Pecularities of fatigue cracks growth in steel and composite sucker rods

Abstract

To predict the fatigue life of sucker rods, it is important to take into account the effect of technological environments under various loading schemes on the conditional fatigue limit of steels used for their manufacture (in particular, such as steel 20N2M), and compare these properties with modern materials for the manufacture of fiberglass and hybrid sucker rods (with metal heads and polymer rod) as prototypes of steel elements of oil-producing equipment.Specimens of sucker rods with technological or operational defects were tested for fatigue life-time in aggressive technological environments. It has been established that the fatigue life-time and fatigue endurance of sucker rods decrease with an increase in their diameter. So, with a test base of 107 loading cycles, the endurance limits of rods with a diameter of 19, 22, and 25 mm were 95, 85, and 80 MPa, respectively. It was shown that technological defects practically did not affect the life-time of the rods. Minor differences were associated with the scatter of the fatigue characteristics of the metal. Defects formed in rods during their operation in wells have a more noticeable effect on their life-time under variable loads. It was found that the resource of sucker rods with a diameter of 19 mm decreased faster compared to rods with a diameter of 22 mm in the presence of operational defects of the same depth in both rods. This is due to the greater sensitivity of smaller diameter rods to defects.According to the curves of the crack growth rate in specimens made of sucker rods with a diameter of 22 mm, it was found that the critical state (according to the jump in the crack growth rate) is reached if the crack length along the perimeter of the rods is within 15...16 mm, and its depth is 4.5 ... 5.6 mm. Such cracks were fixed at a stress range of 100 MPa, which is commensurate with the operating loads of the rods. For rods of other standard sizes, the critical crack depth, which preceded the almost spontaneous destruction of the specimens, was 3...4, 4...5 and 5...6 mm for rods with a diameter of 16, 19 and 25 mm, respectively. In article we also evaluated fatigue life on samples of fiberglass sucker rods under cyclic loading by circular cantilever bending and obtained the corresponding fatigue curves. According to the results of these tests, probabilistic curves of fatigue endurance of samples with steel and fiberglass sucker rods in the technological environment of reservoir water were constructed with different probability of their non-destruction P(N). Fatigue endurance of samples with steel, fiberglass and hybrid sucker rods was compared.