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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The features of the failure of a composite rod sample tested by circular rotary cantilever bending under bending stresses σ = 175 MPa and the number of load cycles to failure N = 1.459×10 6 cycles was also investigated. As a result of the tests, the nucleation and growth of axial cracks were observed, up to the fracture of the shell of the composite sample. As a result of the fatigue tests, the sample was fractured; different angles of its macro view are shown in Fig. 3.

Fig. 3. (a) Broken hybrid rod after fatigue tests; (b) longitudinal cracks in a fiberglass shell.

A feature of this work is the description of fractographic peculiarities at the macro and microscopic levels. It should be noted that already at the macrofracture of the sample, differences in the topography of the fracture were noted in the areas where the fatigue failure originated in the fiberglass outer shell and its subsequent propagation into the carbon-plastic core of the hybrid rod sample (Fig. 3). From the analysis of the macro features of the fracture, it is clear that the fracture started from the outer surface of the hybrid samples in their section where the maximum moment from cantilever bending was reached (Fig. 1c). As a rule, the fracture initiation zones were oriented normally to the axial tensile forces, which (according to the used loading scheme) reached their maximum values exactly in the outer fiberglass layer of the shell. The foci sources of fatigue crack initiation were randomly located along the perimeter of the rod sample. This was evidenced by barely noticeable secondary cracks, which were detected on the normally oriented part of the fracture. Therefore, at the stage of initiation of fracture at the fracture, a normally oriented section within the glass fiber shell of the hybrid sample was recorded in the form of an annular the ring crack, which was interpreted as a fracture associated with cyclic loading. At the same time, almost immediately after the formation of a circular ring crack (starting from a depth of up to 0.5 mm), further propagation of the fracture over the entire cross-section of the sample occurred by a different mechanism. A significant role in this was played by the longitudinal delaminations of the fiberglass shell with their separation from the carbon-plastic core, which divided the perimeter of the sample into separate sectors of fracture. Thanks to them, the reorientation of the fracture took place with the increasing influence of tensile stresses, when the entire cross-section of the sample began to work due to active stretching. Interestingly, the concentration of stresses from the appearance of a normally oriented fatigue crack (as a result of the fusion merging of all fatigue nucleation centers along the entire perimeter of the hybrid sample) was not sufficient for the propagation of fracture deep into the sample in this cross-section. The fracture surface of one of the exfoliated fragments of fatigue crack initiation from the side surface of the rod sample was analyzed (Fig. 4). The obvious phasing of fracture propagation in the fiberglass shell of the rod sample is associated with the nucleation and propagation of the fracture deep into the cross-section of the sample, and towards the neighboring foci sources of fracture initiation until their fronts merge with the formation of a circular ring fatigue crack (Fig. 4a). As a feature of the glass fiber outer layer, it was noted that the glass fibers in the core of the composite rod were not always normally oriented to the applied stresses (Fig. 4a). As a result, the fracture in areas with fibers slightly inclined to the axis of the rod occurred by peeling off the fibers from the matrix with the formation of a corresponding step-like relief on the fracture. At higher resolution, their fragile fracture features became obvious (Fig. 4b-d). The fracture surface of the specimen was significantly veiled by fragments of fibreglass. They lost contact with the fracture, glowed brightly and were perceived as separate fragments. This made it possible to isolate fracture fragments of individual glass fibers, which are usually associated with a brittle cleavage mechanism. This was considered a sign of tensile loads being applied to individual glass fibers to the point of brittle failure. After all, glass, as an amorphous material, is usually destroyed by a cleavage mechanism. Despite this, the