Estimation of Slant Tearing Energy for High-Grade Pipeline Steel From Instrumented Charpy Test Data and its Transferability to Large Structures

Abstract

For several decades, the Charpy upper shelf energy has been used as a fracture controlling parameter to estimate the crack arrest/propagation performance of gas transportation pipeline steels. However, significant discrepancies have been observed between the results of full-scale burst experiments on modern pipeline steels and those predicted by Charpy-based fracture models. This indicates that fracture models calibrated in the past on lower-grade pipeline steels (Charpy toughness below about 100J) cannot be extrapolated beyond their calibration range to assess the fracture behaviour of higher-strength high-toughness steels. One reason for this is the high level of energy often required for crack initiation in these steels. Accordingly, in the short term different correction factors ranging from 1.4 to 2 have been proposed to refine these fracture prediction models. The use of alternative failure parameters like CTOA is currently under review. In this paper a novel experimental technique is given to apportion the upper shelf Charpy fracture energy into its different components, i.e. crack initiation energy and flat and slant tearing energy. The experimental data from instrumented Charpy tests on standard impact specimens made from an X100 grade pipeline steel is used to estimate crack initiation and propagation energy. The areas associated with flat tearing in the centre and slant shearing at the edges of the fracture surface of Charpy test specimens are estimated optically using a fine measurement grid with 0.5 mm spacing. The energy required for generating the flat and slant fracture areas is calculated by the use of associated multipliers, i.e. the specific flat and slant fracture energy (in terms of J/mm2). These are measured separately using flat and slant crack growth data from fracture tests on standard C(T) and modified DCB like specimens. The results showed that the Charpy energy from a test is dominated by non-crack propagation energies. Around 36% of the measured impact energy appeared to be associated with flat and slant tearing processes. As the latter is the important failure micro-mechanism in pipeline steel only that part of the overall Charpy shelf energy which is associated with slant shearing might be used to evaluate the crack growth resistance of modern steels. This suggests the possible use of correction factors for high toughness pipeline steels of the order of 1.7 to transfer the slant fracture energy measured on small-scale specimens to the real structures for predicting their crack arrest/propagation behaviour. The correction factor proposed here from the laboratory test programme agrees with those obtained from costly full-thickness burst experiments on similar class of pipeline steel.