关键词: 热力管道, 应力集中, 土壤参数, 腐蚀疲劳, 疲劳裂纹萌生寿命

Abstract:

To address the challenge of accurately predicting the fatigue crack initiation life caused by surface pitting corrosion in buried thermal pipelines during design, operation, and maintenance, this study employed the finite element method to investigate the influence of pit morphology, defect interaction, and axial loading on the maximum stress concentration factor K_t. An empirical formula for K_t was proposed, and a prediction method for fatigue crack initiation life under soil corrosion was developed. A pipeline in service in Beijing was used as a case study to verify the scientificity and effectiveness of this method. The results indicate that a 10-fold increase in pit depth leads to a 2.84-fold increase in K_t, while a 10-fold reduction in pit circumferential width results in a 4.75-fold increase. Deep and narrow defects characterized by a/c > 0.6 and b/c < 0.6 exert a stronger effect on increasing K_t and significantly shorten crack initiation life. When the defect spacing d=0, K_t reaches 1.03 times that of a single defect. The smaller the defect spacing, the stronger the interaction effects and the lower the crack initiation life. As defect spacing decreases, the fatigue crack initiation life of deep narrow pits is reduced to 0.12 times the original life, whereas shallow wide pits are more sensitive to spacing, with their crack initiation life reduced to 0.83 times the original life. Under conditions of low soil resistivity, low pH, and elevated temperature, crack initiation may occur within 20 years. The crack initiation life of shallow wide pits is more sensitive to soil parameters. Compared with internal pressure loading alone, an axial compressive load of 20 MPa significantly reduces crack initiation life by 0.74 times. Increasing axial tensile load from 20 MPa to 50 MPa results in a further life reduction of 0.82 times.

Key words: thermal pipelines, stress concentration, soil parameters, corrosion fatigue, fatigue crack initiation life