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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anisotropy has not been extensively studied and cannot be effectively taken into account (with rare exceptions, as shown by Diani et al. (2004; 2006); Itskov and Aksel (2004); Marckmann et al. (2016)) when developing the constitutive relations. The main reason for the initial anisotropy is the orientation of the filler in elastomers during the process of calendaring, in which all components are mixed in the matrix. Evidently, the formation of the initial anisotropy in vulcanizates depends on the filler geometry, yet it can also occur in the material containing spherical particles (Diani et al. (2004); Marckmann et al. (2016)). In our research, the experimental study on the homogeneity of mechanical properties of materials was carried out on dumbbell-shaped specimens cut from nanocomposite plates along two mutually perpendicular directions. The induced anisotropy caused by the Mullins softening effect in elastomeric nanocomposites is currently under active study; see, for instance, Diani et al. (2006); Marckmann et al. (2016); Dargazany et al. (2012); Mai et al. (2017); Machado et al. (2012); Mokhireva et al. (2017). For this purpose, the behavior of the material pre-loaded in one direction is investigated to reveal how its properties change in another direction, in particular, perpendicular to the previous one. There are also studies, such as Marckmann et al. (2016); Dargazany et al. (2012); Mai et al. (2017); Machado et al. (2012), where changes in the properties of the material are investigated along various directions. The complexity of such experiments lies in obtaining reliable results, since the small-sized specimens are first cut out at different angles from the material pre-stretched in one direction and then they are deformed one by one. As a result, each specimen recovers its properties in different ways (time-depend effects, as observed by Dargazany et al. (2012); Shadrin (2015)). Moreover, a probability that this set of experiments produce unreliable results remains high because of the possible local defects in specimens or possible experimental errors. Some papers, such as Mai et al. (2017); Machado et al. (2012), present results of experiments conducted under more complex strain histories than successive cyclic loading along each of two orthogonal directions. The initial load is specified not by cyclic uniaxial loading, but by eqibiaxial or unequal biaxial deformation. In any case, the induced anisotropy caused by material softening is most pronounced in high-filled composites and at high stretching levels. The occurrence of this anisotropy depends on the morphology of the filler, its concentration in the matrix and the type of pre-loading. We have investigated the induced anisotropy in experiments conducted with cruciform specimens, as shown by Marckmann et al. (2016); Dargazany et al. (2012); Mokhireva et al. (2017), sequentially loaded along each of two orthogonal axes using a Zwick biaxial testing machine. In this paper, the behavior of several vulcanizates containing fillers of different morphologies (carbon black N330, detonation nanodiamond, graphene, single- and multi-walled carbon nanotubes) but based on the same matrix (styrene-butadiene rubber) was considered. In low-filled systems the amount of each filler was 7 phr and in high filled composites – 50 phr (43 phr of carbon black and 7 phr of one of the nanocarbons examined in this study). Mixing and subsequent vulcanization in all cases were carried out using the same technology. In some cases, the fibrous fillers underwent additional purification prior to their introduction into the matrix. 2. Nanocomposites As an elastomeric base, we have used alpha-methylstyrene-butadiene rubber SKMS-30 ARKM-15 (SBR-1705), which is extended with addition of highly aromatic oil. This general-purpose rubber has found commercial use in the tire, rubber, footwear, cable and other industries. Both carbon black N330 and detonation nanodiamond particles introduced into a matrix are three-dimensional inclusions, but they differ greatly in structure and size. Carbon black is one of the most widely used filler in tires and other rubber products. The particles of denotation nanodiamonds (TAN brand) are ultradispersed diamonds with a typical size of 4 – 6 nm (Dolmatov (2009)). The density of nanodiamond particles is two times higher than that of carbon black particles. This means that the volume fraction of nanodiamonds in the composite is 2 times smaller compared to carbon black. Graphene is often considered as two-dimensional filler; there is one or more layers in every filler particle. The graphene used in the examined composite was manufactured via carbonization of biopolymers during the self propagating high-temperature synthesis (SHS) described by Vozniakovskii et al. (2020); Neverovskaya et al. (2019). Actually, as mentioned by Neverovskaya (2019), the graphene particles are the volume-planar "flaky" structures of sizes up to 20 μ m. Single-walled nanotubes (SWCNT) have a fibrous shape and can be represented as one-dimensional filler. A