(Abstract:)This study developed a new bi-axial tensile test for porous polymer membranes, in order to evaluate the deformation behavior of membranes under bi-axial tension and elucidate the deformation mechanism based on their microstructure. First, uni-axial tensile tests were conducted in each direction to investigate the change in Young's modulus and yield stress depending on the tensile direction. It was found that there was deformation anisotropy in the membrane. Next, we developed a new bi-axial tensile testing machine for porous polymer membranes. Since the present membrane is lower deformation rigidity and the thickness is very thin, gripping of specimen and applying uniform deformation are difficult. This study newly developed the gripping part to enable uniform tensile deformation during the test. Stress-strain curves of each loading axis were obtained by changing the applying strain ratio. Based on the results, a yield surface was created to investigate the deformation behavior under bi-axial tensile loading. We also investigated the deformation mechanism of the microstructure by using SEM observation and FEM computation.(Method:)This study first developed a biaxial tensile testing machine that can be applied to porous polymer membranes. In this machine, two independent stepping motors were employed to apply tensile deformation to the membrane under deformation control. To grip the membrane, four independent gripping parts for one direction were newly developed in order to apply uniform deformation to membrane. In other words, each grip is followed by the deformation of specimen under tensile loading. In addition, the grip part can maintain a strong gripping force. To measure strain of membrane, two-dimensional DIC method was used during the test. To verify the developed machine, uniaxial tensile tests were conducted using a rubber sheet. This loadings with and without transverse constrained were conducted to investigate stress-strain curve and Young's modulus of a rubber sheet. Subsequently, we investigate the deformation behavior of the porous polymer membrane under biaxial tensile load. This study used a PVDF porous membrane, having in-plane anisotropy of elastoplastic deformation. The specimen was cut into squares according to each direction, and biaxial tensile tests were conducted. In this test, nine different strain ratios were tested, including equal biaxial tensile loading.(Result:)From nine different biaxial tensile tests, yield surfaces were created from the obtained yield strengths. The results were agreement with the anisotropic yield surface. It is also found that the yield strength of this material is stronger under bi-axial tensile loading than uni-axial tensile loading. In order to investigate this mechanism, we observed the specimen microstructure after uni-axial and bi-axial tensile tests using SEM. For uni-axial tension, the polymer struts were stretched in the axial direction, and several cracks were generated in several parts, while for biaxial tension, all the polymer struts were equally loaded, and the entire pores were evenly expanded. In addition, a FEM model representing the microstructure of the specimen was created to elucidate deformation mechanism.&nbsp; Uni-axial and bi-axial tensile loadings were computed to investigate microscopic deformation behavior and mechanism of yield surface.(Conclusion:)This study developed a bi-axial tensile testing machine for porous polymer membranes and evaluated the deformation behavior of the membranes under bi-axial tensile loading. The results show that the yield surface is generally consistent with the anisotropic yield surface. It is also found that the bi-axial tensile test is stronger than the uniaxial tensile test in terms of yield strength. The deformation mechanism of the microstructure was clarified by SEM observations and FEM computations.

Development of Bi-axial Tensile Testing for Porous Polymer Membranes and Its Deformation Characteristics

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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