Investigating mechanical behavior of cord-rubber composites by multi-scale experimental and theoretical approach
Pastore Carbone, Maria Giovanna (2011) Investigating mechanical behavior of cord-rubber composites by multi-scale experimental and theoretical approach. [Tesi di dottorato] (Inedito)
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Cord-rubber composites are the skeleton of pneumatic tires. Due to axial stiffness combined with high flexibility, such composites are the perfect material for carcass construction. The availability of detailed information on their mechanical response is of paramount importance in the design of performing tires; hence, both experimental characterization and computer simulation of cord-rubber composites are of great interest to researchers and engineers involved in tire design. However, a macroscopic approach is mostly adopted and the single cord-rubber lamina is treated as an orthotropic material. This approach neglects the phenomena that arise inside the lamina on a mesoscopic and a microscopic scale that are indeed responsible for failure mechanisms and local tire heating. This thesis addresses with an innovative investigation of mechanical response of cord-rubber composites, which is based on a multi-scale approach adopted in both experimental investigation and theoretical modeling. In particular, the mechanical response of the cord-rubber composite and of its constituents has been investigated on both a macroscopic and a microscopic scale. The macroscopic response of the lamina has been characterized by tensile test (both static and dynamic) and has been discussed in the light of the well established approaches based on the rule of mixtures. It has been found that mechanical behavior strongly depends on sample length. According to micromechanical models, such a ‘finite fiber length effect’ is generally ascribed to the efficiency of stress transfer into the uniaxial composite. Actually, it has been theoretically found that this dependence becomes particularly pronounced whether the mismatch between the elastic properties of matrix and reinforcement is considerable, such as for cord-rubber composites. Hence a microscopic investigation has been carried out by micro Raman spectroscopy that has been found to be a very powerful tool for the study of micromechanics of composites. Corded reinforcement has thus been preliminarily examined by investigating molecular deformation processes and evaluating the contributions of single filaments to the response of the overall structure. In addition, since reinforcement can be actually adopted as a strain or stress sensor embedded into the composite, the micromechanical response of the composite has been investigated. In particular, the effect of sample length upon the parameters that govern stress transfer has been examined in detail, providing information on reinforcing efficiency of cord. Results of the micromechanical investigation have then been correlated to the macroscopical findings. A three-dimensional finite element model of the cord-rubber lamina has also been developed by adopting a multi-scale hybrid analytical/numerical approach. Cord has been first modeled by an analytical model that accounts for tension-twisting coupling and relates the constitutive behavior to the hierarchical structure of the cord itself. On the basis of this analytical model, a homogenized cylindrical model of the cord has been implemented in the FEM code, incorporating the relevant mechanical features of the aforementioned cord analytical model. Finally, the overall mechanical model for the composite has thus been implemented by embedding the hybrid cord model into a FEM non-linear hyperelastic matrix. Simulation results have highlighted a significant tension-twisting coupling for the composite and comparison with the commonly adopted orthotropic model have been analyzed.
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