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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for the SENB geometry with respect to crack tip validity criteria and the approach for calculating CTOD. Other approaches were developed by ExxonMobil and CANMET, but they each have differences with respect to approaches for preparing test specimens and carrying out testing. Depending on crack depth (a/W ratio), specimen size, and test temperature, these approaches may result in statistically different measured toughness properties. Many of the organizations that have developed their own procedure for conducting SENT toughness tests express interest in establishing an internationally recognized procedure that would provide consistent guidance on the methods to be used. Without this standardization, comparison of SENT data between laboratories, or even between different materials in some cases, is compromised. Conclusions Fracture toughness in ferritic steels can be influenced by multiple geometric and microstructural factors. It is well known that the material type, processing method, heat input from welding, test temperature, and strain rate can influence toughness results. Lesser known factors that influence toughness properties include: • Specimen size • Effect of local brittle zones on scatter in toughness results • Aspect ratio—(B2B) vs. (BB) • Specimen geometry (SENB vs. SENT) These factors can alter the constraint in samples, which may facilitate fracture initiation at LBZs sooner than the conditions of lower constraint. Many of these effects are more pronounced in the ductile-to-brittle temperature transition region when the likelihood of unstable fracture is present. In most cases, test specimen thickness should represent the material thickness to be used in service. Specimens thinner than the material to be used in service may result in a nonconservative measured fracture toughness due to loss of constraint. Awareness of the various microstructural zones in weld HAZs is critical to properly positioning specimen cracks in the desired region and for recognizing the causes of scatter in HAZ toughness properties. Finally, recognizing the differences between SENB and SENT geometries and their influence on measured toughness can assist in selecting the most representative geometry to replicate the expected loading conditions in service. For more information: Tom McGaughy is director of technology and technology leader, Structural Integrity & Materials Evaluation, EWI, 1250 Arthur Adams Dr., Columbus, OH 43221, 614/688-5054, tmcgaughy@ewi.org, www.ewi.org. References 1. ASTM E1290-08, Standard Test Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurements, ASTM, 2008. 2. BS 7448 Part I: Fracture Mechanics Toughness Tests, Part I, Method for Determination of KIc,, Critical CTOD and Critical J Values of Metallic Materials, The British Standards Institution, 1991. 3. BS 7448 Part 2: Fracture Mechanics Toughness Tests, Part I, Method for Determination of KIc, Critical CTOD and Critical J Values of Metallic Materials, The British Standards Institution, 1997. 4. ISO 12135, Metallic Materials – Unified Method of Test for the Determination of Quasistatic Fracture Toughness, 2002. 5. ISO 15653, Metallic Materials – Unified Method of Test for the Determination of Quasistatic Fracture Toughness of Welds, 2010. 6. Anderson, T.L., Effect of Crack-Tip Region Constraint on Fracture in the Ductile-to-Brittle Transition, U.S. Dept. of Commerce, National Bureau of Standards, 1984. 7. J.M. Barsom and S.T. Rolfe, Fracture and Fatigue Control in Structures Applications of Fracture Mechanics, West Conshohocken, Pa., ASTM, Woburn, Butterworth-Heinemann, 1999. 8. H.G. Pisarski, Influence of Thickness of Critical Crack Opening Displacement (COD) and J Values, Int. J. Fracture, Vol 17, No. 4, p 427-440, 1981. 9. M.G. Dawes, Elastic-Plastic Fracture Toughness Based on the COD and J-Contour Integral Concepts, Elastic-Plastic Fracture, ASTM STP 668, ASTM, Philadelphia, p 307-333, 1979. 10. ASTM E399-09, Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness KIc of Metallic Materials, ASTM, 2009. 11. M.G. Dawes, Presentation at the ASTM E-2408 Committee Meeting, Bal Harbour, Fla., 1980. 12. API Recommended Practice 2Z, Recommended Practice for Preproduction Qualification for Steel Plates for Offshore Structures, 4th ed., 2005. 13. W.A. Sorem, R.H. Dodds, and S.T. Rolfe, An Analytical and Experimental Comparison of Rectangular and Square Crack-Tip Opening Displacement Fracture Specimens of an A36 Steel, WRC Bulletin No. 328, 1987. 14. S. Machida, et al., Study of Method for CTOD Testing of Weldments, Fatigue and Fracture Testing of Weldments, ASTM STP 1058, 1990. 15. J.A. Smith and S.T. Rolfe, The Effect of Crack Depth to Specimen Width Ratio on the Elastic-Plastic Fracture Toughness of a High-Strength LowStrain Hardening Steel, WRC Bulletin No. 358, 1990. 16. Recommended Practice DNV-RP-F108, Fracture Control for Pipeline Installation Methods Introducing Cyclic Plastic Strain, 2006.

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