Indentation failure of polymeric membrane with anisotropic pore structures

Abstract

This study systematically investigates the indentation deformation behavior of polytetrafluoroethylene (PTFE) membranes used for water purification. The examined PTFE membrane has micron-sized pores with an open cell structure. The pore structure is anisotropic in shape owing to its fabrication via the stretching method. A uniaxial tension demonstrates the membranes undergo elastic and plastic deformation with significant anisotropy (as the deformation behavior differed between the longitudinal and transverse directions). To clarify the microscopic deformation mechanism, in-situ SEM observations were carried out during uni-axial tensile loading. It was found that the anisotropic deformation behavior appeared due to the inherent pore structure. Next, to investigate the indentation deformation behavior, small punch (SP) tests using a spherical indenter were carried out. The membrane underwent both elastic and plastic deformations. Finally, a crack nucleated around the indenter contact area and the indenter completely penetrated through the membrane. It was also found that the membrane demonstrates an anisotropic out-of-plane deformation behavior. To clarify these mechanisms, an FEM computation was carried out, with the experimental results of the force–displacement curve and the out-of-plane deformation behavior agreeing with the computational results. The present FEM model enables the prediction of the deformation behavior of the membrane under indentation loading.