The application of 3D-digital image correlation and analytical approaches on the bulge test for biaxial characterization of biocomposite films

Abstract

The mechanical characterization of plastic films is often complicated by their relative frailty and small thickness. When practical applications involve biaxial stress loading, the bulge test seems to be the most simple and suitable approach. In this work, polylactic acid (PLA) film and biocomposites PLA films filled with 1, 2 and 5 wt% of date palm leaf filler (DPLF) obtained by extrusion film blowing were subjected to pneumatic bulge testing. Different analytical approaches proposed in the literature were used to predict the instantaneous bulge radius and the thickness at the top of the dome. These variables were compared with the experimental values measured by 3D-digital image correlation (3D-DIC). A good match between experimental and analytical data was obtained using the Panknin approach to determine the bulge radius, and the Kruglov method to calculate the thickness of the dome apex. The 3D-DIC method was also used in tensile testing, where the samples of PLA and biocomposites films were cut in the machine direction (MD) and transverse direction (TD). This method was used for computing the strain accurately and to evaluate the development of strain concentration during the tensile and bulge tests. Then, mechanical properties in the case of uniaxial and biaxial tests were compared. It was found that both methods produced comparable results for the Young’s modulus and mechanical strength. Meanwhile, the bulge strain at break followed the lower value found by tensile testing in both directions, which corresponded to the elongation at break along the TD.