07 Octobre – Thesis defense - Kaustubha Kane

11 h E24-B02-R3-03 / LAD (50) Manufacture Française des Pneumatiques Michelin - Cébazat

Durability of new metal/rubber assemblies.

Tyres are complex structures with multiple layers of reinforcement such as fabric, polymers and, most importantly, steel cord mesh. As for laminated composites, both tyre strength and rigidity are largely controlled by properties of the metal cord reinforcements. The steel cords embedded inside the rubber matrix form a cord-rubber composite which acts as the skeleton of the tyre. In modern tyres the steel cords are coated with brass that produces strong chemical bonds between Sulphur from the rubber and Copper from the coating during the vulcanisation process. These bonds act as an adhesive interface and undergo complex mechanical loadings, combined with aggressive environmental exposure during the life of a tyre. In this context, extracting detailed information about the mechanical behaviour of this rubber-cord interface is of great importance, both for materials scientists and tyre designers. Traditionally, standard fracture mechanical tests such as peel tests or pull-out tests are used to extract such information. However, these standard tests suffer from many experimental artefacts, and the test results depend on the rubber and cord properties in addition to those of the interface. The peel test cannot mimic the cylindrical nature of the cords whereas pull out tests suffer from friction effects between the fractured faces. These tests therefore fail to provide an intrinsic value of the fracture energy of adhesion.
This PhD thesis aims to design and develop a novel test protocol for quantitative evaluation of the adhesion between tyre rubber and steel cord reinforcement. With this test protocol, referred to as Rubber Cord Adhesion Inflation Test (RCAIT), reproducible test conditions are achieved, and experimental artefacts are found to be minimal. The thesis work involves development of the RCAIT setup, from its design to its execution stage, analytical and numerical treatment of the problem, and calculation of the fracture energy needed for complete interface separation in the test configuration. A Thick Rubber Tube Inflation Model that describes the deformation of the specimens is proposed, in order to perform the analytical and numerical treatment. This model is used to calculate fracture energy or critical strain energy release rate of various rubber-cord composites at different loading rates. The model is initially developed for Mooney – Rivlin and Ogden rubbers, and then extended to other incompressible hyperelastic models that describe the rubber behaviour. A marker tracking technique is proposed, to monitor the crack propagation and to investigate the rubber deformation in the crack process zone region. This analysis is then extended to study the effect of certain experimental conditions on the evaluation of fracture energy. Finally, the theoretical model is used in conjunction with the marker tracking technique to estimate the properties of specimen materials and to evaluate fracture energy. Thus, the rubber-cord interface fracture energy evaluated with this technique is found to be more reliable and exhibit minimal experimental artefacts.

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