Cyclic loadings and crystallization of natural rubber: An explanation of fatigue crack propagation reinforcement under a positive loading ratio (original) (raw)
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Macromolecules, 2010
The mechanism of fatigue crack growth in rubbers under severe loading ABSTRACT. This paper deals with the mechanism of fatigue crack growth in natural rubber submitted to severe relaxing loading conditions. In one mechanical cycle under such loading conditions, the high level of stress at the crack tip engenders high crystallinity, which halts crack growth in the plane perpendicular to the loading direction. Consequently, the crack bifurcates. Then the fracture surfaces tear, slide and relax simultaneously along a highly crystallized crack tip to form striations. The higher the stress level, the lower the crack growth in the plane perpendicular to the loading direction and the greater the bifurcation phenomenon. This explains why the striation shape evolves from triangular to lamellar during crack propagation.
The mechanism of fatigue crack growth in rubbers under severe loading
CRC Press eBooks, 2011
The mechanism of fatigue crack growth in rubbers under severe loading ABSTRACT. This paper deals with the mechanism of fatigue crack growth in natural rubber submitted to severe relaxing loading conditions. In one mechanical cycle under such loading conditions, the high level of stress at the crack tip engenders high crystallinity, which halts crack growth in the plane perpendicular to the loading direction. Consequently, the crack bifurcates. Then the fracture surfaces tear, slide and relax simultaneously along a highly crystallized crack tip to form striations. The higher the stress level, the lower the crack growth in the plane perpendicular to the loading direction and the greater the bifurcation phenomenon. This explains why the striation shape evolves from triangular to lamellar during crack propagation.
Multiaxial deformation and strain-induced crystallization around a fatigue crack in natural rubber
Engineering Fracture Mechanics, 2014
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 setup 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.
Article Influence of Experimental Parameters on Fatigue Crack Growth and Heat Build-Up in Rubber
2013
Loading parameters (frequency, amplitude ratio and waveform) are varied to determine their influence on fatigue crack growth in rubber. Up to three different rubber blends are investigated: one actual engineering material and two model materials. Fatigue crack growth curves and strain distributions of pure shear and faint waist pure shear samples are compared for a model material. Fatigue behavior is studied for three different frequencies (1 Hz, 3 Hz and 5 Hz). Amplitude ratio appears to be another important influence factor concerning fatigue crack growth in rubber. The beneficial effect of positive amplitude ratios (tensional loading conditions) is shown for different materials. However, fatigue crack growth is considerably increased for negative amplitude ratios (tensional-compressional loading conditions). Furthermore, the influence of the waveform is determined for three different waveform shapes. One is sinusoidal, and two have a square shape, including dwell periods and sinusoidal slopes. Special focus lies on heat build-up, which is substantial, especially for large loads, high frequencies and/or highly filled rubber blends. Plateau temperatures are determined for various loading conditions and rubber blends. A very simple linear relationship with dissipated energy per time and unit area is obtained. Results gathered with dynamic mechanical analyses show, likewise, a linear trend, but the heat build-up is very small, due to different sample geometries.
Influence of Experimental Parameters on Fatigue Crack Growth and Heat Build-Up in Rubber
Materials, 2013
Loading parameters (frequency, amplitude ratio and waveform) are varied to determine their influence on fatigue crack growth in rubber. Up to three different rubber blends are investigated: one actual engineering material and two model materials. Fatigue crack growth curves and strain distributions of pure shear and faint waist pure shear samples are compared for a model material. Fatigue behavior is studied for three different frequencies (1 Hz, 3 Hz and 5 Hz). Amplitude ratio appears to be another important influence factor concerning fatigue crack growth in rubber. The beneficial effect of positive amplitude ratios (tensional loading conditions) is shown for different materials. However, fatigue crack growth is considerably increased for negative amplitude ratios (tensional-compressional loading conditions). Furthermore, the influence of the waveform is determined for three different waveform shapes. One is sinusoidal, and two have a square shape, including dwell periods and sinusoidal slopes. Special focus lies on heat build-up, which is substantial, especially for large loads, high frequencies and/or highly filled rubber blends. Plateau temperatures are determined for various loading conditions and rubber blends. A very simple linear relationship with dissipated energy per time and unit area is obtained. Results gathered with dynamic mechanical analyses show, likewise, a linear trend, but the heat build-up is very small, due to different sample geometries.
