Fatigue crack propagation behaviour of pressurised elbow pipes under cyclic bending
Introduction
Currently, the common piping system of China's nuclear power plants is mainly composed of Z2CND18.12N austenitic stainless steels. The nuclear pipeline is served under high temperature and high pressure for a long time, and it is affected by cyclic mechanical load, cyclic thermal load, random load and transient impact. The defects such as inclusions, bubbles and discontinuities usually exist in the processing technology of nuclear pipeline system. Microcracks often emerge from these imperfections and gradually grow up and expand under these complex loads. The fatigue crack growth behaviour of pipeline is related to the safety assessment and life prediction of nuclear power system, which is worth studying.
So far, a lot of researches have focused on ratcheting behaviour of internal pressurised elbow and straight pipes under cyclic bending [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. However, it is also important to study the fatigue crack growth behaviour of nuclear power pipeline. Li et al. [11] proposed an analytical method to calculate the stress intensity factor of circumferential surface crack in steel pipes subjected to fatigue bending. It was shown that the stress intensity factor evaluated by this method matched well with the test results and this analysis method was beneficial to fatigue crack growth assessment and residual fatigue life prediction of cracked steel pipes. Arora et al. [12] studied the fatigue crack growth behaviour of austenitic stainless steel pipe welds. The fatigue crack growth rates at Weld Centre Line (WCL), Heat Affected Zone (HAZ), HAZ-Fusion Line (FL) interface and the parent material were compared. The results showed that the parent material presented superior resistance to fatigue crack growth. The influence of internal pressure, operation time and shape imperfection of the elbow on fatigue crack growth rate was investigated using FE-analysis and experiments at elevated temperature [13,14]. It was found that loading history had a significant effect on the fatigue crack growth rate. The fatigue crack growth rate was a function of the critical zone position of the elbow and loading history. The fatigue property of cracked aluminum-alloy pipe repaired by a shaped CFRP patch [15] and the fatigue crack propagation behaviours of aluminum pipe repaired by composite patch [16,17] were studied by experimental method and the extended finite element method. Fatigue crack growth behaviours of power plant piping materials under corrosive environment were studied by Vishnuvardhan et al. [18]. The results showed that the corrosion environment and the concentration of corrosion solution had a significant effect on the fatigue crack growth rate and fatigue life of the material. Arora et al. [19] studied the fatigue crack growth behaviour of austenitic stainless steel pipe and its weld by two different treatment methods of stress intensity factor, and predicted the fatigue crack growth life of the material. A 3D finite element model was developed to determine stress intensity factors of surface cracked girth welded pipes under membrane tension and bending [20]. The fatigue lives of the straight pipe and weld toe with surface crack were predicted and the reasonable results were obtained. Jeong et al. [21] gave an engineering evaluation method of J-integral and crack opening displacement (COD), and the plastic influence functions were proposed to solve the J-integral of the pipe with complex crack [22]. Vormwald et al. [23] and rabboliniet et al. [24] characterized the crack closure effect by comparing the difference between the global cyclic stress-strain curve and the local cyclic stress-strain curve of the specimen. They provided a method for calculating the effective value of J-integral range. A weight function was proposed for the pipe with high aspect ratio semi-elliptical crack to calculate the stress intensity factor at the deepest point of internal circumferential semi-elliptical crack [25]. The stress intensity factors of semi-elliptical surface cracks with large aspect ratios of pipes were calculated by finite element method [26]. Rahman et al. [27] considered the influence of seismic load on the fatigue crack growth behaviour of pipeline. The results showed that, for the low-cycle fatigue crack growth, the linear elastic fracture mechanics underestimated the ability of crack growth, thus the elastic-plastic analysis was needed. Taylor et al. [28] studied the fatigue crack propagation behaviours of API 5L X-70 steel pipe with the crack in different orientations of matrix and welding consumables. Fatigue crack growth rates of x100 steel welds in high pressure hydrogen gas considering residual stress effects were studied by Ronevich et al. [29]. It was emphasized that the residual stress should be eliminated when measuring the fatigue crack growth rate and the influence of residual stress should be considered when conducting the structural assessment.
Generally, for complex structures, the crack periphery is subject to stronger and more complex constraints and it is affected by the irregular shape of elastic-plastic boundary. Thus, the fatigue crack propagation behaviour of the structure is different from that of the material, and the fracture features of the structure cannot be expressed by the fracture mechanics properties of the material. Therefore, the significance of the fatigue crack propagation behaviour of the nuclear power pipeline system cannot be ignored. This study is devoted to the fatigue crack propagation behaviour of Z2CND18.12N austenitic stainless steel elbow pipe under constant internal pressure of 17.5 MPa and in-plane bending load (the crack at intrados: 10~20 kN, and the crack at crown: -10~-20 kN). The fatigue crack propagation tests of the cracks at intrados and crown of the elbow were designed under notch induction. The evolutions of crack depth, crack length, crack depth growth rate and crack length growth rate with the number of cycles were evaluated. The relation between crack depth and crack length and the macroscopic fracture surfaces of cracks were analyzed. Based on the experimental results, J-integrals of corresponding crack sizes were calculated according to Ramberg-Osgood equation by numerical method. The evolution laws of crack depth growth rate, with J-integral range, were evaluated.
Section snippets
Specimen and experiment details
The elbow pipe specimen used in this study is shown in Fig. 1(a). It consists of an elbow and two straight pipes. Its nominal cross-section specification is mm. Actually, the wall-thickness of the elbow is not uniform due to processing technology. In previous study [6], the wall-thickness distribution of this elbow has been characterized in detail. Fig. 1(a) shows the true wall-thickness distribution of the elbow in the middle cross-section. During the experiment, the load was applied
Experimental results and discussion
The fatigue crack propagation tests of the longitudinal crack at intrados and crown continued until the crack penetrated the inner surface of the elbow, and the hydraulic oil leaked out from the notch. Fig. 5(a) and (b) show the ultimate test stages of the crack at intrados and crown, respectively. During the experiment, the crack depth is not measurable, but the crack length can be observed, as shown in Fig. 5. Through the observation of the crack length, the crack does not extend under low
Numerical process and results
In order to explain the crack growth law and evaluate the fracture characteristics of the elbow pipes, the stress analysis and fracture toughness were evaluated by finite element method. The large-scale finite element software ANSYS 15.0 was used [30].
Conclusions
In this study, the fatigue crack propagation behaviours of Z2CND18.12N austenitic stainless steel elbow pipe were investigated experimentally. Based on the test results, the stress analysis and J-integrals of the elbow were numerically evaluated by finite element method. The conclusions are as follows:
- (1)
Both the cracks at intrados and crown, the evolutions of crack depth with the number of cycles present logarithmic growth. The evolutions of crack length with the number of cycles present
CRediT authorship contribution statement
Caiming Liu: Writing - original draft, Data curation, Software. Bingbing Li: Investigation. Yebin Cai: Validation. Xu Chen: Conceptualization, Writing - review & editing, Supervision.
Declaration of competing interest
None.
Acknowledgment
The authors gratefully acknowledge the National Key Research and Development Program of China (No. 2018YFC0808600).
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