Understanding curly hair mechanics: fibre strength

The relationship between the geometric and mechanical profiles of hair fibres has been studied, with special focus on curly samples. Incidental observations pointed to a significantly different viscoelastic character with varying curliness. Further investigations confirmed initial observations, showing an initial distinct toe-region behaviour for curly-fibres on the stress-strain plot, which is absent for straight fibres. This behaviour suggested a difference in viscoelastic nature of the curly fibre that is linked to mechanical energy stored in the fibre. Results also suggest that the strength of hair depends on two main components, and further pointed out that de facto methods of tensile testing may erode curly fibre strength during preparation. The main outcome of this study is that the tensile strength (σT), of hair fibres is composed of two (rather than one main) components, namely the toe-region (σt) and the elastic region (σε), so that: σ_T=σ_t+σ_ε. For non-curly fibres, the greatest part of fibre strength is derived from σε while σt ≈ 0. For curly fibres, σt (i.e. springiness) adds significantly to the overall strength, even though σε remains the major contributor. Whilst these results require validation in larger studies, they are significant in the current understanding of curly hair. Also, they may represent a fundamental shift from the current understanding of tensile testing of human hair in general. Relevant instrument fibre data was uploaded as txt files for one donor: JID.EC-DimensionData Donor B (280617) - geometric data. JID.EC-StressStrain Data Donor B (280617) - stress/strain data. JID.EC-SF Analysis Donor B (280617) linear regression of tensile data. JID.EC-Tensile Record Donor B (280617) - Tensile record with set parameters. Interpreted data serving as multivariate input data, supporting Figures 3 and 5 are given in JID.EC-Multivariate Data Input.Ref Fig3Fig5. Gathering of data: Hair donors gave informed consent in accordance with this study’s ethical approval by our Institutional Research Ethics Committee. Main sample size (n=25); sample group (n=8); number of fibres assessed per donor depended on calibration, test and validation requirements as described below; fibres obtained per donor (50-100); number of fibres used per donor (10 ≤ fibres ≤ 40). Hair types: straight to very curly; chemically untreated for last 12 months. Curl-type classification was done per donor and per fibre (I-straight to VI-tightly curly), based on a modification of STAM. Each experimental step was photographed as digital evidence and traceability of fibre changes. Fibre preparation: Hair was washed (1% SDS solution); rinsed; air dried (1-4 hours) at 21°C±0.6°C and 45%RH±5%RH; clamped, using the sample-mounting block of the Miniature Tensile Tester (MTT686, Dia-stron) ; (4) Stress-strain behaviour: Miniature Tensile Tester (MTT686, Dia-stron) with load cell: 500 N; strain rate: 12.5 mm/min; max. force: 800 gmf; gauge force: 0 gmf