THE EFFECTS OF MATERIAL PROPERTIES ON CRACK TIP CONSTRAINT AND CTOD FRACTURE TOUGHNESS FOR COMMONLY USED PIPELINES STEELS

As the main transportation carrier of oil and gas, pipelines play a very important role in the petroleum industry. When the crack-containing pipelines subjected to external loads, the cracks may propagate gradually, and result in serious failure eventually. Therefore, accurately obtaining the fracture toughness is very essential for the safety assessment of the crack-containing pipelines. However, the fracture toughness is not a material intrinsic parameter, but heavily depends on the constraint. To obtain the accurate relationship between the constraint and the fracture toughness for different materials, it is necessary to determine the effects of different material parameters on the change characteristics of the constraint and the fracture toughness. In this work, the commonly used pipelines steels are selected as the research materials. The SENB specimens and the complete Gurson model are used to conduct the simulation with ABAQUS. The material parameters analyzed include strain hardening exponent, yield strength and initial void volume fraction. The results show that for the thinner specimen, the higher strain hardening capacity will result in lower constraint. The higher strain hardening capacity will result in higher constraint for the thicker specimen. For the thinner specimen, the constraint is approximately the same for the materials with different yield strength. The constraint will decrease with the increase of yield strength for the thicker specimen. In the middle range of the thickness of specimen, higher initial void volume fraction will result in higher constraint. For the thicker and thinner specimen, the effect of initial void volume is very weak. As the increase of strain hardening capacity and yield strength, the decreasing degree of the fracture toughness becomes higher in the increasing process of the constraint. A higher initial void volume will result in a lower decreasing degree of the fracture toughness. All of the results indicate that the strain hardening capacity is the main factor affecting the constraint and the fracture toughness. The initial void volume also has a significant effect on the fracture toughness. For the relationship between the constraint and the fracture toughness, the main affecting factor is the strain hardening capacity.