Study On Creep Crack-tip Constraint Effect Of Semi-elliptic Surface Cracks In Pipelines

High constraint CT specimen is generally be used to measure the creep crack growth rate (CCG). The high constraint CT specimen data using standard test procedures are conventionally used in creep crack growth assessments of pipeline surface crack with low constraint. It is necessary to investigate and characterize the creep constraint of pipelines with semi-elliptic surface cracks. However, this study is very limited. In this study,the semi-elliptic surface crack-tip constraint of high temperature pipeline was investigated, and the influences of crack sizes, loading modes, load levels, crack direction and creep time on the crack-tip constraint. The main results obtained are as follows:(1) Constraint of semi-elliptic crack front increases with crack depth and length. Constraint parameter R and R*distributions along semi-elliptic are related with the ratio a/c. When a/c is close to1, the peak of constraint occurs near surface point; when a/c is small, the peak of constraint occurs at the deepest point.(2) Constraint R and R*change with time when creep begins (t/trea<0.2). Then constraint does not change until steady creep. So the creep constraint parameter of steady creep can describe the crack constraint during whole creep process.(3) Compared with tensile loading, the bending loading causes higher crack-tip constraint. Constraint parameter R*is load independent for the cracks in pipes. It is further evidence that R*is loading independent parameter. The distributions of R*along circumferential crack front is the same as those along axial crack front and is affected by ratio a/c.(4) Based on the two parameter C*-R*, creep crack growth was simulated. The results show that the creep crack front can’t keep initial elliptic during its growth. Constraint parameter R*and C*have higher values atthe region of crack front with about0=30°and this region has the fastest creep crack growth rate. Constraint R*, C*and creep crack growth rate increase during creep crack growth. Crack growth rate of crack front is lower than that of CT specimen. It is conservative if the CCG data from CT specimen were used in the creep life assessment.