Advanced Materials & Processes

NOV-DEC 2013

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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How Specimen Geometry and Microstructure Influence Fracture Toughness Properties of Ferritic Materials Tom McGaughy Fabian Orth EWI Columbus, Ohio Several subtle factors related to specimen geometry and microstructure are not fully addressed in most test standards, even though they may affect fracture toughness values. F racture toughness test procedures have been used for more than 30 years and are broadly standardized by ASTM, BSI, ISO, DNV, and others. While these test standards have been carefully developed and revised over the years, there are several subtle factors related to specimen geometry and microstructure that are not fully addressed in most test standards and which may influence the measured value of fracture toughness. Guidance provided by these standards does not address these factors in detail, which could affect resulting toughness values if not fully understood by the laboratory carrying out the test procedure. This article identifies some of these factors and discusses how they may influence results on ferritic steels. In particular, test temperatures within the ductileto-brittle temperature transition regime and test specimens intended to measure the toughness of weld metal or heat-affected zone regions are considered. Fracture toughness facts Fracture toughness is a material property that characterizes the material's resistance to crack propagation when under load or stress. In more precise terms, it refers to the resistance of a preexisting crack to extend either under unstable (i.e., brittle fracture) or by stable tearing means (i.e., ductile fracture). Experimental methods for characterizing fracture toughness play a critical role in applying fracture mechanics to integrity assessment, fitness-for-service evaluation, and limit state analyses for a wide variety of engineering structures. Fracture toughness properties are frequently used as a basis for material selection, material qualification programs, and quality assurance for critical structures such as high-pressure gas and liquid transmission pipelines, pressure vessels, nuclear reactor components, petrochemical processing vessels, and aircraft. Fracture toughness test methods were first used in the 1940s and 50s with the development of purely linear-elastic test methods that produced toughness parameters such as KIC and JIC. These methods were limited to situations where fracture occurred by unstable crack extension and were not suitable for more ductile materials. As steel quality improved in the fol- Nomenclature a Crack length B Specimen thickness W Specimen width a/W Ratio of crack length to the specimen width (B2B) CTOD specimen with rectangular cross section (BB) CTOD specimen with square cross section t Thickness of the material specimen K Stress Intensity Factor, fracture toughness of the material y Yield strength of the material CTOD Crack-tip opening displacement HAZ Heat-affected zone EDM Electro-discharge machine HSLA High-strength low alloy SENT Single edge notch in tension SENB Single edge notch in bending C-T Compact tension LBZ Local brittle zone CGHAZ Coarse-grain HAZ IRCG Intercritically reheated CGHAZ SRCG Subcritically reheated CGHAZ FGHAZ Fine-grain HAZ ICHAZ Intercritical HAZ SCHAZ Subcritical HAZ lowing decades, these linear-elastic test methods were shown to be too conservative with steels commonly in use by the 1970s due to their inability to characterize toughness when brittle fracture was not the primary failure mode. This led to development of elastic-plastic toughness test methods such as crack-tip opening displacement (CTOD) and J-integral methods, which characterize fracture toughness when ductile behavior is present. During the 70s and 80s, ASTM and BSI developed detailed fracture toughness test methods that were widely adopted in many international codes and standards. Common fracture toughness test standards used today include ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 13

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