Issue 28

D. Gentile et alii, Frattura ed Integrità Strutturale, 28 (2014) 42-50; DOI: 10.3221/IGF-ESIS.28.05 43 always assumed to be placed symmetrically with respect to the bending plane because the maximum crack opening and COA occur for this configuration [5]. In the literature, based on finite element analyses, several solutions for the maximum crack opening have been reported and the COA is calculated assuming an elliptical distribution of the COD along the crack length. An example of these solutions can be found in design code such asGE-EPRI [4, 6]. In real cases and formany reasons, such as fabrication imperfections, vibrations, load transient, etc., the crack can be off- centered and located in different positions around the pipe circumference. In these conditions, for leak rate and applied load identical to that of symmetrical centered crack, these solutions lead to: 1. a smaller crack opening area that drive to a larger detectable flaw size (detrimental effect). 2. a higher pipe capability to carrying load (beneficial effect). Therefore, finite element analysis becomesmandatory to determine the effective leak area for a generic off-centered crack configuration [7]. When the crack is off-centered with respect to the bending plane, the COA is reduced with respect to that of the reference centered configuration and the shape of theCODdistribution along the crack is no longer elliptical. Bonora [1] proposed theHodographConeMethod (HCM) inwhich theCOD distribution (shape and amplitude) and the leak area are predicted, as a function of the crack length and off-center angle, only by means of geometrical considerations. Later, Rahman et al. [8] performed further numerical investigations on off-centered crack configurations, andFirmature andRahman [9] investigated the behavior of off-center cracks in elastic-plastic regime. At the present, the HCM method is the only available solution that does not require extensive use of finite element simulation and allows quick estimation of the COD distribution and associated COA, given the crack length and the off- axis angle, and the maximum COD for the prescribed load in the centered crack configuration. For this reason, it is attractive for its potential use in design-by-analysis procedures. This model has been validated by means of finite element simulation on selected flawed pipe configurations [10]. More recently, experimental measurements of the COD profile for off-centered circumferential crack under bending have been presented [11]. In this work, an extensive experimental work, to investigate the evolution of COD profile with the off- center angle in different pipe configurations (diameter and thickness) under bending, was carried out. These results have been used toprovide a physical evidence for the qualificationof theHCM. M ATERIALANDMETHODS n this study, the pipe selected for the CODmeasurement were made of twomaterials: FE360 steel for the pipe 3.4 mm thick, and ST52 for 2mm and 5mm thick pipes. Threemean pipe radius/thickness ratios have been investigated. These ratios were selected to compare the results with the GE-EPRI solution which has the following limit 5<R/t<20. A summary of the pipe geometries, off axis angle and selected load is given inTab. 1. Test Configuration Mean radius [mm] t [mm] R m /t a/  off-axis angle  (°) LOAD [N] 2  50T2 29 2 14.5 0.139 0 30 60 500 2  90T2 29 2 14.5 0.25 0 30 60 500 2  90T3.4 28.3 3.4 8.32 0.25 0 30 60 500 2  50T5 27.5 5 5.5 0.139 0 30 60 1500 2  90T5 27.5 5 5.5 0.25 0 30 60 1500 Table 1 : Test configuration: three different R m /t and twodifferent crack length. Pipe used in this workwere obtained by cold drawing ensuring constant thickness and small ovalization. On each pipe the cracks wasmachined using a circular sawwhich generate a squared crack tipwith an initial opening of 1.0mm. No fatigue pre-cracking was performed to sharp the tip radius since the interest is in the COD distribution along the crack length. However, preliminary 3D FEM analysis was performed in order to verify if an initial blunting could influence the COD I