||The thesis aims to investigate the phenomenon of wrinkles and fatigue behavior of the central hole arrangement in rectangular Polyproplyene(PP) thin films at room temperature. The number of holes and the arrangement of holes in the central position of the rectangular PP films were used to perform the stretching tests. The wrinkles and mechanical properties and tensile data of the material were obtained. In order to understand the fatigue characteristics of PP films, the tension-tension low cycle fatigue tests by using strain control were conducted to receive the results of fatigue properties and life. |
All samples were bonded on the cardboard frames and clipped in an MTS Model42 micro-tensile system. Round holes with diameters of 10 mm were cut centrally in PP films with the thickness 8 μm. The holes in the films were arranged in three ways. One was to cut a hole in center, second were to cut two holes with a distance of 20mm apart around the center vertically (2V), horizontally (2H), third type were four holes with a distance of 20 mm apart around the center just like symbol + (4+) and x (4x).
The experimental evidences show that the evolution of wrinkles can be divided into two stages from the elastic state of in-plane deformation to the plastic state of out-of-plane deformation. At elastic state the in-plane deformation was too hard to be observed, therefore, our focus was on exploring the out-of-plane deformation stage.
According to the fatigue test, the elastic and plastic curves of the PP film were obtained, and the two curves were combined as a complete strain-life curve. One hole and two holes types specimens, the first wrinkle appears near the hole, then wrinkles expand slowly to both sides of the specimen. For four holes samples, the wrinkles will grow in the adjacent side of the hole, grow to both sides afterwards.