This paper presents a composite model of the natural keratin fibres (wool and hair) whichconsists essentially of isotropic viscoelastic filaments, oriented parallel to each other in the fibreaxial direction, embedded in an isotropic viscoelastic matrix. The model accurately fits the exper-imental data on the fibre axial stress relaxation moduli and provides upper and lower bounds forthe initial/final values for the fibre transverse tensile and shear stress relaxation moduli andtransverse Poisson’s ratio. The partially water penertrable filament phase of the composite modelis identified as the microfibrils in the fine structure of keratin. The strong anisotropy of keratinsin mechanical properties and hygral/thermal expansion is analyzed in terms of composite struc-ture and mechanical as well as the thermal/hygral properties of the two constituent phases.
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