A detailed study of the initiation of fatigue cracks in carbon black–filled natural rubber is conducted. Interrupted fatigue tests are performed and fatigued samples are observed with a scanning electron microscope. This procedure first enables the quantification of the morphology, spatial distribution, and evolution of crack initiation sites for different strain levels, which gives some statistical data for each strain level. It also permits analysis of the chemical nature of inclusions inducing crack initiation thanks to energy dispersive spectrometry of X-rays. It is shown that fatigue damage initially occurs generally on carbon black agglomerates or oxides such as ZnO. However, those two types of inclusions correspond to different crack initiation mechanisms, and most of the time, only the initiations on carbon black agglomerates are followed by crack propagation that leads to failure. This difference is probably because carbon black agglomerates have a stronger cohesion than ZnO inclusions and a stronger adhesion to the matrix.

In previous papers, a protocol using thermal measurements to predict the fatigue lifetime throughout an energy-based criterion afforded a very efficient prediction of the deterministic Wöhler curve with only one sample, within less than 1 day. Nevertheless, these papers investigated a limited number of materials because the fatigue campaigns required to provide a validation of this fast prediction are very demanding in time and specimens. In this study, the PROFEM project (supported by the French National Research Agency) allowed us to investigate a very wide range of materials including natural rubber (NR) and synthetic compounds (SBR). The investigation therefore considers either crystallizing (NR) or noncrystallizing (SBR) materials for several carbon black amounts (0, 20, 43, 58 phr) and types (N220, N326, N375, N550, N772). As a first step, this study presents the application of the fast prediction protocol on one compound and investigates its robustness (repeatability, influence of the N frequency, sensitivity to the graphical analysis). Then, the method is challenged on the 11 other compounds, and the comparison of the predicted fatigue curve with the Wöhler curves obtained by classical campaigns highlights a very good agreement for most of the materials.