Fracture Initiation in Low Strength Steel Pressure Vessels

Tests to failure have been made on 5 ft dia × 1 in. thickness steel pressure vessels containing longitudinal slits through wall thickness representing natural defects. Steels of from 33,000 to 70,000 psi yield strength have been tested over a range of temperatures with defects of 6, 12 and 24 in. length. When the toughness exceeds 20/30 ft lb Charpy V-notch energy, failure occurs by plastic collapse, and failure conditions can be predicted from the tensile properties of the steel. At lower levels of toughness the concept of a constant crack opening displacement (COD) at fracture initiation of defects in pressure vessels and in notched bend specimens can be used to predict vessel failure conditions. Recognition must be made of the variables found in COD notched bend testing, and tests are necessary to determine the most embrittled area of a pressure vessel. Examples are given of the method of application of COD and the degree of embrittlement possible in a low alloy steel.