基于内聚区模型的连续油管韧性断裂研究

Study on ductile fracture of coiled tubing based on cohesive zone model

依据连续油管用CT80钢的三点弯曲试样的断裂实验,通过基于改进Levenberg-Marquardt优化算法的反演分析确定了CT80钢的内聚区模型参数,为连续油管的韧性断裂研究提供了有效的内聚区模型。将所确定的最优内聚区模型通过UMAT嵌入ABAQUS软件,建立了井筒内含环向穿透裂纹连续油管的有限元模型,模拟了有内压和无内压条件下裂纹韧性的扩展规律。通过研究含裂纹连续油管的加载面轴向力随加载位移的变化,确定了连续油管的临界穿透裂纹尺寸,为连续油管的"先漏后断"评定提供依据。结果表明:内压使裂纹更容易扩展,且裂纹的初始尺寸越小,内压的作用越明显;内压对临界穿透裂纹尺寸会产生显著影响,承受30MPa内压的连续油管,在轴向极限拉力作用下的临界穿透裂纹尺寸仅为无内压情况下的52.7%。

Based on a fracture test of three-point bending specimen of CT80 steel used for coiled tubing, the cohesive parameters of CT80 steel are determined by inverse analysis based on modified Levenberg-Marquardt optimization algorithm, which provides an effective cohesive zone model for the study of ductile fracture of coiled tubing. Embedding the determined optimal cohesive zone model into ABAQUS commercial software via a user defined subroutine UMAT, the finite element model of the coiled tubing with circumferential through-wall crack is established and used to simulate ductile crack propagation. The method to determine critical size of the through-wall crack of the coiled tubing is put forwarded through the study of the variation of axial force on loading surface of coiled tubing with loading displacement. The result may be used as the basis for the leak-before-break assessment of the coiled tubing. It is found that internal pressure makes the crack propagate easier. The smaller the initial size of the crack is, the more obvious effect of internal pressure on crack propagation will be. The internal pressure has a significant impact on the critical size of the through-wall crack. Under the action of axial limited tensile force, the critical size of the through-wall crack corresponding to internal pressure of 30 MPa is only 52.7% of that without internal pressure.