چكيده به لاتين
Rupture assessment of rubbers weakened by crack and notch is the main aim of the present thesis. In this regard, the aim is pursued in two fields. First and in the experimental phase, because of the lack of sufficient experimental data for rubbers weakened by stress concentrator (especially those containing a notch) in the open literature, some experiments are performed. On the other hand, because of great attention to nano-technology during the recent decades, addition of nano-particles into the rubber to improve its properties has been an active field of science during the recent years. Therefore, some fracture tests on nanoclay-reinforced rubber weakened by crack and notch are also conducted in this study.
After the experiments and in the second phase of thesis, three fracture criteria for load prediction of cracked and notched rubbers with or without nanoclay are presented. These criteria are: (1) Effective stretch (ES) criterion which is based on the physics of rubber and micromechanics of chains, (2) Maximum tangential logarithmic strain (MTLSt) criterion, and (3) Averaged strain energy density (ASED) criterion extended to be used in hyperelastic materials. The key point in the presentation and extension of the criteria, which is proved via finite element modeling (FEM), is the nearly uniaxial nature of stress fields in the proximity of the crack/notch tip.
The experimental results of this study reveal that independent of type of stress concentrator (crack or notch) and mode of loading (mode I or mixed mode I/II), the rubber nearly ruptures in a horizontal path and perpendicular to the loading direction. Moreover, it is seen that the addition of nanoclay to rubber affects the tearing (i.e, rupture) displacement of notched samples more than that of cracked ones. This result is justified by comparing of the stress state near the notch and crack tip with the state of stress in a uniaxial tensile test. Furthermore, the validation of the triple presented criteria with the results of various experiments shows that the maximum average percentage difference for a typical criterion is less that 7 %, which confirms very good efficiency of the criteria. Additionally, good agreement between the estimations of triple criteria and the experimental results of nanoclay reinforced rubber is obtained, which also approves the accuracy of the criteria in the case of nanocomposite rubbers. However, the ES criterion, which is based on the physics of rubber, has the best performance in load assessment of cracked and notched rubbers. After a thorough analysis of triple presented criteria from different aspects (i.e., for different types of rubbers, mode of loading, and type of stress concentrator), finally, the MTLSt criterion which provides very good predictions for both load carrying capacity and rupture initiation angle is selected as the most appropriate criterion.
