Failure analysis on the fracture of disc cutter in EPB shield machine

Disc cutter is a tool that EPB shield machine uses in fragment hard rocks in the construction of a subway tunnel in Jinan. Many disc cutter rings fractured during the tunnelling process in April to May 2018. Based on the force characteristics of the cutter ring, a comprehensive analysis was performed to investigate the base materials, residual stress, mechanical properties, metal flow line, and micro morphologies of the fracture cutter ring to identify the fracture causes. The cutter ring fracture type was quasi-cleavage fracture. Moreover, the test result showed that tensile strength and toughness of the cutter ring was lower than the regular cutter ring. The direction of the initial crack is perpendicular to the cutter ring edge. A collection of banded precipitates and oxides were distributed among the grains by metallographic test, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). It increased internal defects in the cutter ring and reduced the mechanical properties of the cutter ring. These defects are the main factor that causes the cutter ring fracture during karst formation tunnelling with large impact loads. Besides, excessive residual stress was another of the cutter ring fracture. The uneven distribution of residual stress and cutter ring hardness also indicated uneven heating of the cutter ring during manufacturing and heat treatment. Finally, some suggestions which aimed at EPB shield machine tunnelling progress and cutter ring manufacturing were put forward as well.