Accurate 3D reconstruction of a rubber membrane inflated during a Bulge Test to evaluate anisotropy

This paper describes a methodology for carrying out an accurate mechanical characterization of an amorphous hyperelastic rubber-like material (carbon black filled natural rubber) by a custom-made experimental setup for bulge testing. Generally, during sample testing the slight anisotropy of the internal polymer structures, primarily due to the calendering process is neglected. This methodology is able to evaluate these effects. A hydraulic circuit inflates a thin disk of rubber blocked between two clamping flanges with adjustable flow rate, thus controlling the speed of deformation of the sample. The device has a sliding crossbar, which moves proportionally as the membrane inflates. A stereoscopic technique is able to capture with pixel precision and identify the strain on a silk-screen grid printed on the upper surface of the sample. For each acquisition step, the epipolar geometry of the image pairs is represented in a single absolute reference system integral to the experimental setup. The acquired images are processed using geometrical algorithms and different filters. In this way an extremely precise 3D reconstruction of the sample is created during the bulge test. Slight anisotropic behaviors due to the rubber calendering process have been observed and measured since the first steps of the bulge test, where the strains are minimal and principal strain direction in equibiaxial tension test are determined.