Relationship Between Apparent (Total) Charpy Vee-Notch Toughness and the Corresponding Dynamic Crack-Propagation Resistance

Abstract

The consequences of a dynamic fracture in a gas-transmission pipeline require that pipelines be designed to avoid such incidents at a high level of certainty. For this reason, the related phenomonology has been studied since the early 1970s when the possibility of a dynamic ductile fracture was recognized. Full-scale experiments were done to characterize the fracture and gas dynamics associated with this process and empirical models were developed as a means to represent these experiments in a design or analysis setting. Such experiments focused on pure methane gas, and in the early days used steels with toughnesses less than 100 J, consistent with the steel making capabilities of the 1970s. Subsequently, interest shifted to larger diameter, higher pressure, higher BTU “rich” gases requiring higher toughness steels. The full-scale tests conducted to validate the arrest toughness levels determined that these empirical models were non-conservative. This paper presents a relationship between the dynamic crack propagation resistance and the apparent crack propagation resistance as measured by Charpy vee-notch (CVN) test specimens. This relationship is used in conjunction with the existing Battelle empirical criterion for dynamic-fracture arrest to determine the apparent toughness required to arrest a propagating ductile fracture in gas-transmission pipelines. The validity of this relationship is illustrated by successful predictions of arrest toughness in pipelines under a range of conditions including rich gases and high-toughness steels, including those showing a rising upper-shelf behavior.

Arresting Propagating Shear in Pipelines

“…3, b it is apparent that the one-to-one correlation can overestimate fracture resistance to an extent that increases as toughness increases. This tendency is consistent with the 'Leis correction factor' (LCF) for the BTCM [13], which was based on instrumented CVN testing that showed the relative fraction of the energy dissipated in crack propagation decreased nonlinearly as the toughness increased. As noted elsewhere [14], these trends suggest a simple correction to the BTCM based on the trend in Fig.…”

“…As time passed, it became clear that not only the BCTM and BSE results showed this trend, but that all SEs that had emerged to quantify fracture arrest for single-phase gases that were calibrated in reference to the CVN specimen shared the same trend: all became increasingly non-conservative as the toughness increased beyond about 75 ft-lb (~100 J) [20]. Fig.…”

“…Alternative schemes were sought by the Alliance Consortia. Eventually, consideration given to an emerging concept developed under IR&D funding at Battelle in the early 1990s [13], which provided a physically sound basis to offset the cause for the 'bent-over' trend seen in Fig. 9.…”

State-of-the-art review of fracture control technology for modern and vintage gas transmission pipelines

“…At that time, Fearnehough et al [36] and Wilkowski et al [37] first observed that the Charpy energy was not linear with the propagating fracture toughness for higher toughness steels. Soon it was found that the BTCM and all simplified models predicted non-conservative arrest toughness in comparison to measured Charpy energy from a full-scale gas burst test for high-strength pipeline steels with CVN larger than 70 ft-lb (or 95 J) [38]. Therefore, work was initiated in the decades to improve the BTCM and its predictions of arrest toughness for modern high-strength pipeline steels with grade X70 and above, and a variety of corrections, correlations, and modified or alternative methods have been proposed, and are reviewed next.…”

“…Note that the initiation energy denotes the area under the instrumented load-displacement curve up to the maximum load, and the propagation energy refers the area under the load-displacement curve after the maximum load. Accordingly, Leis [38] proposed a correction to improve the BTCM. It assumes that the CVN arrest toughness is the same as that determined by the BTCM if the measured CVN energy is less than 95 J (or $70 ft-lb); otherwise, the following correction is needed to determine arrest toughness in the SI units:…”