Analysis of the Thermomechanical Behaviour of SMP in Equi-Biaxial Condition by Means of Hydraulic Bulge Test

In the last few decades, the shape memory polymers have gained growing interest and relevance in many application fields, which include textiles, aerospace, biomedical devices, etc. SMP are materials with stimuli-sensitive switches that are able to change their geometry from a primary shape to a secondary shape-and vice versa-in response to external stimuli. This phenomenon is called shape memory effect and, generally, is triggered by heat. The heat stimulation, in fact, alters the internal structure of the polymer: by exceeding the glass transition temperature Tg, it is possible to program the shape of the component. Then, by cooling the material below Tg and imposing a fixed deformation, the polymer can reach its temporary shape. The original shape can be recovered by heating again the material above Tg without any constraints. Recent research works are mostly focused on thermomechanical uniaxial characterization of these materials, aimed to obtain the material main memory effect parameters (namely the Young modulus above/ under glass transition temperature and shape fixed/recovery ratio). In this work the authors propose a non-conventional characterization approach for the investigation of SMP behaviour under equi-biaxial stress state. In particular, the main idea is to carry out the hydraulic bulge test with a thermomechanical cycle on thin sheets of thermoplastic polyurethane shape memory polymer (TPU-SMP). Full-field measurements on the specimen surface are used for retrieving the material shape, this latter by employing the Digital Image Correlation (DIC) technique. The proposed configuration makes the test suitable for determining the biaxial-stress strain curve and the thermomechanical cycle, also providing data for inverse calibration methods such as the Virtual Fields Method (VFM) and the Finite Element Model Updating (FEMU).