Material Fracture Toughness Evaluation Of The Two Models

Resistance to tear is one of the most important mechanical properties of flexible materials such as thin polymer sheets, rubbers etc. In the earlier studies, there were several methods like linear elastic fracture mechanics and J Integral method which were fully developed for describing the tearing behavior of polymer materials. However, there were no suitable theories for thin flexible polymer sheets until EWF (Essential Work of Fracture) theory was proposed in 1970s. EWF theory has drawn more and more attention for material evaluation abroad because it's convenient to do the test, could reflect the deformation and tearing behavior of materials in different ways.The thesis takes an insight look into two different models (two-zone and three-zone models) for describing the deformation and fracture behavior of the tearing ligament. By earlier works, these two different models based on Essential Work of Fracture approach were proved to be capable for predicting the specific total work of fracture along the tear path across all plastic zones. A comparison of these two models was made based on the results from an early experiment done by Kim and Karger-Kocsis. The investigation was based on two-leg trousers tear specimen with constant pre-crack length and material thickness. The materials used in this work were PET and laminates composed of PETG and PET.Tearing resistance of both two materials got by using these two different models compared with the experimental results and it's concluded that these two models both are capable of predicting for the specific essential work of fracture, specific total work of fracture and specific non-essential work of fracture of thin polymer sheets. For material PET-0.5, the mean value of relative error by using three-zone model is 4.37%, but 8.45% by using two-zone model; For material PET/PETG-2.1/18,the mean value of three-zone model is 5.5% and 9.62% by using two-zone model. So it's easily can see that the three-zone model has a much better accuracy with respect to the two-zone model. And the other interesting phenomenon is both three-zone model and two-zone model are more suitable for mono-layer material compared to the laminates.A finite element simulation is presented also to show the stress around the crack tip and the deformation of the tearing specimen. The commercial software of ABAQUS is selected to performance the simulation. The results shows that when only consider the materials to be elastic, the shear stress is decreased along the crack path and it means that the shear stress is the closer the bigger to the crack tip. When plastic is also under considered, along the direction of crack propagation the first stage stress S12 is increasing and then start to decrease which means that the biggest shear stress isn't at the crack tip but very close to it. In this case the biggest shear stress occurred around 6mm to the crack tip.