Unified correlation of constraints with crack arrest toughness for high-grade pipeline steel

doi:10.1016/j.ijpvp.2021.104454

International Journal of Pressure Vessels and Piping

Volume 193, October 2021, 104454

https://doi.org/10.1016/j.ijpvp.2021.104454 Get rights and content

Highlights

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Sensitivity analysis for crack arrest toughness of different specimens to in-plane and out-of-plane constraints.
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A new unified parameter η for characterizing both in-plane and out-of-plane constraints is proposed.
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A linear relationship between the dimensionless fracture toughness CTOA_C/CTOA_ref and η^1/2 was found.

Abstract

There is much controversy surrounding using laboratory specimens to predict the fracture behavior of high-grade pipeline steel due to their different constraints. Since there is a lack of unified parameters to quantitatively characterize the constraint effects, a model for the transformation of crack arrest toughness between different specimens and pipes has not been formed. In order to solve this problem, the evolution law of the plastic zone of the crack tip with the crack tip opening degree under different in-plane and out-of-plane constraints is explored using the finite element simulation technology. On this basis, a constraint parameter η which is based on the area A_PEEQ surrounded by the isoline of equivalent plastic strain ε_p at the crack tip is proposed. The feasibility of this parameter to characterize both in-plane and out-of-plane constraints is discussed, and a quantitative correlation model between it and the crack arrest toughness, critical crack tip opening angle (CTOA_C), is established. The results show that the proposed parameter can be used to characterize the overall constraints caused by different specimens with different loading forms and geometries. The normalized arrest toughness, CTOA_C/CTOA_ref, has a linear relationship with η^1/2, and the correlation line is only related to the material. The quantitative correlation model has universal applicability for different specimens and is expected to predict the transformation of crack arrest toughness between laboratory specimens and pipelines.

Introduction

As the demand for natural gas continues to grow rapidly, the application of high-grade pipelines has become an inevitable trend during pipeline engineering development [1,2]. Although the high-grade pipeline has good toughness and high strength, ductile fracture incidents still occur endlessly, which can easily lead to serious safety problems and catastrophic consequences [3,4]. Therefore, effective control of pipeline ductile fractures is the core of safe natural gas transportation. Crack arrest toughness, as a measure of the ability of the pipeline to resist ductile crack propagation, should be carefully determined in the design of new gas pipelines.

The crack tip opening angle (CTOA), as a manifestation of local characteristics of the crack tip, can characterize the stress-strain state of the crack front well. It can meet the needs of crack arrest toughness prediction for high-grade pipeline steel and has a broad application prospect [5,6]. Design criteria based on CTOA are usually written as follows:CTOA_max < CTOA_Cwhere CTOA_max is the maximum crack driving force calculated according to the specimen size, material properties and pipeline operating conditions; CTOA_C is the resistance of material, and its experimental measurement is usually based on standard specimens in-plane strain states. However, a large number of experiments and theoretical analyses have shown that CTOA_C is highly related to the constraint and its value based on laboratory specimens cannot be directly used for actual pipelines [2,[7], [8], [9]].

The constraint, which is defined as the resistance of the structure to plastic deformation at the crack tip region, is a critical factor affecting the fracture mechanical behavior of materials. It is mainly determined by the specimen or structure size, crack depth, loading mode et al., and includes in-plane constraints and out-of-plane constraints, existing in real structures at the same time [10,11]. The current research mainly focuses on two aspects to realize the matching of constraints between laboratory specimens and pipelines, and transferring the value of CTOA_C between them accurately.

One is to design specimens with different geometries, sizes, and loading modes, so that they have similar constraint forms and plastic zones with pipelines as much as possible, such as Modified Double Cantilever Beam (MDCB) specimen [7] and Modified Compact Tension (MCT) specimen [12], etc. However, it is unclear whether their test results are consistent with the pipeline. Although some researchers believe that the CTOA obtained from full-thickness DWTT can be directly used in pipelines [13,14], Shibanuma et al. [9] found that the CTOA in a burst test is generally different from that in the steady-state region of DWTT. Xu et al. [15] believed that the above numerical difference is due to the ignorance of the tilt angle of pipeline deformation. Up to now, there is no unified conclusion. Thus, the transferability of CTOA test results between the laboratory specimens and the pipeline is still controversial.

