PSI - Issue 32

Kseniia A. Mokhireva et al. / Procedia Structural Integrity 32 (2021) 137–143 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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significant changes in properties could be seen when nanodiamonds are added in combination with a large amount of carbon black. Nanodiamonds were expected to be able to fill the voids inside the carbon black agglomerates. However, our experience shows that this filler does not participate in the formation of the mechanical properties of the composite. If carbon black (43 phr) and purified single-walled nanotubes (7 phr) are introduced, then the orientation of the nanotubes is preserved and the anisotropy becomes much more pronounced. Note that the stress level in the specimens cut along the calendering direction increases by an order of magnitude as compared to the nanocomposite filled only with carbon black (50 phr) only. There is also a significant increase in the stress level for the specimens cut in the perpendicular direction. For the purified multiwalled carbon nanotubes introduced into a highly filled with carbon black particles, the isotropy of properties is retained within the experimental error. 3.2. Induced anisotropy The experiments aimed to studying the induced anisotropy were carried out on cruciform specimens with multiple slits parallel to the arms sides, and a nearly homogeneous stress-strain state was obtained in the central measurement part of the specimen, as pointed by Marckmann et al. (2016); Dargazany et al. (2012); Mokhireva et al. (2017). First, the specimens were cyclically stretched along the OX-axis up to 50% deformation. At the each step of load change, time delays were set so that the time processes do not greatly affect the material behavior. The cross shaped specimen loaded along the OX-axis was deformed along the OY-axis so that the load along this axis was close to zero over the entire loading range along the OX-axis. Then, without removing from the grips, the specimen was twice subjected to cyclic stretching along the orthogonal axis OY. In this case, the OX-axis remained free of load. After sequential training of the specimen in two mutually perpendicular directions OX and OY, the cruciform specimen was again subjected to deformation along the OX-axis. Note that the study does not involve specimens made of anisotropic materials – nanocomposites filled with purified single-walled nanotubes – because of the material anisotropy. The results of experiments obtained for low-filled vulcanizates are not of particular interest due to the low proportion of filling; any changes in the mechanical properties did not actually appear. The loading curves are shown in Figure 4 for materials with a high content of granular fillers. Under cyclic loading along the OX-axis, elastomeric composites soften only along this direction. That is, in a small range of deformations, the loading history of specimens along one direction does not actually affect their mechanical properties in the other direction. At the same time, returning to deformation along the OX-axis after training along both axes, the material shows initial properties in this direction due to the closure of defects caused by deformation in the opposite direction. As the deformation increases, the material exhibits a softening behavior, which can be attributed to serious changes in the structure.

Fig. 4. Loading curves for cross-shaped specimens at 50% cyclic deformation caused by loading along two mutually perpendicular directions. Solid black line – loading along the OX-axis, dashed blue line – loading along the orthogonal OY- axis, solid red line – reloading along the OX axis. Filler: a) carbon black (50 phr) and b) carbon black (43 phr) + nanodiamonds (7 phr).