13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Draft: SI8-043 Fatigue Behavior Investigation and Micro-Analysis of X80 High-Strain Line- pipe Yang Li1,2 *, Weiwei Zhang2, He Li2, Likang Ji2, Chunyong Huo2, Helin Li2,3 1 School of material science and engineering, University of Science and Technology of Beijing, Beijing 100008, China 2Tubular Goods Research Institute of CNPC, Xi’an 710065, China 3School of material science and engineering, Xi’an Jiaotong University, Xi’an 710049, China * Corresponding author: liyang011@cnpc.com.cn Abstract Symmetrical push-pull low circle fatigue tests were carried out with conventional and high-strain X80 linepipe material respectively. Micro-analysis was performed for deformation mechanism as well. Results showed that high-strain X80 linepipe material exhibited a lower cyclic softening rate and a longer low-circle fatigue life than those of conventional X80 linepipe material. Cyclic response curves showed that the cyclic softening occurred at all strains amplitude (0.4%~1.4%) on two X80 line pipe materials. For conventional X80 linepipe material, cyclic softening was found after slight cyclic stress saturation at high strain amplitude of 1.0% ~1.4%. however, for high-strain X80 linepipe material, cyclic softening was observed after slight cyclic hardening at high strain amplitude of 1.0% ~1.4%. Fractography analysis suggested that transgranular fracture with well-developed fatigue striations and obvious second crack is the predominant failure mode. The amount of second crack was fewer detected in high-strain X80 linepipe material than that in conventional X80 linepipe material. TEM examination reveals that the primary deformation mode of two kinds of material was dislocation slipping. Abundant dislocation glide bands and dislocation cells were formed at grain boundary as strain amplitude increased. The number of dislocation blocked and wall thickness of dislocation cells in conventional X80 linepipe material are much greater than that in high-strain X80 linepipe material. The better cyclic deformation resistance of high-strain X80 linepipe material was ascribed to relative more M/A islands distributed inside the grains or grain boundaries. Keywords High-strain line pipe; Low-circle fatigue; Strain amplitude; Dislocation; M/A 1. Introduction Fatigue is a common failure mode of oil & gas pipeline. Fatigue fracture of pipeline is mainly caused by various alternating stress, which derives from pressure oscillation, layering of gas medium and all sorts of external variable load, such as vibration led by vehicle on the buried pipeline, fluvial abrasion of river, quicksand migration in desert and waves wash in offshore pipeline [1, 2]. Alternating stress can lead to local and alternative plastic deformation where stress concentration occurs, then fatigue failure of pipeline. In addition, since a lot of long distance, high pressure and large throughput pipeline set up in the harsh geological area, such as landslide, permafrost and seismic, buckling always caused by serious local plastic deformation of pipeline by bending, axial compression and axial tension stress. However, fracture can not be accomplished only once, low circle fatigue always becomes the final reason for the fracture of pipeline [3, 4]. Therefore, investigation on fatigue behavior of pipeline, especially low circle fatigue behavior is very significant for providing the theoretical suggestion for design, material selection and lifetime prediction of pipeline operating in the harsh condition. In this paper, the difference of low-circle fatigue performance between two X80 linepipe materials has been discovered by sub-structure evolution observation as well. 2. Experimental The material of X80 conventional and high-strain line pipe used in this paper is obtained by TMCP and UOE forming technology. The microstructure of both X80 linepipe material mainly consists of granular bainite, polygonal ferrite and M/A as show in figure 1. While comparing the detailed
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