Fatigue failure modeling in rubber stator of downhole motors: Dependency to materials and geometry, and its application to life prediction

Abstract

A numerical crack growth analysis is here combined with the experimental fatigue and tearing data to fully characterize the crack growth performance of the rubber liners of downhole motors. Findings consistently revealed that the geometry of “uniform liner” has a more than three times higher J-integral than the “conventional liner” type and the crack growth in the former is remarkably affected by changing the material type, the crack length, and the loading conditions. By increasing the interference fit from 0.05 to 0.3 mm, the energy release rate changes in a range of 0.1–0.3 for the conventional liner, but it surprisingly reaches a value of 3.0 for the uniform liner. The significance of reinforcing rubber liners with either a particulate filler or a polymeric resin was identified. Very intensified fluctuations in the tearing force of samples containing the polymeric resin were observed. The fracture mechanics approach to fatigue was adapted to compare the performance of liners. Under similar conditions, it was observed that growth in crack length from 0.5 to 1.5 mm occurs in a span of 8.65 × 105 and 2.11 × 107 cycles, respectively for the uniform and conventional liners.