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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Keywords: fatigue strength, composite hybrid sucker rod, durability, fracture, carbon fibers, glass fiber, fatigue crack, delamination.

1. Introduction Sucker rods are widely used in industry due to their many advantages, including relatively low cost, well established technology, availability, and a wide range of applications (Takacs (1997), Martin (2012), Takacs and Mihaly (2014, 2017), Teodoriu and Pienknagura (2018), Kopei 1 et al. (2023)). Sucker rods are one of the main components of the pumping unit, and therefore their fracture leads to the failure of the entire hydrocarbon extraction system. In addition, the rods work in dynamic mode, that is, they are in constant accelerated motion. They are also operated under conditions of high temperatures, maximum loads, and under the influence of corrosive-active environments. The important properties of the hybrid sucker rod, in particular carbon and glass fibres as its component, are strength, stiffness, and density, as well as resistance to corrosive influences, wear, and fatigue resistance (Mouritz et al. (1999), McIlhagger et al. (2015), Li et al. (2022), Krechkovska et al. (2022), Kopei 2 et al. (2023)). Regarding the fatigue resistance of carbon fibres, a material with identically oriented fibres can be cyclically loaded for up to 10 8 cycles without failure at a load close to 90% of its initial strength Mouritz (2010). Such high resistance to fatigue is explained by the presence of equally oriented carbon fibres in the rod body. Thus, the breakage of one carbon fibre does not lead to a concentration of load on neighboring fibres. At the same time, a crack in the steel creates instability in the form of a stress concentrator, which contributes to the propagation of the crack under the influence of each subsequent load cycle, which ultimately leads to the failure of the rod. 2. Materials and methods For fractographic studies, a specimen after fatigue strength tests was used, made of a hybrid rod with a carbon fiber core and a fiberglass shell, the diameter of which was 19 mm and the length was 220 mm. The sample was loaded by rotating cantilever bending on the ZKSh-25 stand at a stress of 175 MPa and a frequency of 16 Hz. To ensure the loading of the sample by rotating cantilever bending, it was fixed on the shaft in a collet clamp, which rotated due to friction with the rolling supports mounted on the risers of the stand. The shaft was connected to the electric motor by a finger coupling (Fig. 1). The bending moment on the sample was created by the loads that were placed on the suspension plate fixed on its loading head. The installation was mounted on a frame with shock absorbers. At the moment of fracture of the sample, the limit switch, which was activated under the pressure of the lever, automatically turned off the electric motor. The number of load cycles was recorded according to the readings of the counter. The length of the crack was monitored on the side surface of the rod (Kopei et al. (2019)). The installation was periodically stopped to record the lengths of fatigue cracks around the joints of the composite hybrid sample with the steelhead (Fig. 2c). The length of the cracks was measured with an accuracy of 0.1 mm.

Fig. 1. Sample of the sucker rod for fatigue tests.

To prevent melting of the bond between the glass and the carbon fibers of the rod, and to improve conductivity and the quality of fractographic images, the fracture surface of the samples obtained after testing in laboratory conditions was previously coated with a thin layer of gold using a magnetron-type JFC-1600 Auto Fine Coater