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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). Methods:The basis for our study are data for the tensile properties (wet and dry) of a set of untreated and bleached hair tresses, which were submitted to the same schedule of thermal treatments. As characteristic tensile parameters, we chose modulus (E), break extension (BE), and break stress (BS). First, parameters were analysed across treatments for the correlations between wet and dry data.Second, we applied two-factor analysis of variance to assess the effects of the factors and their potential interaction.Results: Correlations for the dry versus wet data show only a weak relationship for E, while coefficients of determination (R 2 ) are quite high for BE and BS.Two-factor ANOVA enables to quantify the various contributions to the Total Sum-of-Squares for all three parameters. We show that the parameters respond quite differently to the chemical and the thermal treatments as well as to testing conditions (wet or dry). It is of interest to note that the interaction between the chemical and the physical treatment is generally quite weak. For the interpretation of the results, we use the concept of the humidity-dependent as well as strain-induced glass transition of the amorphous matrix. Conclusions:The independence hypothesis for dry and wet tensile measurements only applies for modulus. Overall, we consider modulus (wet) as the best tensile measure of fibre damage when assessing chemical and/or physical treatments. Under ambient conditions (dry), break stress is shown to be a feasible alternative measure.Conclusions: L'hypothèse d'indépendance pour les mesures de traction sec et humide ne s'applique qu'au module. Dans l'ensemble, nous considérons le module (humide) comme la meilleure mesure de traction des dommages des fibres, lors de l'évaluation des traitements chimiques et/ou physiques. Dans des conditions ambiantes (sèches), le stress de rupture est une mesure alternative réalisable.

The shape of the mature hair shaft (straight, wavy and curly) is predetermined in the living bulb of the follicle.S1 , S2 Once the hair reaches the transition zone between the living and the "dead" mature hair, the mechanisms to impart the shape of the mature hair are triggered. 1 The decisive trigger is the loss of water, which induces, namely, lateral contraction. This happens when proteins are cross-linked and the cells die in the transition zone between the living and the mature hair. Curliness CORRESPONDENCE groups with different IF arrangements move towards the fibre 3The efficacy of this curl-inducing mechanism thus relies fundamentally on the presence of structurally sound IFs.