Di Vito, Donato
(2017)
Mechanical modeling and experimental characterization of filled elastomers through generalized biaxial testing.
[Tesi di dottorato]
Abstract
Elastomer mechanical characterization and modeling are subjects that attract the scientific community since more than 50 years ago. However, despite the ample literature and the related scientific and industrial activity in this field, many different phenomena are not yet consolidated.
With this Thesis, then, the Author wants to try to fill in some of the gaps while trying to use a 'hybrid' approach, for what concerns modeling and multi-axial characterization of filled elastomers by trying to use forefront techniques like Digital Image Correlation (DIC) together with mechanical testing machines able to perform non-standard biaxial tests, in static and dynamic loading conditions. With this purpose, a significant part of the research activities focused on the use and the partial redesign of some components of the BiaxTester, a planar biaxial testing device used to perform mechanical testing on polymeric materials.
The PhD Thesis is then divided in two different sections: the first is composed by a brief introduction on elastomers and on active reinforcing fillers, together with some background about mechanical properties of filled rubbers and about the evolution of modeling of this kind of materials. Afterwards, some remarks of Continuum Mechanics in a nonlinear framework are included, with the purpose of underlining some of the aspects that will be encountered through the Thesis.
The second part of the Thesis is instead based on the different research activities developed during the PhD: in Chapter 2 there is a first introduction to biaxial testing and about the aforementioned BiaxTester, followed by a section regarding the sample and clamping system optimization and results comparison obtained by using the optimized shape with some more conventional tests. In this section, the need to redesign the measurement process and control of the boundary conditions is shown to be fundamental in order to obtain reliable results in terms of stress-strain curves; the results obtained gave then the opportunity to choose the optimal configuration terms of homogeneity of deformations, with the aim of performing different types of tests on such materials. Successively, a novel approach to characterize mechanical behavior of elastomers through by imposing a planar biaxial loading state that follows certain paths of deformation, showing that different phenomena, such as relaxation and connected dissipative phenomena characteristic of the tires are not negligible in the case of loads in complex combinations. With this objective, it is shown how, by imposing displacements of the different clamps in two orthogonal directions (the two directions in which it is possible to impose deformations on these samples) different resulting forces will be registered, even in limit cases in which such complex loading combinations are reduced to equibiaxial loading. Moreover, it is shown how these phenomena are gradually more pronounced when increasing the reinforcement fraction and for bigger 'distances' of the biaxial path from the one imposed in the two orthogonal directions during the test.
Chapter 3 focuses instead on the description of the Mullins effect in filled rubbers: for this reason, some basic knowledge about pseudoelasticity theory is given, in order to then focus on a model able to describe different phenomena found in filled elastomers, such as stress softening, hysteresis and residual strain after loading application. This is done by following some other approaches found in literature, and by revising some parts of the Dorfmann-Ogden model, based on internal variables which, together with the hyperelastic modeling, allows to describe the aforementioned phenomena.
On the basis of the previously obtained results, Chapter 4 is focused on the crack propagation characterization in filled elastomers; this problem is of great interest due to the crucial importance of generalizing the concepts of stress intensification and of crack growth in complex loading conditions, with the objective of foresee and extend the fatigue life of filled elastomers undergoing cyclic solicitations also in case of complex geometries, such as car tires. Within this framework, the BiaxTester can be an interesting instrument thanks to its flexibility of applicable conditions; this will be shown in this work by comparing crack propagation data obtained through conventional testing setup and through the BiaxTester.
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