PSI - Issue 59

Halyna Krechkovska et al. / Procedia Structural Integrity 59 (2024) 292–298 H. Krechkovska et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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As the fatigue failure propagated deep into the cross-section of the composite rod, the bond between the glass fibers and the polymer matrix continued to break. But at the same time, an increase in stress concentration at the tip of a ring fatigue crack and the contribution of the static stress component of axial stresses contributed to their propagation deep into the cross section of the rod. As a result, an oblique fracture began to form in the shell with the failure by a shear macromechanism. With the transition to the core of the rod, the fracture became very finely dispersed and it was difficult to identify the boundaries of the carbon fibers with the polymer that bound them into a single unit (core) (Fig. 5). Only individual fragments of carbon fibers sometimes stood out against the background of a monotonous fine-dispersed relief of fractures, which made it possible to determine their true sizes. It was found that their diameter near the fractures did not exceed 4 μm. Analysis of the central part of the fracture of the composite rod, corresponding to the final fracture zone of the hybrid sample, showed that the carbon fibers in the center of the carbon-polymer core are separated from the matrix due to decohesion, forming colonies of broken fibers (Fig. 5b-d). The diameter of the carbon fibers is significantly smaller (by at least 2.5 times) compared to the glass fibers in the surface layer of the composite sample (Fig. 5d). Visually, they can be compared by size in Fig. 5b. Moreover, the layered structure of carbon fibers is evident (Fig. 5d). Thus, fractographic studies of the fracture of a hybrid rod showed that the determining factor in its fatigue life is the surface layer reinforced with glass fibers. Under the influence of cyclic loads, part of the fibers under the side surface of the rod peeled off from the matrix (this was evidenced by secondary cracking along their borders). After this, the exfoliated fibers were free to stretch (regardless of the matrix) in each loading cycle until their fracture. At the next stage of development of the hybrid rod failure, a ductile failure of the polymer matrix bridges between the exfoliated fibers occurred. Therefore, the concentration of stresses at the tip of a fatigue crack formed in such a sequence of separate acts of fracture created the prerequisites for the propagation of fracture deep into the cross section of the rod due to the stretching of glass fibers in front of its tip with the formation of an oblique fracture. The core of the rod, reinforced with carbon fibers, was destroyed due to stretching, which ended with the pulling out of part of the carbon fibers from the polymer matrix. This indicates the proportionality of tensile stresses at the final stage of fatigue failure of the rod with the adhesion of carbon fibers to the matrix. 4. Summary The fatigue strength of a composite sucker rod is critical to its durability. The fracture of the hybrid rod under cyclic loading occurs gradually with a sequential alternation of stages of damage. In particular, the initiation of a normally oriented transverse fatigue crack at the site of the maximum bending moment of the specimen was accompanied by the appearence of axial cracks on the surface of the fiberglass shell, the growth of which was caused by the delamination of part of the surface layers of the fiberglass shell. Its growth caused the detachment of part of the surface layers of the shell on the sample and their stretching to destruction due to the plastic failure of the polymer bond in the fiberglass shell. Under the action of cyclic tensile stresses accompanying the final stage of fatigue, the carbon fibers in the core of the hybrid rod were destroyed or pulled out of the polymer binding them. 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