FRACTURE-MECHANICS INVESTIGATIONS ON A PIPE WITH A CIRCUMFERENTIAL FLAW SUPPORTED BY FEM

The transferability of crack resistance properties obtained from fract ure mechanics specimens to analyse stable crack growth of a 120-degree s surface flaw in a pipe of large diameter under pure bending is discu ssed supported by results of an elastic-plastic FEM calculation. The r atio of triaxiality, hydrostatic stress and the von Mises effective st ress, is about 2.6 and does neither depend on the location PHI at the crackfront (if PHI < +/- 45-degrees) nor on the bending stress (if sig ma(b) > 100 MPa). Thus, stable crack growth of a circumferential surfa ce 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 show s that the distribution of the bending stress across the cross-section is getting asymmetrical due to the ovalization of the pipe. The evalu ation of the J-integral was performed using the bending moment versus crack mouth opening displacement curves measured at various locations along a 120-degrees circumferential notch under four-point bending (no tch 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 t he worst case because in the vicinity of the connection between pipe a nd pressure vessel the high local stiffness of the system will prevent ovalization of the pipe. An estimate of the crack resistance under fo ur-point bending with ovalization indicates that the J(R)-curve for th e 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 wi th a notch of 0.1 mm resp. 0.25 mm notch radius.