Fracture mechanics investigations on a pipe with a circumferential flaw supported by FEM

Abstract

The transferability of crack resistance properties obtained from fracture mechanics specimens to analyse stable crack growth of a 120°C surface flaw in a pipe of large diameter under pure bending is discussed supported by results of an elastic-plastic FEM calculation. The ratio of triaxiality, hydrostatic stress and the von Mises effective stress, is about 2.6 and does neither depend on the location Φ at the crackfront ( if Φ < +-45°C) nor on the bending stress ( if σ b > 100 MPa). Thus, stable crack growth of a circumferential surface flaw in a pipe under bending may conservatively be estimated using the J R -curve of a large or side-grooved CT specimen. An elastic FEM analysis of the pipe under four-point bending according to a test shows that the distribution of the bending stress across the cross-section is getting asymmetrical due to the ovalization of the pipe. The evaluation of the J- integral was performed using the bending moment versus crack mouth opening displacement curves measured at various locations along a 120° circumferential notch under four-point bending (notch radius 0.25 mm). The result reveals a much higher J- integral level caused by the same bending moment when the ovalization of the pipe is not taken into account. Thus, the question may be raised whether the four-point bending test on large diameter pipes with flaws will meet the worst case because in the vicinity of the connection between pipe and pressure vessel the high local stiffness of the system will prevent ovalization of the pipe. An estimate of the crack resistance under four-point bending with ovalization indicates that the J R - curve for the circumferential notch corresponds better to that of a CT-25 specimen with a fatigue precrack than to the J R - curve of a CT-25 specimen with a notch of 0.1 mm resp. 0.25 mm notch radius.