A dual-scale failure evaluation method for Carbon-glass hybrid Fiber sucker rod and their joints under complex loads

A hybrid fiber-reinforced polymer (HFRP) continuous sucker rod, comprising a carbon fiber-reinforced polymer (CFRP) core layer, a glass fiber-reinforced polymer (GFRP) winding layer, and a GFRP coating layer (CFRP:GFRP = 2:3), has been developed and widely used in oilfield extraction due to its lower specific gravity, enhanced corrosion resistance, and superior strength. However, HFRP rod joints and their adjacent sections are prone to multi-mode failures, including fracture, debonding, and cracking. Due to the complexity of joint structure and the coupling of tension, bending, and torsion, the failure mechanism is unclear. To address this issue, a dual-scale failure assessment methodology for HFRP rods was proposed, utilizing both macro and meso finite element models (FEM). This methodology was validated through tensile and bending experiments, which yielded critical loads for the φ22 mm HFRP rod: a tensile load of 340.2 kN, a torque of 132.3 N m, and a bending moment of 1192.4 N m. Additionally, a comprehensive FEM of the joint was established, which identified potential failure points at the necking of the rotary joint, resin adhesive and the HFRP rod cross-section at the first groove tip. These failure modes closely matched the experimental observations. Furthermore, the simulation results show that stress concentration at the joint reduced the tensile, bending, and torsional strengths of the HFRP rod to 61%, 12%, and 82% of their original values, respectively. The effects of bending moments and torque on the tensile strength of HFRP rods were subsequently explored, leading to the development of an equivalent fatigue assessment method for HFRP rod joints. This method, based on the fatigue characteristics of HFRP rods and joint components, reveals that the primary cause of joint failure is the susceptibility of both the joint and the HFRP rod to bending moments and torque induced by dynamic buckling of the sucker rod string (SRS). Using this method, the fatigue ultimate axial force of the φ22 mm HFRP joint was determined to be 91.5 kN, with corresponding fatigue ultimate torque and bending moment under an axial force of 62.4 kN being 89.3 N m and 71.5 N m, respectively. Finally, a design method incorporating a concentrated weighting strategy for HFRP-steel mixed rods was proposed to enhance their service life, and its effectiveness was demonstrated through on-site testing.