Evaluation of the pipe crack resistance by impact toughness and DWTT test

The physical nature of impact toughness and the fraction of the ductile component in the fracture of the sample when assessing the crack resistance of the pipe were under study. These parameters are found to depend on the stressing rate, on the speed of the crack propagation, on the size of the sample (scale effect), on the simulation of the actual process of destruction of pipelines and other factors. The samples were tested following the scheme of impact bending loading, when the conditions of occurrence of the significant compressed zones in the sample were created causing the artificial increase of the criterion of impact toughness and the fraction of the ductile component in the fracture of the sample. The process of the pipeline destruction goes the following way: first the stage of crack initiation comes; then, the crack growth to a critical size under conditions of static stresses; the final stage of the process is the appearance of fracture under high-speed propagation of cracks. The destruction of the underground pipeline is caused by circumferential stresses that are below the metal yield strength and, as a rule, in conditions of tensile stresses with no compression stress. The process of pipeline destruction cannot be imitated when testing a small standard sample of KCU or KCV types, as well as a sample of the DWTT type on impact bending. When testing the samples the process of crack initiation and its growth to a critical size is in the dynamic mode. It does not coincide with actual conditions. Only the stage of crack propagation corresponds to the process of pipeline destruction. In reality the criteria of impact toughness and DWTT tests cannot be used for the quantitative evaluation of the crack resistance of gas and oil pipes.