Fracture Mechanics Tesing of Weldments – Through Thickness vs. Surface Notch

Christian Thaulow

Norwegian University of Science and Technology, Dept Engineering Design and Materials, Trondheim, Norway

Erling Østby, Hans Iver Lange, Synnøve Åldstedt and Odd Magne Akselsen SINTEF Materials and Chemistry, Trondheim, Norway

ABSTRACT The requirement to structural steels increases as the oil and gas industry goes north. It can be challenging to achieve high fracture toughness in the Heat Affected Zone and the weld metal at very low temperatures, and the present practice with lower bound fracture toughness testing is challenged. In this paper the effect of surface- vs. through thickness notching in fracture mechanics testing of the HAZ of weldments is examined experimentally and with FE (Finite Element) modeling. The standardized test specimen for lower bound fracture toughness testing is the deeply notched through thickness B*2B specimen. The B*B surface notch has, however, always been considered as an alternative to the through thickness notching as a more realistic and practical testing procedure. Both elder and new experiments revealed that the through thickness notched specimens have lower toughness than the surface notched, with a shift in transition temperature up to 50°C. KEY WORDS: Fracture toughness; steel; weldments; testing; surface notch; through thickness notch INTRODUCTION In fracture mechanics testing of weldments, low fracture toughness is frequently experienced in the HAZ. The low toughness depends upon the detailed interaction between the crack tip and the embrittled microstructural constituents in the HAZ. It has further become evident that the large scatter often observed in the fracture mechanics testing of weldments not only is due to the distribution and size of these zones with embrittled microstructure, but also largely depends upon the global and local distribution of mechanical properties and specimen geometry , Toyoda M et al (1994) and Thaulow C et al (1994). The standardized test specimen for lower bound fracture mechanics testing is the deeply notched B*2B geometry, Figure 1. This specimen provides the highest constraint and will also have a high probability of hitting a brittle area because the through thickness notch (TTN) will intersect the HAZ of several weld layers, API (2005), British Standard

(2005).

Figure 1 Notch orientation in weldment testing. Through Thickness

Notch (left) and Surface Notch (right). The position of Local Brittle Zones (LBZ) close to the fusion line is indicated.

But in practice there will seldom be through thickness cracks, and the fusion line will usually be slant with respect to the specimen surface. A surface notched specimen (SN) could therefore be a more realistic geometry with respect to real cracks in structures, and the question is if the TTN specimen introduces an unrealistic degree of conservatism. A large fracture mechanics testing programme was performed in the nineties to examine the effect of notch positioning, the results, however were not published to a wider audience , Norwegian –Japanese Research Programme (1990-94). But with the renewed interest in understanding low temperature fracture toughness of steels in conjunction with the oil and gas exploration and production in the arctic region, we now revisit the results and compare them with resent test results from steels developed for application under arctic conditions. C-Mn steel base materials usually exhibit acceptable toughness even down to -60°C, due to the careful manufacturing processes and small

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Proceedings of the Twenty-first (2011) International Offshore and Polar Engineering ConferenceMaui, Hawaii, USA, June 19-24, 2011Copyright © 2011 by the International Society of Offshore and Polar Engineers (ISOPE)ISBN 978-1-880653-96-8 (Set); ISSN 1098-6189 (Set); www.isope.org