The other research focus is developing quantitative characterization parameters for constraint effects. The parameters proposed earlier mainly represent in-plane (T [16,17], Q [18], A₂ [19]) or out-of-plane (T_Z [20]) constraints respectively. Unfortunately, the sensitivity of the above parameters to both constraints is different thus the overall constraint level of structural cracks cannot be well described. In order to investigate the synergistic effect of the two constraints on fracture behavior, Brocks et al. [21] proposed a three-dimensional stress parameter h which was further proved far from satisfactory. Based on this parameter, Han [8] proposed a crack tip opening displacement (CTOD) correction criterion considering the constraint effect caused by SENB specimen thickness while Zhen [2] established a similar failure criterion for CTOA. However, the parameter h cannot fully take into account the influence of in-plane and out-of-plane constraints. Mostafavi et al. [22] defined a constraint characterization parameter ϕ according to the area of the plastic zone at crack initiation to solve this problem. The connection between constraint and the size of the plastic zone has been proved by Lv et al. [23] and a mechanistic explanation has also been provided. Although the parameter can represent in-plane and out-of-plane constraints simultaneously, it works only under the circumstances of small-scale yielding. Yang et al. [10,[24], [25], [26], [27], [28]] modified this parameter by replacing the area of the plastic zone with the area enclosed by the equivalent plastic strain (ε_p) isoline in front of the crack tip and established a quantitative correlation between it and the fracture toughness J_C. However, the premise of determining the parameter is the accurate calculation of J_C, which is complicated especially for non-standard specimens. Furthermore, it is unknown whether the parameter can reflect the influence of in-plane and out-of-plane constraints on the crack arrest toughness of natural gas pipelines. The problem of accurate transfer of fracture arrest toughness between laboratory specimens and pipelines has not been solved.

In this paper, the critical crack tip opening angle (CTOA_C) was used to describe the crack arrest toughness of X80 pipeline steel. Based on the experimental calibration of material parameters, the specimens with different loading modes (SENB, CT, MDCB) were studied. Firstly, the distributions of the plastic zone around the crack tip of the above specimens under in-plane and out-of-plane constraints were investigated by numerical technology for crack simulation. Then, the area A_PEEQ surrounded by the isoline of equivalent plastic strain ε_p of the specimen corresponding to CTOA_C was proposed as a constraint parameter. The feasibility of it to characterize both in-plane and out-of-plane constraints and their interaction was discussed later. By describing the relationship between this constraint characterization parameter and crack arrest toughness CTOA_C, a quantitative correlation model was established and its wide application was verified finally.

Section snippets

Materials and specimens

X80 pipeline steel was used in this paper. With the aid of the universal material test machine, the tensile test and three-point bending test were conducted. At the same time, the Digital Image Correlation (DIC) method was used to record the whole deformation field. The engineering and true stress-strain curves are illustrated in Fig. 1 while the determined fundamental material parameters of X80 pipeline steel are listed in Table 1. Meanwhile, the load-displacement curve of the three-point

Numerical results and discussion

Typical simulation results of SENB, CT, and MDCB specimens are shown in Fig. 6. It can be seen that stress concentration exists at the crack tip and the loading position of each specimen, especially a large amount of plastic strain is accumulated at the crack tip.

According to the definition in Fig. 7, the values of CTOA during the whole loading process of different dynamic crack propagation models were taken and illustrated in Fig. 8, where d was set as 1 mm. For 3D FE models, CTOA is

Unified characterization parameter and its correlation with arrest toughness

SENB specimen with W = 32 mm, a₀/W = 0.5, B/W = 1/2 was set as the reference specimen. The different equivalent plastic strain distribution at CTOA = 10° and 20° obtained at the symmetrical plane along the thickness direction are shown in Fig. 10. The phenomenon of stress concentration around the crack tip and loading position is obvious. When CTOA is equal to 10°, the plastic region around the crack tip looks relatively small. When CTOA reaches 20°, the specimen yields completely, while the

Conclusions

In this paper, the quantitative correlation between the characterization parameter of specimens with different loading modes or geometric shapes and the corresponding crack arrest toughness CTOA_C was investigated. The conclusions are drawn as follows:

(1)
For SENB and CT specimens, the arrest toughness CTOA_C of X80 pipeline steel is not sensitive to the change of in-plane constraint but increases with the increase of out-of-plane constraint until the ratio of specimen thickness to width B/W reaches

Credit author statement

Zhen Ying: Methodology, Software, Writing-Reviewing and Editing, Validation. Cao Yuguang: Conceptualization, supervision. Jiao Zhenghao: Software, Validation, Editing. Niu Ruiyan: Methodology, Validation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This paper was funded by the Shandong Provincial Natural Science Foundation, China (ZR2020ME093), the National Key R&D Program of China (2016YFC0802306), the National Key R&D Program of China (2016YFC0802105), the Fundamental Research Funds for the Central Universities (18C×06036A), National Natural Science Foundation of China (11472309), Sichuan Science and Technology Program (2019YJ0400).