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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sample coating unit. A Zeiss EVO-40XVP electron microscope was used to determine the fracture mechanisms of fractures obtained as a result of tests in laboratory conditions. 3. Results and discussion The features of the fracture were studied at the fracture of a composite two-layer rod consisting of carbon fibers inside the core and glass fibers in the shell. The damage of the rods depending on the number of loading cycles by rotating cantilever bending was evaluated by the loss of stiffness, and their fatigue strength was evaluated by the fracture due to the appearance of cracks. In the work, Kopei et al. (2022) described the data for a composite sucker rod under fatigue failure. The number of cycles before damage to the rod was determined at an applied stress of 140 MPa. Damage in the rod originated and spread as a result of cracking and fracture of the polymer matrix, which bound the glass fibers in the shell as a single unit (Fig. 2a). Fatigue from bending loading began when the deflection of the hybrid rod exceeded 42% of the deflection corresponding to the strength level of the rod under its active cantilever bending. Fatigue damage reached a saturation point along the line of contact between the fiberglass shell and the carbon fiber core due to the stress concentration occurring at the interface of the composite, which led to the asymptotic behavior of the rod in the load cycle due to the asymmetry of its stiffness loss. Since the damage in the rod did not extend into the carbon-plastic core, the mechanical properties under static load remained at the level of ~ 85% of the strength limit or even more. Diagrams of the growth of six axial fatigue cracks (Fig. 2b) and a diagram of their location along the perimeter of the rod before its fracture (Fig. 2c) were obtained. Obvious asymmetric growth of fatigue cracks due to one-sided loss of stiffness of the rod during the critical growth of cracks 1 and 2 in the scheme of Fig. 2c, which caused the initiation of cracks 3 first, and later 4 – 6, the growth of which occurred much faster than cracks 1 and 2. In this case, the sample was cyclically loaded for 1.3 ٠ 10 6 cycles until the cracks nucleated (Fig. 2), and after that, when 1.5 ٠ 10 6 cycles were reached, its shell was destroyed with a corresponding loss of stiffness. In another sample, tested for rotational bending before losing its rigidity, 6 fatigue cracks of approximately the same length were also found (Table 1) (Kopei et al. (2017)).

Fig. 2. (a) Axial cracks in a composite sucker rod specimen during cantilever rotational bending tests; (b) Dependencies of the length l of six axial fatigue cracks (in points of 1-6) on the outer surface of the fiberglass shell of the composite sample on the number of load cycles of this sample N . The final length of each of the six cracks extending from each of the six points along the perimeter of the bar specimen was 23, 23, 18, 7, 14, and 12 mm, respectively; (с) a scheme of the location of all six fatigue cracks on the surface of the rod along its perimeter. The cracks on the specimen surface appeared after 1. 3·10 6 loading cycles and they propagated up to the loss of the sucker rod stiffness at 1.6 10 6 cycles.

Table 1. Results of tests of a sample from a hybrid composite rod by rotary bending.

N, 10 6 cycles

Number of cracks

l, mm

The state of the specimen

σ, MPa

140

1.437

6

21; 20; 17; 15; 12; 10

Broken