The Spb method used to estimate crack extension for coiled tubings fracture toughness tests

Abstract

Coiled tubings are widely used in oil and gas industries. Fracture toughness test of coiled tubing is a challenge. Non-standard specimen has to be used due to its geometrical difficulties, ie small thickness and diameter. Taking into account these factors, fracture toughness resistance curves, J-R curves, of coiled tubings were determined using the Spb method for the first time. The separability parameter, Spb is defined as the load ratio between two specimens at constant displacement, a precracked one that exhibits crack growth during its test and a blunt notched specimen with constant crack length when tested. Spb method, which uses Spb parameter, estimates stable crack growth in fracture toughness tests. It has been applied to different standardized specimen geometries. In this work, the method is applied to non-standard coiled tubing specimens. The geometry function was experimentally obtained testing four-point bending specimens. Stable crack growth results are compared to those measured using DC potential drop method used as reference method. Encouraging results were obtained.