Characterization of the fracture behavior of NBR and FKM grade elastomers for oilfield applications

Abstract

The monotonic fracture behaviour of two elastomer compounds (HNBR and FKM) was characterized in this study. These elastomeric materials are frequently used in many oilfield applications (e.g., seals and hoses) and are exposed to a complex combination of mechanical, thermal and environmental loads. Furthermore, rapid gas decompression failure of these elastomeric materials was observed under real service conditions. A phenomenon termed rapid gas decompression (RGD) damage occurs when elastomer seals exposed to high gas pressure fail in a brittle manner upon the sudden release of the gas pressure. Hence, to support both material development and design efforts, it is of prime theoretical and practical importance to characterize the fracture behaviour of these materials. Fracture mechanics tests using a faint waist pure shear specimen (FWPS) configuration were performed over a wide loading rate range. First, global force and displacement values were measured and single parameter fracture mechanics values in terms of peak tearing energy, T p values were calculated. Furthermore, the crack growth process and the local crack tip deformation were characterized by non-contact optical devices. The digital image correlation technique used allows for the determination of displacement and full-field strain up to a high strain and crack growth rate. Based on above measurements, crack growth resistance curves in terms of tearing energy ( T − Δ c) and crack tip opening angle ( CTOA − Δ c) functions were derived. While a continuous crack growth was observed for HNBR at all loading rates, a discontinuous crack extension was observed for FKM. Finally a phenomenological model was deduced for describing the rapid monotonic crack extension process for these elastomeric compounds.