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RESEARCH PAPERS

# Significance of Fracture Toughness Test Results of Beam Welds in Evaluation of Brittle Fracture Performance of Girth Welded Pipe Joints

[+] Author and Article Information
Mitsuru Ohata

Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka, 565-0871, Japanohata@mapse.eng.osaka-u.ac.jp

Masao Toyoda

Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka, 565-0871, Japantoyoda@mapse.eng.osaka-u.ac.jp

Steel Research Laboratory, JFE Steel Corporation, 1 Kokan-cho, Fukuyama, 721-8510, Hiroshima, Japannob-ishikawa@jfe-steel.co.jp

Toyohisa Shinmiya

Steel Research Laboratory, JFE Steel Corporation, 1 Kokan-cho, Fukuyama, 721-8510, Hiroshima, Japant-shinmiya@jfe-steel.co.jp

J. Pressure Vessel Technol 129(4), 609-618 (Feb 19, 2007) (10 pages) doi:10.1115/1.2767652 History: Received December 01, 2005; Revised February 19, 2007

## Abstract

High power beam welds, such as electron beam welds or laser welds, sometimes provide fracture path deviation (FPD) in standardized Charpy V-notch fracture toughness testing due to narrow bead profile together with higher overmatching in strength between weld metal and base metal. Moreover, it should be noted that these typical features of beam welds might result in a plastic constraint loss around both the notch and crack tip in fracture toughness test specimens. Even in the temperature range where FPD would not occur, the fracture toughness test results could not necessarily be an intrinsic value of such beam welds. These fracture properties make it difficult to evaluate fracture performance of girth welded pipe joints. In this paper, the estimation method of intrinsic fracture toughness of beam weld metal itself using standard toughness test specimens is proposed on the basis of “Weibull stress criterion.” The predicted intrinsic fracture toughness was found to be lower than the test results both in standard Charpy specimen and in three-point bend specimen with fatigue precrack. The assessment of brittle fracture performance of girth welded pipeline was conducted from the estimated intrinsic fracture toughness of girth welds by means of Weibull stress criterion. It was demonstrated that the low intrinsic fracture toughness of beam welds could not directly lead to the low fracture performance of a pipe joint under tensile loading. This is because of a lower plastic constraint compared to a three-point bend specimen due to difference in loading mode together with constraint loss in pipe joints and shielding effect of straining in weld metal due to highly overmatched narrow welds.

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## Figures

Figure 1

Cross sections for all beam welds

Figure 2

Features of all beam welds

Figure 3

Standard Charpy test results for beam welded specimens with notch in weld metal center: (a) for 50L welds, (b) for 65L welds, and (c) for 80L welds

Figure 4

Classification of fracture transition behavior in Charpy testing of beam welds by combination of 2H and Sr

Figure 5

Three-point bend test results for beam welded specimens with fatigue precrack in weld metal center: (a) for 50L welds, (b) for 65L welds, and (c) for 80L welds

Figure 6

Joint and all WM FE models for Charpy and three-point bend specimens

Figure 7

Comparison between equivalent plastic strain distributions around notch tip in joint and all WM models of Charpy specimens: 50L and 65L welds at −20°C

Figure 8

Comparison between maximum principal stress ahead of notch tip in joint and all WM models of Charpy specimens: 65L welds

Figure 9

Comparison between equivalent plastic strain distribution around crack tip in joint and all WM models of three-point bend specimens: 65L at −20°C

Figure 10

Comparison between maximum principal stress ahead of crack tip in joint and all WM models of three-point bend specimens: 65L welds

Figure 11

Critical Weibull stress distribution for weld metal of 65L welds, m=20.2

Figure 12

Calculated Weibull stress at various temperatures for joint models of Charpy specimens as a function of plastic strain energy

Figure 13

Predicted critical energy for Charpy specimens of 65L welds based on Weibull stress criterion compared with experimental results

Figure 14

Comparison between Weibull stress for joint and all WM models of 65L welds

Figure 15

Predicted intrinsic fracture toughness of weld metal in 65L welds compared to experimental results: (a) for Charpy specimens and (b) for three-point bend specimens

Figure 16

Configuration of girth welded pipe with surface crack in beam weld metal used for FE analysis

Figure 17

One example of mesh division for pipe with surface crack

Figure 18

Comparison between Weibull stress for joint and all WM models of 65L pipe, together with that for All WM model of three-point bend specimen

Figure 19

Plastic constraint loss in pipe joints due to heterogeneity in beam welds, and comparison with that in three-point bend specimen

Figure 20

Procedure for transferring the intrinsic fracture toughness to critical CTOD for pipe joints in consideration of plastic constraint loss on the basis of Weibull stress criterion

Figure 21

Estimated critical CTOD at −20°C for pipe joints with different size of crack

Figure 22

Effect of heterogeneity in beam welds on CTOD design curves for pipe joints, and on effect of crack size

Figure 23

Estimated pipe performance in terms of critical overall strain from intrinsic fracture toughness of beam weld metal

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