Fracture-Toughness Requirements for Steels

Abstract

Abstract For certain structural applications, steels having high levels of toughness are desirable. For these cases, steels that are extremely tough at service temperatures are available. However, because the cost of these steels generally increases with their ability to perform satisfactorily under more severe operating conditions, the designer should not arbitrarily specify more toughness than is required for the specific application. How much toughness is sufficient is a difficult question to answer and establishing the fracture-toughness requirements and the concomitant inspection requirements for various structural applications has long been a problem for engineers. Using concepts of fracture mechanics, a quantitative approach to the development of toughness requirements for steels is presented. This approach is based on the requirement that in the presence of a large sharp flaw, through-thickness yielding should occur before fracture. KIc and Charpy V-notch impact tests were conducted on various steels having yield strengths in the range 40 to 250 ksi. Various empirical correlations were developed between the KIC and Charpy test results at room temperature and in the transition-temperature region so that the fracture-mechanics criterion could be expressed in terms of Charpy test results. Using these empirical correlations, and the proposed toughness requirements, relations were developed between the required Charpy V-notch impact values for through-thickness yielding before fracture, yield strength of the steel, and section thicknesses up to 2 inches. The criterion presently developed establishes quantitatively the fracture-toughness requirements for structural applications in which through-thickness yielding should occur prior to fracture in the presence of a large sharp flaw. This criterion is probably conservative for many applications but is believed to be desirable for critical applications. Introduction Fracture toughness may become an important design parameter for certain critical structural applications for which high levels of toughness are necessary. For example, the increasing offshore oil-well activity and new pipeline systems being planned for service under arctic weather conditions may require steels with quite low ductile-to-brittle transition temperatures. In addition, certain components of large offshore drilling structures may require steels that have very high levels of resistance to shear fracture because these structures may be subjected to dynamic loading (waves, earthquakes, ice movement). For these special cases, steels that are extremely tough at all service temperatures are available. However, because the cost o£ these steels generally increases with their ability to perform satisfactorily under more severe operating conditions, the designer should not arbitrarily specify more toughness than is required for the specific application. How much toughness is sufficient is a difficult question to answer and establishing the fracture-toughness requirements and the concomitant inspection requirements for various structural applications has long been a problem for engineers. Ideally, the fracture-toughness requirements for different structural applications should be based on correlations between laboratory-test results and service experience. An extensive investigation1) of steels from World War II tankers showed a correlation between the Charpy V-notch (CVN) impact energy and service performance. Plates in which brittle fractures had initiated generally absorbed 10 ft-lb or less at the service temperatures7 plates in which brittle fractures had been arrested generally absorbed more than 10 ft-lb.