Fatigue crack initiation around inclusions for a carbon black filled natural rubber: an analysis based on micro-tomography

The fatigue properties of filled elastomers are strongly related to the inclusions' population induced by their complex recipes and the successive stages of the manufacturing process (mixing, injection and curing). The improvement of these properties involves at first an ability to describe the statistical features of these inclusions' population in terms of size, geometry, nature and spatial distribution. Then a detailed understanding of the damage mechanisms is required in order to define the mechanical criticality of inclusions according to their characteristics under cyclic loading. The aim of the study is to take advantage of tomography observations that enable a three dimensional vision to access the statistical description of the inclusions population and of the related damage mechanisms. First of all, a tool to detect and characterize accurately inclusions and cracks is presented. Then, this tool is applied to an interrupted specimen in fatigue, for a compound filled with carbon black. A statistical analysis is performed on the nature of inclusions leading to cracks, on the kind of damage mechanisms (polar crack, cavitation between 2 inclusions or failure of an inclusion) and on the density and the geometric properties of activated inclusions. An additional focus is given on the inclusions leading to the biggest cracks in order to assess more precisely the features (nature, size ....) of the most critical inclusions for fatigue.