Multiaxial deformation and strain-induced crystallization around a fatigue crack in natural rubber

Abstract

, 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.

Crack precursor size for natural rubber inferred from relaxing and non-relaxing fatigue experiments

“…The influence of the crack precursor size on natural rubber fatigue life was studied, and the result shows that the inferred mean size of crack precursors in the natural rubber component is around [30][31][32][33][34][35][36][37][38][39][40] lm, and the fatigue life fluctuation is due to the inhomogeneity of crack precursor sizes except the factors such as unavoidable variations in testing conditions and specimen variations.…”

“…This interesting result can be explained as follows: The maximum and minimum strains increase as the positive R ratio increases for a given strain range; therefore, there is more energy for the crack growth with a higher R ratio, resulting in the increase of crack growth rate and the decrease of fatigue life. However, owing to the effect of strain-induced crystallization, the higher the maximum strain, the higher the tearing energy, the larger the crystallized zone at crack tip and the higher crystallinity, the lower the fatigue crack growth rate in natural rubber [35], resulting in the increase of fatigue life. That is the reason why fatigue life may increase or decrease when R ratio increases for given strain range and crack precursor size.…”

“…Finally, the quantitative effect of initial crack size on fatigue life was studied. We found that the inferred mean size of crack precursors in the rubber component is around [30][31][32][33][34][35][36][37][38][39][40] lm under both relaxing and non-relaxing loading conditions, and the fluctuation of fatigue life is due to the inhomogeneity of crack precursor size except the factors such as unavoidable variations in testing conditions and specimen variations. The good agreement of inferred crack precursor sizes from different R ratio loading conditions is a strong indication that the Mars-Fatemi model provides a proper accounting for the effects of strain crystallization, and it confirms yet again the understanding that nucleated cracks originate from similarly sized precursors in both relaxing and non-relaxing fatigue experiments.…”

Phase field modelling of strain induced crystal growth in an elastic matrix

“…Although the precise mechanisms involved are still a matter of debate, Strain Induced Crystallization (SIC) confers to Natural Rubber (NR) or more generally crystallisable elastomers, a good resistance to crack [1][2][3][4] and an excellent fatigue behavior. 3,5,6 Since the discovery of SIC in NR by Katz in 1925, 7,8 many experimental 1,2,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] and theoretical [27][28][29][30][31][32] studies have been made in order to understand it.…”

“…3,5,6 Since the discovery of SIC in NR by Katz in 1925, 7,8 many experimental 1,2,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] and theoretical [27][28][29][30][31][32] studies have been made in order to understand it. Experimentally, several techniques have been used with NR, synthetic rubber, and NR carbon-filled, such as birefringence, 12,13 infrared absorption, 14,15 differential scanning calorimetry (DSC), 16 and X-ray diffraction.…”