Zhuojin Pipe Fitting Co., LtdConstitutive Relationships and Fracture Criteria of X90 Ultra-High Strength Gas Transmission Steel Pipe
Literature Overview
This paper by Yang Fengping et al. (2017), published in Acta Petrolei Sinica, presents a comprehensive study on the material constitutive relationships and fracture criteria of X90 ultra-high strength gas transmission steel pipe. The research was conducted at the State Key Laboratory of Pipeline Materials and Equipment Materials Service Behavior and Structural Safety, affiliated with the CNPC Petroleum Tube Engineering Technology Research Institute, and was supported by multiple funding sources including the National Natural Science Foundation (No. 51404294), Shaanxi Provincial Natural Science Foundation (2014JQ2-1004), and a CNPC major science and technology project (2012E-2801).
Material Characterization and Constitutive Modeling
The study conducted quasi-static tensile tests on five different round bar notch specimens to determine the stress triaxiality and its effect on fracture behavior. The key finding was that the presence of notches increased the stress triaxiality by 2.43 times, reduced the fracture strain by 29%, and decreased the damage strain energy by 71%. These results highlight the significant influence of stress state on the ductility and fracture resistance of X90 pipeline steel.
| Test Condition | Stress Triaxiality | Fracture Strain | Damage Strain Energy |
|---|---|---|---|
| Smooth specimen (baseline) | 1.00 (reference) | Baseline value | Baseline value |
| Notched specimen (5 types) | 2.43× baseline | -29% | -71% |
| Quasi-static loading | Moderate | Higher ductility | Higher energy absorption |
| High strain rate (Hopkinson bar) | Moderate | ~10% variation | Comparable to quasi-static |
The strain rate sensitivity of X90 steel was found to be relatively low, with a maximum difference of approximately 10% between quasi-static and high strain rate conditions. This finding is important for the design of pipeline systems that may experience impact loading or rapid pressure transients.
Fracture Criteria Development
The authors developed two fracture criteria based on damage mechanics theory:
- Plastic Uniform Extension Strain (PUE) based criterion: This criterion relates the critical plastic strain to the stress triaxiality and strain rate, providing a practical tool for fracture assessment under complex stress states.
- Damage Strain Energy based criterion: This criterion uses the critical damage strain energy density as the fracture criterion, which accounts for the combined effects of stress triaxiality, strain rate, and specimen geometry.
The fracture characteristic length was derived based on the assumption of constant critical material damage strain energy density, establishing a relationship between the fracture characteristic length, stress triaxiality, and specimen diameter. This relationship is particularly useful for scaling laboratory test results to full-scale pipeline components.
Constitutive Model Parameters
| Parameter | Symbol | Typical Value for X90 Steel |
|---|---|---|
| Yield Strength | σ_y | 620-690 MPa |
| Tensile Strength | σ_uts | 720-800 MPa |
| Strain Hardening Exponent | n | 0.20-0.25 |
| Strain Rate Sensitivity | m | 0.005-0.015 |
| Damage Evolution Parameter | A | Material specific |
| Fracture Strain at σ* = 1.0 | ε_f | 0.15-0.20 |
| Fracture Strain at σ* = 2.43 | ε_f | 0.10-0.14 |
The Johnson-Cook constitutive model was employed to describe the strain rate-dependent plastic behavior of X90 steel, while a modified Johnson-Cook failure model incorporating stress triaxiality was developed for fracture prediction. These models provide a robust framework for finite element analysis of pipeline components under complex loading conditions.
Engineering Practice and Welding Implications
X90 pipeline steel is widely used in high-pressure gas transmission systems, where the operating pressure can reach 12 MPa or higher. The welding of X90 steel pipes requires careful attention to the heat-affected zone (HAZ) properties, as the high strength of the base metal makes it susceptible to HAZ cracking and reduced toughness.
Key welding considerations for X90 pipeline steel include:
- Preheat temperature of 100-150°C to prevent cold cracking
- Interpass temperature control below 250°C to avoid excessive grain growth in the HAZ
- Use of low-hydrogen filler metals (e.g., E71T8, ER80S-D2) with hydrogen content below 1.5 mL/100g
- Post-weld heat treatment (PWHT) at 550-650°C for stress relief and HAZ softening mitigation
- Charpy V-notch impact testing at the lowest design temperature (typically -20°C to -40°C)
- Hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) resistance testing per NACE MR0175/ISO 15156
The fracture criteria developed in this study can be directly applied to the assessment of welded joints in X90 pipelines. The stress triaxiality effects are particularly relevant at weld toe locations, where the geometric discontinuity creates high stress concentrations and elevated stress triaxiality. The damage strain energy criterion provides a more comprehensive fracture assessment than simple stress-based criteria, as it accounts for both the stress magnitude and the strain state.
Key Questions and Reflections
The study addresses several critical aspects of X90 steel behavior that are essential for pipeline engineering. The finding that stress triaxiality significantly reduces fracture strain and damage strain energy has direct implications for the design of pipeline components with geometric discontinuities, such as welds, fittings, and pipe terminations. The relatively low strain rate sensitivity is reassuring for pipeline design, as it suggests that impact loading will not drastically reduce the fracture resistance of X90 steel.
However, the study primarily focuses on the base metal properties, and the effect of welding on the fracture behavior remains an important area for further investigation. The HAZ of X90 welds typically exhibits reduced toughness and altered microstructure, which may significantly affect the local fracture resistance. Future research should extend the fracture criteria to welded joints, incorporating the heterogeneity of the weld metal, HAZ, and base metal regions.
Study Insights
This paper provides a rigorous framework for the constitutive modeling and fracture assessment of X90 ultra-high strength pipeline steel. The development of stress triaxiality-dependent and strain rate-dependent fracture criteria represents a significant advancement in pipeline engineering practice. For engineers involved in the design, fabrication, and inspection of X90 pipelines, the findings emphasize the importance of considering the stress state in fracture assessments and provide practical tools for predicting the fracture behavior of pipeline components under complex loading conditions. The fracture characteristic length relationship offers a valuable scaling tool for translating laboratory results to full-scale pipeline applications, bridging the gap between material characterization and structural assessment.