Evaluation and interpretation of ductile crack extension in SENT specimens using unloading compliance technique M.A. Verstraete ⇑ , S. Hertelé, R.M. Denys, K. Van Minnebruggen, W. De Waele Laboratorium Soete, Ghent University, Faculty of Engineering and Architecture, Technologiepark Gebouw 903, 9052 Zwijnaarde, Belgium article info Article history: Received 22 April 2013 Received in revised form 18 September 2013 Accepted 1 November 2013 Available online 15 November 2013 Keywords: SENT Unloading compliance 3D FEA Negative crack growth abstract Resistance curves obtained from Single Edge Notch Tensile (SENT) specimens gain interest for defect assessment in pipelines. To determine these resistance curves, the unloading compliance technique is a commonly applied single specimen technique. A frequently reported problem is the occurrence of an initial decrease in the compliance, resulting in an apparent negative crack growth. To overcome this problem an offset in combination with a blunting correction is proposed. This approach is evaluated by finite element simu- lations. Then the proposed approach is validated experimentally. The presented approach is simple in use and provides an acceptable estimation error when it comes to constructing resistance curves. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Harsh environments such as regions prone to landslides or earthquakes, can impose large deformations to pipelines. In addition, the pipeline might as well be subject to large deformations during installation, as is the case for offshore reeling. Such loading conditions, also referred to as strain-based conditions [1,2], require an improved understanding of the struc- tural integrity in the presence of girth weld defects. Several strain-based defect assessment approaches are available now- adays [3–7]. Many of these procedures rely on the use of crack growth resistance curves obtained from low constraint clamped Single Edge Notch Tension (SENT – Fig. 1) specimens [8,9]. A resistance curve expresses the material’s fracture toughness as a function of the ductile crack extension. In essence, two approaches are available to construct a resistance curve, namely the multiple specimen technique and the single specimen technique. For the multiple specimen technique, prescribed by the DNV recommended practice DNV-RP- F108 [10], a set of SENT specimens (minimum six) is loaded to different load levels. Subsequently, these specimens are bro- ken up in liquid nitrogen and the ductile crack extension is measured. Although successful applications are described in lit- erature [11,12], such procedure involves high labor and material costs. On the other hand, the single specimen technique aims to monitor the defect size during the test. Since this approach is more practical it gains interest. Several defect size mea- surement techniques can be applied. The unloading compliance and direct current electric potential techniques are the most widely used [13–15]. This paper focuses on the use of the unloading compliance technique. This technique has been standardized for high con- straint Single Edge Notch Bending (SENB) specimens and Compact Tension (CT) specimens [14]. Furthermore, it has been successfully applied for Centre Cracked Tension (CCT) specimens [16,17]. Recently, a lot of work has been published on the application of the unloading compliance technique for SENT specimens [18–21]. 0013-7944/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfracmech.2013.11.004 ⇑ Corresponding author. E-mail address: Matthias.Verstraete@UGent.be (M.A. Verstraete). Engineering Fracture Mechanics 115 (2014) 190–203 Contents lists available at ScienceDirect Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech