International audienceNatural rubber (NR) exhibits great fatigue properties which are usually explained by its ability to crystallize under strain. Nevertheless, strain-induced crystallization of NR in fatigue has never been investigated. We perform original in situ fatigue tests during which the degree of crystallinity, and the number and volume of crystallites are measured by synchrotron wide angle X-ray diffraction. For all loading conditions, the number of crystallites is constant. The evolution of their volume depends on the minimum stretch ratio achieved at each cycle. The results show that cyclic loading conditions modify the macromolecular structure of the material, in particular of its amorphous phase

cuta, et al.. Characteristics of strain-induced crystallization in natural rubber during fatigue testing: in situ wide-angle x-ray diffraction measurements using synchrotron radiation. Rubber Chemistry and Technology, American Chemical Society, 2014, 87 (1), pp.184-196. 10 ABSTRACTStrain-induced crystallization of carbon black-filled natural rubber is investigated by wide-angle X-ray diffraction (WAXD) during in situ fatigue tests using synchrotron radiation. Thanks to an original experimental method, we measure the evolution with the number of cycles of: (i ) the index of crystallinity, both (ii ) size and (iii ) orientation of the crystallites, and finally (iv) the lattice parameters. It is shown that when the minimum stretch ratio of the fatigue test is lower than the onset of melting of the crystallites, then the index of crystallinity and the size of the crystallites decrease, whereas they increase when the minimum stretch ratio is higher than the onset of melting. For all the fatigue tests, the misorientation of the crystallites slightly decreases and the lattice parameters remain constant with the number of cycles.

A micro-tensile testing machine placed in the chamber of a scanning electron microscope is used to perform in situ fatigue tests on a 43 phr carbon black-filled cis-1,4-polyisoprene rubber; the crack tip is observed in real-time during crack propagation. These observations lead to a detailed description of the crack tip morphology; the crack front is a regular pattern of diamond-shaped zones delimited by extended straight ligaments. Fatigue crack growth is driven by the ligaments breakage, which occurs in all the zones of the crack front surface continuously in time but at different velocities. This phenomenon of nonlocalized damage explains initiation and limited propagation of branches which deviate from the main crack. All those mechanisms are sources of energy dissipation which explains the great fatigue properties of NR. Finally, from similar experiments conducted on styrene butadiene rubber, it is established that the peculiar morphology of the crack tip and mechanism of crack propagation in NR are due to strain-induced crystallization.

A micro-tensile testing machine placed in the chamber of a scanning electron microscope is used to perform in situ fatigue tests on a 43 phr carbon black-filled cis-1,4-polyisoprene rubber; the crack tip is observed in real-time during crack propagation. These observations lead to a detailed description of the crack tip morphology; the crack front is a regular pattern of diamond-shaped zones delimited by extended straight ligaments. Fatigue crack growth is driven by the ligaments breakage, which occurs in all the zones of the crack front surface continuously in time but at different velocities. This phenomenon of nonlocalized damage explains initiation and limited propagation of branches which deviate from the main crack. All those mechanisms are sources of energy dissipation which explains the great fatigue properties of NR. Finally, from similar experiments conducted on styrene butadiene rubber, it is established that the peculiar morphology of the crack tip and mechanism of crack propagation in NR are due to strain-induced crystallization.

A micro-tensile testing machine placed in the chamber of a scanning electron microscope is used to perform in situ fatigue tests on a 43 phr carbon black-filled cis-1,4-polyisoprene rubber; the crack tip is observed in real-time during crack propagation. These observations lead to a detailed description of the crack tip morphology; the crack front is a regular pattern of diamond-shaped zones delimited by extended straight ligaments. Fatigue crack growth is driven by the ligaments breakage, which occurs in all the zones of the crack front surface continuously in time but at different velocities. This phenomenon of nonlocalized damage explains initia-tion and limited propagation of branches which deviate from the main crack. All those mechanisms are sources of energy dissipation which explains the great fatigue properties of NR. Finally, from similar experiments conducted on styrene butadiene rubber, it is established that the peculiar morphology of the crack tip and mechanism of crack propagation in NR are due to strain-induced crystallization.