Browsing by Subject "Hyperelastic"
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Item A fracture mechanics approach to accelerated life testing for cathodic delamination at polymer/metal interfaces(2013-05) Mauchien, Thomas Kevin; Liechti, K. M.This work presents a fracture mechanics analysis of the cathodic delamination problem for the polyurethane/titanium and polyurea/steel interfaces. The nonlinear behavior of both polymers was investigated. The recent Marlow model was used to define the strain energy function of the polymers. Viscoelastic effects of the polyurea were also studied. The Marlow model was associated with a nine-term Prony series. This model was seen to represent experimental data relatively well for a wide range of strain rates both in tension and compression. The driving force for delamination, the strain energy release rate G, is presented for both interfaces. Cathodic delamination data for several temperatures are presented as crack growth rate as a function of crack driving force. The approach recognizes that both temperature and stress can be used as accelerated life testing parameters.Item Stretch-induced wrinkling of thin sheets(2013-08) Nayyar, Vishal; Huang, Rui, doctor of civil and environmental engineering; Ravi-Chandar, K.Thin sheets and membrane structures are widely used in space applications such as solar sails, sunshields and membrane optics. Surface flatness over a large area is one of the key requirements for many applications using the flexible thin structures. However, wrinkles are commonly observed in thin sheets. It is thus important to understand the mechanics of thin sheets for practical applications that require reliable control of surface wrinkles. In this study, a model problem of stretch-induced wrinkling of thin sheets is considered. First, a two-dimensional (2-D) finite element model was developed to determine stretch-induced stress distribution patterns in hyperelastic thin sheets, assuming no wrinkles. As a prerequisite for wrinkling, development of compressive stresses in the transverse direction was found to depend on both the length-to-width aspect ratio of the sheet and the applied tensile strain. Next, an eigenvalue analysis was performed to predict the critical conditions for buckling of the elastic sheet under the prescribed boundary conditions, followed by a nonlinear post-buckling analysis to simulate evolution of stretch-induced wrinkles. Experiments were conducted to measure stretch-induced wrinkling of polyethylene thin sheets, using the three-dimensional digital image correlation (3D-DIC) technique. It was observed that the wrinkle amplitude first increased and then decreased with increasing nominal strain, in agreement with finite element simulations for a hyperelastic thin sheet. However, unlike the hyperelastic model, the stretch-induced wrinkles in the polyethylene sheet were not fully flattened at high strains (> 30%), with the residual wrinkle amplitude depending on the loading rate. The hyper-viscoelastic and the parallel network nonlinear viscoelastic material models were adopted for finite element simulations to improve the agreement with the experiments, including the wrinkle amplitude, residual wrinkles and rate dependence. Finally it is noted that wrinkling is sensitive to defects and material inhomogeneity in thin sheets. By varying the elastic stiffness in a narrow region, numerical simulations show drastically different wrinkling behavior, including the critical strain and evolution of wrinkle amplitude and wavelength. In conclusion, a comprehensive understanding of stretch-induced wrinkling is established, where geometry, material, and boundary conditions all play important roles.