Characteristic time of strain induced crystallization of crosslinked natural rubber

Abstract

Real time Wide-Angle X-ray Scattering (WAXS) measurements during cyclic tensile tests at high strain rates (from 8 s À1 e280 s À1 ) and at room temperature on crosslinked Natural Rubber (NR) are performed thanks to a specific homemade device. From the observed influence of the frequency on the crystallization index at the maximum sample elongation, a characteristic crystallization time is deduced. This is done taking into account the material self-heating during such unusually high strain rates. Two regimes for the dynamic process of strain induced crystallization are evidenced. For the NR tested, the obtained characteristic time is around 20 ms when the material average elongation during the cyclic test is above a critical elongation value l c . l c is the minimum elongation needed to induce crystallization during low strain rate tensile tests. Moreover, a rapid increase of this characteristic time is found when the average elongation decreases below this critical value.

In situsynchrotron wide-angle X-ray diffraction investigation of fatigue cracks in natural rubber

“…0.1 Hz. As SIC is a rapid process (Albouy et al, 2012;Candau et al, 2012;Bruning et al, 2012), we assume that the results observed at 0.1 Hz would be similar to those observed at 2 Hz, even if the crystallized zone is probably larger at 0.1 Hz because of the time-dependency of SIC. Considering that the exposure time is 1 s, a trigger is sent by the stretching machine to the beamline 0.5 s before each maximum displacement.…”

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

“…It means that SIC still exists at 2 Hz especially when the distance to the crack tip is reduced because the stretch ratios are very high. However, according to Candau et al [21] this characteristic time can rise to 200 ms when the stretch ratio is close to the onset of crystallization, which means that the crystallized zone is smaller in real fatigue conditions (2 Hz) compared with the one measured at 0.1 Hz.…”

“…Furthermore, it is to note that the frequency decrease (from 2 to 0.1 Hz) applied to perform the acquisition and the mapping might have a slight effect on the observed results. Recent studies dealing with the kinetics of SIC [21][22][23] have highlighted that the crystallization characteristic time in NR is short, around 20 ms according to Candau et al [21], which is much smaller than the duration of a half fatigue cycle at 2 Hz (250 ms). It means that SIC still exists at 2 Hz especially when the distance to the crack tip is reduced because the stretch ratios are very high.…”

Strain-induced crystallization of carbon black-filled natural rubber during fatigue measured by in situ synchrotron X-ray diffraction

“…They are performed at conventional frequencies for rubber, i.e. around 1 Hz, which limit self-heating to about 3°only [18]. As the minimum exposure time to record a diffraction pattern is 1 s (about the duration of a full cycle), it is not possible to record the diffraction patterns while the actuators are in motion.…”