L51474 Brittle Fracture Initiation of Heavy-Wall Components

Pipeline valve bodies, fittings, flanges, and numerous other components are typically made of cast or forged low-alloy steels. The large wall thicknesses necessitated by the complex shapes of these components usually are not conducive to ductile fracture behavior. Nevertheless, there is a distinct need to have sufficient toughness in such components so that they will not be susceptible to catastrophic failure from defects that may remain after manufacturing and hydrostatic testing. The present practice for controlling toughness in such components consists of specifying arbitrary values of Charpy energy or percentage shear area at a given temperature to be determined on a coupon of the material made in a manner similar to the parent cast or forged component. These values are usually decided by agreement between the purchaser and manufacturer.

This report describes experimental research aimed at correlating typical impact tests to more fundamental fracture mechanics tests using various available correlations and to predict full-scale fracture behavior of heavy-walled pipeline components using the fracture mechanics parameters obtained. The components examined in this research task are: (1) a 4-inch, 600 pound class valve, (2) a quarter section of a 24-inch Arctic grade valve, and (3) a 16-inch diameter by 0.75-inch wall, 45 degree elbow. Small-scale tests conducted on the 4-inch valve material were standard Charpy V-notch impact, precracked Charpy impact, precracked Charpy slow bend, dynamic tear, and three-point bend – J/COD tests. Only Charpy V-notch impact and J/COD tests were performed on the 24- inch Arctic Grade valve and the 16-inch elbow.

This report describes a conservative approach to estimating failure stresses and tolerable flaw sizes to predict brittle fracture in heavy-wall components.