טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentLev Yoav
SubjectDeformation and Failure of Rubber-like Materials
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Konstantin Volokh
Full Thesis textFull thesis text - English Version


Abstract

Thanks to the stable production and process control available today, rubber-like materials are increasingly used and produced in large quantities with consistent properties. The correct description of rubber-like materials for modelling is crucial but there are some aspects that are still absent in the literature. In recent years, Volokh has proposed a new approach to modelling rubber fracture based on elasticity with energy limiters. Our work is based on this theory. We first implement the description into a FE software and present simulations of cracks running in a thin pre-stretched rubber sheet. Aspects of the running crack are compared to published tests. It is observed that changing the element size in simulations changes the crack path, indicating the mesh sensitivity on the results. There is still no consensus on the regularization solution. Volokh's 'Fracture as a Material Sink' theory offers a potential for regularization. This description couples between the mass and momentum balance equations. This idea is illustrated by solving a uniaxial stretch case using a finite difference (FD) method. Another regularization may be derived from the unique thermo-elastic characteristics of rubber-like materials. No experiments considering strength under heating were found in the literature. Thus, we have planned the set-up of uniaxial and equi-biaxial experiments subjected to temperatures in the range of 25°C to 90°C. We consider in tests three common rubber-like materials; Nitrile Butadiene Rubber (NBR), Neoprene and Silicone. The equi-biaxial tension conditions are found indirectly from the bulge test or inflation balloon test. Iterative finite element simulations are done up to a sufficient fit to the bulge experiment results. The material constants derived are used to describe the equi-biaxial stretch state. These material constants are used for further calibration of the material model based on the simultaneous fit of uniaxial and equi-biaxial data. It is found that strength might be significantly decreased by heating while the stiffness only slightly depends on the temperature alterations. The calibration is done using a new suggested form of the thermal energy in the material model, and a new relation for the temperature related ultimate values. This generalizes the theory and may serve as a design consideration. An analytical application of the energy limiter theory to analysis the cavitation expansion problem has been considered. The literature offers an explanation for this phenomena using only the neo-Hookean model. According to this, the resulting plateau yield-like region on the stress--stretch curve explains the failure observed in tests. We show, that the occurrence of the plateau yield-like region depends on the choice of material model made. By integrating the energy limiter theory into the cavity problem, we observe an instability in the form of a yield-like region on the stress-stretch curves even for material models that exhibit stiffening due to the unfolding of the long molecules. Using the generalized thermo-elastic energy limiter theory developed, we found that the cavitation process was affected by the interplay between the reduced material strength and the increased material stiffness as a result of heating.