Slow-Bend KIc Testing of Medium-Strength High-Toughness Steels

Linear-elastic fracture mechanics (LEFM) has been used extensively to analyze the fracture behavior of steels having yield strengths greater than 200 ksi. Although theoretically LEFM can be used to analyze the behavior of all steels, it has not been used extensively to analyze the behavior of medium-strength (and lower-strength) high-toughness steels, because large plastic-zone sizes develop prior to fracture of these steels and test results are therefore difficult to analyze. Therefore, to establish the limitations of the LEFM approach to the analysis of medium-strength steels, a series of slow-bend KIc tests were conducted on various steels, with different levels of toughness. The present paper describes the test procedures and analyses used in slow-bend KIc tests of steels having yield strengths from 110 to 246 ksi and Charpy V-notch impact energy absorption (+80 F) in the range 16 to 89 ft • lb. The range of KIc values for various pop-in criteria is presented as well as a discussion of the various factors affecting the slow-bend KIc testing of medium-strength high-toughness steels. The results indicate that LEFM can be used to analyze the fracture behavior of medium-strength high-toughness steels. KIc values ranging from 87 to 246 ksi in. were obtained for the eleven steels investigated. Although the specimen sizes tested generally 2 in. thick) did not satisfy completely the recommended ASTM Committee E-24 size requirements (in some cases, the suggested sepcimen thicknesses were greater than 6 in.), the KIc values are considered to be representative on the basis of general strength and Charpy V-notch toughness levels and expected service performance. Therefore, the measured KIc values are estimated to be reliable for general engineering use, although some of the specimen sizes did not satisfy the current ASTM Committee E-24 requirements completely. When KIc and Charpy V-notch energy-absorption values were normalized on the basis of yield strength, an empirical correlation was established over a wide range of yield strengths. Although such a correlation increases the confidence in both types of approaches to fracture analysis, the use of Charpy tests to predict KIc values is not recommended until additional test and service experience is obtained.