Papers by Keyword: Delamination Crack

Abstract: The effect of weak interfacial orientation, thickness and the main crack depth on the initiation of delamination crack and main crack in three-point bending specimens of X80 pipeline steel are investigated, using 3D finite element method. Considering the different fracture mechanism of delamination crack and main crack, two different criteria are adopted for them in the simulation. The results reveal that, when the delamination cracks initiates, the initiating position is fixed and the Jz integral of the main crack is a constant for specific weak interfacial orientation specimens with different main crack depth and a certain thickness. When specific weak interfacial orientation specimens have different thickness and a certain main crack depth, the initiating position of delamination crack is equal to the above mentioned, but the Jz integral of the main crack has a ripad decline with the increasing of thickness and then tend to stable. In particular, the delamination crack will not appear for specimens with thin thickness. The thicker the specimen is, the higher the ultimate weak interface strength is needed to prevent the initiation of the delamination crack for specimen with a certain main crack depth. The larger delamination crack will generate, when the specimens have a lower ultimate weak interface strength, a smaller Jz integral of main crack and a larger thickness.

Abstract: Single or multiple of delaminations have been found frequently on the fracture surface of X70 pipeline steel. In this study, the delamination cracks and their influence on the fracture of pipeline are investigated by both experiment and three-dimensional fracture analyses. It is shown that the three-dimensional stress state is prerequisite for delamination crack and the strength distribution of material influences the form and direction of delamination crack. The delamination cracks are produced on the weak interfaces among the material by the tensile stress perpendicular to them before the fracture passes. The direction of delamination crack depends on the three-dimensional stress fields and strength distribution of material near the crack tip or notch root. The delamination cracks of the fracture through thickness of pipe wall make the effective thickness decrease and the delamination cracks of surface crack are perpendicular to the direction of fracture propagation direction. The delamination cracks reduce the stress triaxiality near crack tip and in turn, improve the fracture toughness of X70 pipeline steel.

Abstract: The Charpy V notch specimens of X70 pipeline steel with different notch orientation are tested under the static and dynamic conditions at different temperatures. By analyzing the load versus displacement curves and fracture appearances of specimens the relation of fracture behavior and loading rate is investigated. The maximum load increases and the displacement corresponding to maximum load reduces with test temperature decreasing. Both under the static conditions are larger than that under the dynamic conditions. The fracture sections of all test specimens are reduced in the thickness direction and quantity reduced depends on the load rate, notch orientation and test temperature. At the higher temperature, delamination cracks are found on the fracture surface both under the static and dynamic conditions, which are perpendicular to the thickness direction in T-L specimen and perpendicular to the main crack propagation direction in T-S specimen. Influence of loading rate on the delamination crack size and amount is obvious. The fracture initiation energy and total fracture energy increase with test temperature increasing. Influence of loading rate on the total fracture energy is unobvious at the higher temperature, but is obvious at the lower temperature. So the loading rate effects on total fracture energy are relevant to test temperature and notch orientation.

Abstract: The delamination cracks and its effects on the fracture of pipeline steel are investigated experimentally by using of Drop-Weight Tear Test (DWTT). The delamination cracks are produced by the stress perpendicular to the weak interfaces before main crack beginning or accelerating, no new delamination crack is produced during the stabile propagation of fracture. The quantity, splay degree of delamination crack and the space between two delamination cracks are influenced by the stress state of the crack tip at beginning or accelerating point of main crack and the length of delamination crack is influenced by the stress state of the crack tip during the propagation of fracture. The surface of delamination crack is cleavage fracture appearance with large cleavage facet. There is no delamination crack on the brittle fracture surface below the brittle-to-ductile temperature or on the brittle fracture region of mix-mode fracture surface with ductile and brittle region. The part of fracture surfaces with delamination crack ought to be evaluated as the shear area because the delamination cracks are produced only on the ductile fracture surface or on the ductile part of fracture surface.

Abstract: The effects of thickness, notch orientation and delamination cracks on the impact toughness of X70 pipeline steel are investigated experimentally by use of the instrumented Charpy impact tests at different temperatures. The couple effect of delamination cracks, thickness, notch orientation and temperature is discovered. The delamination cracks have certain direction, and their amount and size are related to the temperature and the specimen thickness. Though the delaminating orientations of T-S and T-L specimen are not same, the reasons for both T-S and T-L specimen delaminating are that the weak interfaces in the specimens are pulled apart by the stress perpendicular to them. The delamination cracks can improve the actual impact toughness of X70 pipeline steel both T-L and T-S specimens. The effect of delamination cracks on the actual impact toughness changes with the thickness and the temperature. The couple effect of wall thickness, defect orientation and working temperature of pipeline must be taken into account in safe assessment of pipeline.