PSI - Issue 5

Yoichi Kayamori et al. / Procedia Structural Integrity 5 (2017) 279–285 Yoichi Kayamori et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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1. Introduction

An elastic-plastic fracture mechanics parameter, CTOD, has been widely used for the evaluation of fracture toughness and structural integrity, where CTOD fracture toughness is calculated according to CTOD testing standards. Some conventional CTOD testing standards such as BS7448 (1991) and ISO 12135 (2008) employ the plastic hinge model for estimating the plastic part of CTOD, but ASTM E1290 (2002) adopted a different method of calculating the plastic CTOD, namely the J -conversion CTOD. After the advent of the J -conversion CTOD, the fracture toughness evaluation committee of JWES precisely measured and analyzed the shape of deformed crack tips, and proposed a new CTOD calculation formula of standard single edge notch bend (SE(B)) specimens (2014), where CTOD is affected by the size and Y/T of the specimens. This new CTOD formula has been utilized in WES 1108 (2016). On the other hand, the CTOD design curve approach is used for the assessment of flaws in welded structures, where a fracture driving force of CTOD in a welded joint is not measured directly, but is estimated by means of a CTOD design curve. A Fitness-For-Service procedure prescribed by JWES, WES 2805, has been adopted the CTOD design curve approach since 1980, and the latest issue (2011) produced a CTOD design curve, which is composed of a couple of equations for elastic and plastic local strains. In the equations, the effect of welded joint size on CTOD is considered in the calculation of the effective crack size parameter, c , and the local strain acting on the assumed crack,  . However, the effect of Y/T on CTOD is not considered directly in both CTOD design curve equations. As a result, there is a mismatch in CTOD calculation between the fracture toughness of SE(B) specimens and the fracture driving force of welded joints. This could lead to confusion for structural integrity. Under this circumstance, the formulation of an improved CTOD design curve has been required for considering the effect of Y/T on CTOD in welded joints. Firstly, conventional CTOD design curves in JWES were reviewed. Secondly, a new CTOD design curve was formulated by using several fracture mechanics equations. Finally, CTOD estimated by the new design curve was compared with CTOD obtained by finite element analysis for a corner boxing fillet welded joint in this study.

Nomenclature a

crack depth

effective crack size parameter

c

Young’s modulus

E F G K m J

shape parameter for stress intensity factor

energy release rate

J -integral

stress intensity factor

dimensionless constraint factor

Y/T

yield to tensile ratio

crack tip opening displacement local strain acting on an assumed crack

     y    y

yield strain

stress corresponding to boundary force

yield stress

2. CTOD design curves

2.1. Conventional CTOD design curves in JWES

The CTOD design curve approach is applied to the evaluation of brittle fracture initiation from a crack or a crack like planar flaw in the plastically deformed and highly stressed region surrounded by the elastic stress field, where CTOD is estimated by using a CTOD design curve derived from the relation between  and CTOD. The first form of the JWES CTOD design curve was given by the following equation, where  is the CTOD: