Impact Toughness and Brittle Failure of Carbon Steels

Brittle failure of pressure containing equipment can lead to catastrophic consequences and must be prevented. ASME pressure vessel codes have developed clear rules on toughness requirements based on both long term safe and reliable experience and fracture mechanics concepts. The essence of the toughness rules in ASME Boiler and Pressure Vessel (B&PV) Codes Section VIII, Division 1 and Division 2 is a set of impact test exemptions curves and impact toughness requirements as a function of materials’ yield strength and thickness. Considering the limitations of the impact toughness in characterizing the resistance to brittle fracture, a high level of conservatism is built into the toughness rules. It should be emphasized that impact tests are satisfactory as a rule making tool for quality control, but impact toughness values by themselves cannot be reliably used to predict materials’ resistance to brittle failure. Fracture mechanics analysis should be performed to investigate the root cause of a brittle failure, instead of relying solely on impact toughness values.