Mechanical property evolution and service life prediction of pultruded carbon/glass hybrid rod exposed in harsh oil-well condition

A glass fiber shell (GFS) and carbon fiber core (CFC) hybrid rod (HFRP) was produced through pultrusion technology for an alternative to traditional steel sucker rod for petroleum production. The mechanical properties of a really used HFRP rod closed to the ground (25 °C exposed temperature, 0.5 MPa hydraulic pressure and ~70 kN cyclic loading) and in the deep oil-well up to 2600 m (90 °C exposed temperature, 18 MPa hydraulic pressure and ~50 kN cyclic loading) were investigated as a function of service time as long as 553 days. The mechanical, thermal properties and microanalysis tests of HFRP rods were conducted to obtain the long-term evolution of HFRP rods under the above service environments. Exposure closed to the ground caused a maximum degradation rate of short beam shear strength (SBSS) up to 18.92%. Meanwhile, the maximum degradation rates of interface shear strength (ISS) were 28.24% for carbon fiber/resin, 16.88% for glass fiber/resin and 36.07% for CFC/GFS interface. In comparison, exposure in the deep oil-well brought in an extra degradation rate from 4.34% to 11.11% for ISS, and the higher cyclic loading (~70 kN) played an insignificant effect on the degradation. The degradation mechanism of mechanical properties for HFRP rods was attributed to plenty of bound water combined with the resin through the hydrogen bond and brought about the hydrolysis of resin and the increase of pore volume content (~5%), leading to the debonding of fiber/resin. The long-term prediction of interface shear strength shows the degradations of CFC and CFC/GFS were more sensitive to the cyclic load (70 kN) compared to no cyclic load (0 kN), and an extra degradation rate up to 31.3% was observed compared to the no load condition, and the accelerated degradation was weakened with the increase of service temperature and hydraulic pressure. In addition, the exposure at the increased service temperature and hydraulic pressure in the deep oil-well led to an additional degradation rate of ISS in comparison to the ground exposure: 10.70% (CFC), 13.25% (CFC/GFS) and 4.81% (GFS).