experiments are widely regarded to reflect the properties of the inner-most portions of the hair structure (the cortex), and have no significant contribution from the outer cuticle layer. Instead, the outer portions of the hair structure are taken to play a greater role in bending and twisting (torsion). Therefore, surface coatings or materials that only penetrate into the outer portions of the hair may be expected to register larger effects in such measurements.