Fatigue crack growth rate of API X70 steel pipelines under spectrum loading S.M. Beden a, b, * , S. Abdullah a , A.K. Ariffin a a Department of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia b Department of Mechanical Engineering, Diyala University, Baquba, Iraq article info Article history: Received 20 December 2011 Received in revised form 20 February 2012 Accepted 7 March 2012 Keywords: Fatigue crack growth Modelling Pipeline Variable amplitude loading X70 steel abstract Pipelines offer the most efficient way to transport bulk quantities of gas and oil, either from points of production to storage locations or from storage locations to distributed points of end use. As one of the main materials of westeeast gas transmission pipes, X70 pipelines usually serve under variable ampli- tude loading (VAL). Base on the importance of in-service API X70 pipelines, it is important for the safe operation of this system to know its behaviour under VAL. This paper focuses on the ability of using the NASGRO model to predict the fatigue crack growth (FCG), based on investigation with the modified Wheeler model and experimental data. The results show that the NASGRO model give a fatigue life near by to that published in literatures and also showed the FCG rate response of X70 pipeline steels when exposed to VAL with different overload values. Extra modification to the NASGRO model may lead to better representing of FCG rate. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Most structural components such as pipeline systems are sub- jected to cyclic loading and fatigue fracture is the most common form of failure. In order to operate safe and reliable gas and oil pipeline networks, a better understanding of the mechanisms responsible for the FCG is required, and more empirical data for a range of pipeline steels and test parameters must be collected. Discussion of the mechanism(s) for FCG in pipeline steels is complicated by the fact that the mechanism for FCG under constant and variable amplitude loading in these materials is not completely understood [1e3]. Furthermore, our discussion will be primarily restricted to what is commonly called “Stage II” growth, which tends to be transgranular in the absence of environmental effects. A number of equations have been developed to describe the sigmoidal da/dNeDK relationship. Paris and Erdogan [4] were apparently the first to discover the power law relationship to describe the stable crack growth in that region. Many models have been developed to predict the fatigue life of components subjected to VAL [1e3,5]. The earliest of these are based on calculations of the yield zone size ahead of the crack tip and are still widely used. The Wheeler model [6] and Willenborg et al. model [7], for example, both fall into this category. Another category models based on the crack closure approach, which considers plastic deformation and crack face interaction in the wake of the crack, was subsequently proposed by Elber [8]. These facts have been used to model crack growth rates under variable amplitude loads [9e14]. Other models have been devel- oped to describe the fatigue crack growth behaviour after over- loading [15e22]. Elber [23] employed the crack closure concept to explain the crack growth retardation after overloading. More recent proposals include combinations of the Wheeler model with the Newman crack closure model [24] and model based on the strain energy density factor [25]. Based on details of plastic flow at the tip of a propagating fatigue crack, both Laird [26] and Pelloux [27] have proposed fatigue crack propagation mechanisms Laird’s model involves crack advance through local plastic flow during the crack blunting process, with sharpening and work hardening of the crack tip region during crack closure. Pelloux’s model is also based on details of plastic defor- mation at the tip of a propagating crack, but focuses on the irre- versibility of crystallographic slip at the crack tip. Tomkins [28] showed that an analysis of crack tip micro plasticity could be used to predict fatigue life. Despite the extensive work on crack growth, there is still a need for a satisfactory and generally applicable method to predict the fatigue crack propagation rate to consider various effects. In the current investigation, systematic crack growth modelling were conducted on 350 WT steel material that used by Yuen and Taheri [29] and X70 steel pipe material based on the NASGRO model. The present work aimed to improve the accuracy in * Corresponding author. Department of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia. Tel.: þ603 8921 6518; fax: þ603 8925 9659. E-mail address: sabah_19592007@yahoo.com (S.M. Beden). Contents lists available at SciVerse ScienceDirect International Journal of Pressure Vessels and Piping journal homepage: www.elsevier.com/locate/ijpvp 0308-0161/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2012.03.001 International Journal of Pressure Vessels and Piping 96-97 (2012) 7e12