Fatigue crack propagation in reinforced natural rubber
CRC Press eBooks, 2013
In this paper, dynamic fatigue crack propagation properties of natural rubber/silicone rubber (NR/VMQ) composites are studied under constant tearing energy (G) input. Through dynamic fatigue crack growth testing, it is found that with the increase of VMQ fraction, NR/VMQ exhibits a lower crack growth rate (dc/dN). The viscoelastic parameters have been recorded in real-time during crack propagation, including the storage modulus E 0 , loss factor tan d, and loss compliance modulus J 00 , and their relationships with crack propagation behaviour have been established. The improved crack propagation resistance is attributed to the reduced J 00 , resulting from the synergistic effect of increased E 0 and decreased tan d, and thus more energy dissipation occurred in the linear viscoelastic region in front of the crack tip, which consumed part of the energy for crack growth. Finally, good correlation between dc/dN and J 00 could be successfully established.
Fatigue of natural rubber under different temperatures
International Journal of Fatigue, 2019
Natural rubbers have extraordinary physical properties, typically the ability to crystallize under tension. Especially, they exhibit a high fatigue resistance. Furthermore, strain-induced crystallization (SIC) is a high thermo-sensitive phenomenon. Better understanding how SIC reinforces fatigue life and how temperature affects this property is therefore a key point to improve the durability of rubbers. The present study investigates temperature effects on the fatigue life reinforcement due to SIC for nonrelaxing loadings. After a brief state of the art that highlights a lack of experimental results in this field, a fatigue test campaign has been defined and was carried out. Results obtained at 23 • C were first described at the macroscopic scale. Both damage modes and number of cycles at crack initiation were mapped in the Haigh diagram. Fatigue damage mechanisms were then investigated at the microscopic scale, where the signature of SIC reinforcement in the crack growth mechanisms has been identified. Typically, fatigue striations,wrenchings and cones peopled the fracture surfaces obtained under non-relaxing loading conditions. At 90 • C, fatigue life reinforcement was still observed. It is lower than at 23 • C. Only one damage mode was observed at the macroscopic scale. At the microscopic scale, fracture surfaces looked like the ones of non-crystallizable rubbers. At 110 • C, the fatigue life reinforcement totally disappeared.
2012
A homemade stretching machine has been developed to perform fatigue tests on natural rubber in the synchrotron facility Soleil. Strain induced crystallisation is investigated by wide angle X-ray diffraction during in situ fatigue tests of different minimum and maximum strain levels. The index of crystallinity x decreases with the number of cycles when the minimum strain level reached during the fatigue test is lower than the critical stretch ratio for melting l M. On the contrary, when the stretch ratio is maintained higher than the critical stretch ratios for melting l M and crystallisation l C , x increases with the number of cycles. The size of the crystallites has the same evolution during fatigue testing as x. On the contrary, the misorientation of the crystallites does not depend on the minimum strain level reached and decreases with the number of cycles for all the fatigue tests.
Multiaxial Fatigue Crack Initiation on Filled Rubbers: Statistical Aspects
Fracture of Nano and Engineering Materials and Structures
White reinforcement fillers such as precipitated silica inclusions are traditionally used in order to increase tear resistance and reduce internal heating of technical rubber components. In this work, two precipitated silicas, which essentially differ in their specific surface area, are incorporated into a natural rubber matrix and investigated according to their mechanical behaviour and fatigue lifetime. To this end, cyclic tension/compression and torsion tests have been carried out on dogbone shaped specimens. All tests have been performed until a visible millimetric crack appears on the outer surface of the specimens. Microscopic examinations are utilised in order to find out the crack growth mechanisms in both materials. While a significant difference has been observed in terms of mechanical properties, the fatigue curves seem to merge into a unique diagram for both reinforced rubber materials. The location and the orientation of the cracks are taken into account by using the largest principal stress. Furthermore, a statistical analysis is performed based upon the amount of specific inclusions per cm 3 of the rubber compound.