Measurement and analysis of deformation behavior of thermoplastic elastomer tube subjected to biaxial tensile stress under loading and unloading

A material testing apparatus for measuring the biaxial deformation behavior of a polymer tube is developed to quantitatively evaluate the deformation behavior of polymeric materials. The testing apparatus can apply axial force and internal pressure to a tubular specimen. A noncontact strain measurement system, where the biaxial strain components and the radius of curvature in the axial direction of the specimen are continuously measured to control the stress path applied to the specimen, is also developed. Thermoplastic elastomer tubes with an outer diameter of 15 mm and a thickness of 2 mm are used as test samples. The samples are subjected to linear stress paths with stress ratios of ({sigma }_phi : {sigma } _heta =1:0, 4:1, 2:1, 4:3, 1:1, 3:4, 1:2, 1:4), and 0:1, where ({sigma }_phi ) and ({sigma } _heta ) are the axial and circumferential stress components, respectively, applied to the center of the bulging specimen. Biaxial stress–strain curves are measured for each linear stress path at a nearly constant logarithmic strain rate of 1 × 10–3 s−1. The contours of equal plastic work plotted in the ({sigma }_phi -{sigma } _heta ) stress space show significant anisotropy of the test material. A material model for accurately reproducing both the work-hardening behavior and deformation behavior of the test samples is proposed.