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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SENT versus SENB specimen geometry There is an increasing trend, particularly in the pipeline industry, to use SENT rather than SENB specimens. This trend was implemented to support strain-based design principles for pipeline systems that are desired to tolerate plastic strains in service. Under these high-strain conditions, the pipeline, and more specifically girth welds, are primarily subjected to tensile loads. Crack-tip constraint conditions and applied loading orientation in the SENT specimen more closely replicate the stress conditions in girth welds, as loading is primarily in pure tension. SENB specimens have a higher level of constraint than SENT specimens of the same thickness. Consequently, SENT specimens provide a more representative measure of fracture toughness than the highly constrained SENB specimen geometry in this situation. Figure 8 illustrates standard SENT geometry where B corresponds to material thickness. While the use of SENT specimens has increased significantly over the last few years, limited guidance exists regarding appropriate specimen preparation and data analysis methods. This has led to a scenario where results from different labs can be difficult to compare, and increases the possibility of incorrect laboratory practices that negatively influence the accuracy of measured toughness values. One limitation is the lack of guidance on calculating toughness either in terms of CTOD or J for the SENT geometry. Standard equations for either approach have not been broadly adopted by any major standardizing body. In addition, guidance on using fixed-end or clamped speci- (B×B) (B×2B) Fracture toughness etry were associated with LBZs associated with increased grain size in the HAZ of the weld located at the precrack tip. However, the effect of additional constraint created by both the (B2B) specimen geometry and the effect of weld metal mismatch on the lower bound results was not investigated. Based on the higher constraint in the (B2B) specimen geometry, lower CTOD results could be associated with a combination of higher stress from the increased constraint and presence of LBZs at the crack tip. Based on these investigations, the constraint in the (B2B) geometry is slightly higher than the (BB) geometry. The effect of the aspect ratio on toughness is minimal except in the transition regime area. The increased constraint may increase the probability of a LBZ fracturing, thereby resulting in a lower toughness result in a (B2B) geometry compared to a (BB) geometry. Figure 7 illustrates the effect of the (B2B) geometry compared to (BB) geometry in the transition regime. As stated previously, the specimen aspect ratio to be used to measure the material's toughness properties should be based on the type and orientation of the cracks that are expected to occur in service. In addition, the use of (BB) specimens on heavy sections (i.e., thicker than 2 in.) will have less effect on measured toughness compared to specimens extracted from thinner material. Test temperature Fig. 7 — Effect of specimen aspect ratio on fracture toughness behavior[9, 10, and 15]. P B Gripped area Daylight between grips, H Fig. 8— Standard single edge notched tension (SENT) geometry. W a Gripped area P mens versus pin-loaded specimens, which are free to rotate, has not been fully developed. This difference regarding in-plane rotation may have a significant impact on the actual toughness value measured. Further, the rigidity of the test frame may potentially affect the specimen's inplane rotation. Another issue is the loading orientation for SENT tests. In SENB geometry tests, loading is in three-point bending, which produces a high level of constraint at the specimen crack tip for standard specimens notched to a depth of a/W = 0.5 (i.e., notch depth is equal to half the specimen width). However, under tension loading in the SENT geometry, constraint is lower for the same test specimen thickness. Consequently, when given the choice between using either SENB or SENT specimen geometries, the SENT geometry will likely result in a higher level of toughness for specimens of similar size and tested at the same temperature. Therefore, it is critical to understand these differences and select the specimen geometry that most closely matches the anticipated loading conditions in service to ensure representative toughness properties are measured. The only published standard for SENT testing is DNVRP-F108[16], which is essentially based on procedures used ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 17

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