To cite this version:Samy Mzabi, Daniel Berghezan, Stéphane Roux, François Hild, Costantino Creton. A critical local energy release rate criterion for fatigue fracture of elastomers.

The concomitant appearance of crystallites and nanocavities under uniaxial strain is investigated by X‐ray scattering in a model natural rubber system. The nanocavities appear after crystallization and only when the true stress is above a critical cavitation stress σCav. The presence of crystallites alone does not influence the calculation of the void volume fraction ϕvoid. The nanocavities formed are 20–50 nm in size with a constant aspect ratio. The presence of filler shifts the critical crystallization extension ratio λCry, λCav, and σCav to lower values. The clear correlation between σCav and the crystallinity at the onset of cavitation χC(λCav) implies that the crystallites take most of the mechanical loading thus delaying the cavitation in the amorphous phase. Under cyclic loading, nanocavitation is significant only in the first loading and in the successive loadings if the extension ratio is above its maximum historical value. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1125–1138

, et al.. Multiaxial deformation and strain-induced crystallization around a fatigue crack in natural rubber. Engineering Fracture Mechanics, Elsevier, 2014, 123, pp.59-69. 10.1016/j.engfracmech.2014 Multiaxial deformation and strain-induced crystallization around a fatigue crack in natural rubber The study of fatigue crack propagation in elastomers is an essential prerequisite to improve the service life of tire products. Natural rubber is a key compound in tires, because of its unique mechanical properties and more particularly its remarkable resistance to fatigue crack growth as compared to synthetic rubbers. To explain this resistance, the literature often mentions the phenomenon of strain-induced crystallization which takes place at fatigue crack tips in natural rubber and then reinforces it. In the present study, an original experimental set-up that couples synchrotron radiation with a homemade mechanical fatigue machine is developed to investigate both strain-induced crystallization and deformation multiaxiality around fatigue cracks in natural rubber. During uninterrupted fatigue tests, recording of wide-angle X-ray diffraction patterns is performed in the crack tip region providing the two-dimensional spatial distribution of both crystallinity and principal strain directions. In particular, the influence of loading conditions on the size of the crystallized zone is investigated and related to fatigue crack growth rates. Finally, measurements of deformation multiaxiality, i.e. principal strain directions and change in thickness, obtained by this method are successfully compared with digital image correlation results.

, et al.. In situ synchrotron wide-angle X-ray diffraction investigation of fatigue cracks in natural rubber. Journal of Synchrotron Radiation, International Union of Crystallography, 2013Crystallography, , 20, pp.105-109. 10.1107 In situ synchrotron wide-angle X-ray diffraction investigation of fatigue cracks in natural rubber Natural rubber exhibits remarkable mechanical fatigue properties usually attributed to strain-induced crystallization. To investigate this phenomenon, an original experimental set-up that couples synchrotron radiation with a homemade fatigue machine has been developed. Diffraction-pattern recording is synchronized with cyclic loading in order to obtain spatial distributions of crystallinity in the sample at prescribed times of the mechanical cycles. Then, real-time measurement of crystallinity is permitted during uninterrupted fatigue experiments. First results demonstrate the relevance of the method: the set-up is successfully used to measure the crystallinity distribution around a fatigue crack tip in a carbon black filled natural rubber for different loading conditions.

We have investigated the structural changes occurring in highly crosslinked and carbon-black filled natural rubber under uniaxial extension by small-and wide-angle X-ray scattering using synchrotron radiation. The experiments focused on strain-induced crystallization (SIC) and nanocavitation and were carried out on a model series of materials as a function of temperature and aging conditions. We find that for all materials both SIC and cavitation decrease markedly with temperature and aging. However, the presence of carbon black filler shifts the ceiling temperature where SIC is observed to at least 120 C, presumably by a nucleating effect, maintaining the high strength of the elastomers. Interestingly, although in pure elastomers, the cavitation strength decreases with temperature, we find that in these filled elastomers the critical stress for the onset of cavitation increases significantly with temperature strongly suggesting that cavitation is due to the local confinement between fillers and supporting the idea of a glassy layer near the filler. Aging for 10 days at 110 C in oxygen-free conditions decreases both SIC and cavitation and reduces the strength of the elastomer at high temperature. This is attributed to the formation of sulfur side chains hindering the crystallization.Such conditions are found between steel plies of the carcass of a truck tire. The need to bond to the steel cord imposes high levels of added sulfur and therefore a high crosslink density, while the position of the rubber inside the tire results in nonoxidizing aging conditions and a temperature of the order of 100 C for highway driving. The combination of high stress, high temperature, and high confinement makes this spot a critical safety spot for the tire and materials requirements are particularly stringent.The main results of previous studies on SIC of natural rubber can be summarized as follows. SIC is a thermodynamically driven phenomenon triggered by a decrease in entropy of the polymer chains as they are stretched, which reduces the barrier to form crystals. Within that framework, the higher the deformation of the chains and the lower the temperature, and the more chains can crystallize along the stretching direction. This has been demonstrated in lightly crosslinked unfilled natural rubbers. However, the effect of increasing the crosslink density is less obvious to interpret. For peroxide crosslinked natural rubber increasing the crosslink density clearly favors SIC in terms of onset of crystallinity and the crystallization "rate," that is, the increase in crystallinity with increasing stretch λ. 7,15 However, for the much Additional Supporting Information may be found in the online version of this article.