International Journal of Pressure Vessels and Piping 84 (2007) 244–255 Failure assessment of cracked square hollow section T-joints Z.M. Yang à , S.T. Lie, W.M. Gho Maritime Research Centre, School of Civil and Environmental Engineering, Nanyang Technological University, 50, Nanyang Avenue, Singapore 639798, Singapore Received 23 February 2006; received in revised form 19 September 2006; accepted 19 September 2006 Abstract Three full-scale static strength tests were carried out on pre-cracked square hollow section (SHS) T-joints. In accordance with the experimental results, an accurate crack model for welded SHS joints is proposed in this paper. Based on this numerical model, the load–displacement curves and the stress intensity factors (SIFs) along the crack front are calculated. It is found that the SIF varies greatly along the crack front, and the highest value is located at the brace corner. From Charpy V-notch impact test results, anisotropic fracture toughness was found, and influenced the failure behavior of the damaged joints. Ductile tearing was found to initiate from the crack front parallel to the chord side wall where the fracture toughness was smaller, and not from the crack front at the corner where the SIF value was the highest. Using the plastic collapse load obtained via the twice elastic compliance technique and the fracture toughness obtained from the Crack tip opening displacement (CTOD) tests, the BS7910 Level 2A Failure Assessment Diagrams (FAD) for the three cracked joints are plotted. It is confirmed that the standard BS7910 Level 2A curve gives a conservative assessment for cracked SHS T-joints under brace end axial loads. r 2006 Elsevier Ltd. All rights reserved. Keywords: Failure assessment diagrams; Fatigue crack; Fracture; Plastic collapse; Square hollow section T-joints 1. Introduction In service, the safety of any welded structure depends on periodic non-destructive inspection to detect cracks before they develop to a critical size, and hence to permit structural repair or replacement to be carried out before failure. To determine the critical crack size, the structure should be assessed according to knowledge of the service stresses and the fracture properties of the material. API RP579 [1], BSI Document BS7910 [2] and the R6 procedure [3] give guidance for assessing the acceptability of defects in welded structures based on the failure assessment diagram (FAD) method. The FAD method was originally derived from the two-criterion approach [4]. This approach stated that structures can fail by either of two mechanisms, brittle fracture or plastic collapse, and that these two mechanisms were connected by an interpolation curve based on the strip yield model [5]. If the service point falls inside the assessment curve, the structure is considered safe, otherwise, the structure is deemed unsafe. The use of the FAD for the assessment of flawed structure is shown in Fig. 1. This method enables the analyst to go directly from linear elastic fracture mechanics (LEFM) calculations to plastic instability calculations [6]. For different materials and geometries, the assessment curves are different. In the standard codes, lower-bound curves are used to assess all types of damaged structures. These codes are intended for general applications and do not necessarily give optimal solutions for all types of structures, such as welded tubular joints. Hence, the use of the standard curves should be validated for assessing the integrity of cracked tubular joints. Because of the complexity of the tubular joint geome- tries, the chord wall is in combined bending and tension near the brace–chord intersection where the crack is located. Therefore, the crack tip material behaves in a mixed mode of tension and shear, resulting in the crack growing in a non-planar surface, i.e. growing along a ARTICLE IN PRESS www.elsevier.com/locate/ijpvp 0308-0161/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2006.09.020 à Corresponding author. Tel.: +65 6790 6199; fax: +65 6790 6620. E-mail address: zmyang@ntu.edu.sg (Z.M. Yang).