Comparing hair tensile testing in the wet and the dry state: Possibilities and limitations for detecting changes of hair properties due to chemical and physical treatments.

Objectives: This investigation focuses, first, on the question to which extent wet and dry tensile tests on human hair may be considered as leading to independent results. Second, we try to assess the sensitivities of wet and dry-testing to detect changes of mechanical properties. Specifically, we were interested in separating changes, which were induced by a combination of a chemical (oxidation/bleach) and a physical treatment (heat).

The information content of tensile tests of human hair (wet) is limited: Variables mainly cluster in just two principal components.

Tensile testing of keratin fibres, such as wool and hair, in the wet state is a well established tool in the academic as well as applied portfolio of mechanical analyses. For the tensile curve of a hair fibre, fourteen material-specific stress-, strain, and work-related variables were identified. Analysing twelve samples of cosmetically untreated hair, we show that the variables show good precision and a satisfactory degree of homogeneity across samples.