Fracture by cavitation of model polyurethane elastomers

Abstract : The main objective of this thesis was to establish the role played by the composition and crosslinking structure of an elastomer on its resistance to cavitation under a predominantly hydrostatic pressure. We prepared three polyurethane model networks from triisocyanate and monodisperse polyether diols (PPGs) of various molecular weights, under well controlled conditions and carried out a complete characterization of the reagents and of the networks by using NMR, FTIR, analysis of the solfractions and DMA. We evaluated the nonlinear elastic and linear viscoelastic properties of the three model networks and determined their fracture toughness in mode I with notched samples. We performed cavitation experiments at different strain rates and temperatures on the transparent samples using an original setup developed specifically for the thesis combining a well controlled confining geometry and real-time optical visualization. Failure occurred in two steps: small and stable precritical cavities appeared during loading before catastrophic fracture occurred by the rapid growth of a single large cavity at or near the center of the sample. This implied the existence of two separate criteria: one for the nucleation and one for the growth. The critical cavitation stress was found to decrease significantly with decreasing strain rate and increasing temperature in clear disagreement with existing cavitation models by cavity expansion or by fracture. The analysis of the cavitation results strongly suggests a beneficial influence of a high mode I fracture toughness GIc and of a marked strain hardening of the network on the critical cavitation stress. Moreover, the existence of time dependence of the nucleation rate of the cavities under stress has been pointed out. In terms of materials, we mainly demonstrate that the presence of entanglements toughen the material which stabilizes the crack growth even in confined conditions. We concluded that the cavitation strength depends not only on the modulus but on the mode I fracture toughness and that the distribution of defects and subcritical crack growth is important for the nucleation rate.
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Chemical Physics. Université Pierre et Marie Curie - Paris VI, 2009. English


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Submitted on : Monday, March 21, 2011 - 10:57:09 AM
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Antonella Cristiano. Fracture by cavitation of model polyurethane elastomers. Chemical Physics. Université Pierre et Marie Curie - Paris VI, 2009. English. <pastel-00578499>

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