Analysis of abnormal fracture occurring during drop-weight tear test of high-toughness line-pipe steel

In this study, various types of drop-weight tear test (DWTT) were conducted on a high-toughness line-pipe steel in order to analyze abnormal fracture appearance occurring in the region impacted by a hammer. A pressed notch or a chevron notch was introduced into DWTT specimens, some of which had a back slot designed to decrease the inverse fracture area. After the DWTT, percent shear area, area of inverse fracture surface, and hardness and work hardening exponent of the hammer-impacted region were measured. The results indicated that the shear area of the pressed-notch DWTT specimens was larger than that of chevron-notch DWTT specimens. In the hammer-impacted region of all the DWTT specimens, abnormal inverse fracture having a cleavage fracture mode appeared, and its fracture area was correlated well with hardness and work hardening exponent measured by an indentation test. The formation of this inverse cleavage fracture raised the fracture propagation transition temperature (FPTT) by decreasing the shear area measured in accordance of the API 5L3 specification, which was confirmed by relating to the Charpy impact test data of pre-strained steel specimens. Thus, it was desirable to exclude the inverse fracture formed in the hammer-impacted region when calculating percent shear area of DWTT because compressive pre-strain induced by work hardening did not exist in actual applications of line-pipe steels.