A Single Specimen CTOA Test Method for Evaluating the Crack Tip Opening Angle in Gas Pipeline Steels

Failure information from recent full-scale burst experiments on modern TMCP gas pipeline steels having a yield strength level of 690MPa and higher has shown that the CTOA fracture criterion can be effectively used to predict the arrest/propagation behaviour of the pipe against possible axial ductile fractures. The use of CTOA as an alternative or an addition to the Charpy V-notch and DWTT fracture energy in pipelines is currently under review. A significant difficulty currently limiting the more extensive use of CTOA in pipeline assessment is its practical evaluation either in the real structure or in a laboratory scale test. Different combinations of experimental and finite element analyses have been proposed for the measurement of the CTOA of a material. Although most of these models are able to predict the CTOA effectively, their implementation requires extensive calibration processes using the test load-deflection data. The authors have recently developed a novel test technique for direct measurement of the steady state CTOA using a modified double cantilever beam geometry. The technique uses optical imaging to register the uniform deformation of a fine square grid scored on the sides of the specimen. The slope of the deformed gridlines near the crack tip is measured during crack growth from captured images. Its value is a representative of the material CTOA. This paper presents recent results from the implementation of the technique to determine the steady state CTOA (steady state in this work refers to regions of ductile crack growth where CTOA values are constant and independent of crack length) of API X80 and X100 grade gas pipeline steels. In each case the approach was able to produce large amounts of highly consistent CTOA data from both sides of the test sample even from a single specimen. This extensive data set allowed an evaluation of the variance of the stable CTOA as the crack grew through the microstructure. The test method generated a steady CTOA value of 11.1° for X80 and 8.5° for X100 steels tested, respectively.