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.

本研究探讨了毛发纤维的几何和力学特性之间的关系，特别是针对卷曲纤维样本进行了深入研究。偶然的观察揭示了随着卷曲程度的增加，其粘弹性特性存在显著差异。进一步的调查证实了初步观察结果，显示卷曲纤维在应力-应变图上呈现出初始的明显趾区行为，而直纤维则不存在这种特性。这种特性表明卷曲纤维的粘弹性特性与纤维中储存的机械能相关。研究结果表明，头发的强度取决于两个主要组成部分，并进一步指出，实际上在拉伸测试中的方法可能会在制备过程中削弱卷曲纤维的强度。本研究的主要成果是，毛发纤维的拉伸强度（σT）由两个（而非一个主要）组成部分构成，即趾区（σt）和弹性区（σε），因此：σ_T=σ_t+σ_ε。对于非卷曲纤维，纤维强度的最大部分来源于σε，而σt（即弹性）对整体强度贡献显著，尽管σε仍然是主要贡献者。尽管这些结果需要在大规模研究中进行验证，但它们对于当前对卷曲头发的理解具有重要意义。此外，它们可能代表着从当前对人类头发拉伸测试的一般理解中的根本转变。与一位捐赠者（JID.EC-DimensionData Donor B，280617）相关的仪器纤维数据以txt文件的形式上传，包括几何数据（JID.EC-DimensionData Donor B）、应力/应变数据（JID.EC-StressStrain Data Donor B）、拉伸数据的一元线性回归分析（JID.EC-SF Analysis Donor B）以及拉伸记录（JID.EC-Tensile Record Donor B），其中包含了设定的参数。作为多元输入数据，解释数据支持图3和图5。数据收集过程中，头发捐赠者根据本研究伦理批准由我们机构研究伦理委员会批准的知情同意书进行了同意。主要样本量（n=25）；样本组（n=8）；每位捐赠者评估的纤维数量取决于校准、测试和验证要求，如以下所述；每位捐赠者获得的纤维数量（50-100）；每位捐赠者使用的纤维数量（10 ≤ 纤维 ≤ 40）。头发类型：从直发到非常卷曲；过去12个月内未进行化学处理。每位捐赠者和每根纤维根据STAM的修改版进行了卷曲类型分类（I-直发到VI-紧密卷曲）。每个实验步骤均进行拍照，作为纤维变化的可追溯性和数字证据。纤维制备：头发用1%的SDS溶液清洗；冲洗；在21°C±0.6°C和45%RH±5%RH的条件下，于21°C±0.6°C和45%RH±5%RH的条件下自然风干（1-4小时）；使用微型拉伸试验机（MTT686，Dia-stron）的样品安装块夹紧；（4）应力-应变行为：微型拉伸试验机（MTT686，Dia-stron）配备500N的力传感器；应变率：12.5 mm/min；最大力：800 gmf；量程力：0 gmf。