Failure Mechanisms in Cutter Tools for Tunnel Boring

Abstract

Tunnel boring machines (TBM) utilize cutter discs as a primary tool for cutting through hard rock. The discs are mounted on a rotating, circular cutter head which is pressed against the rock. A microscopical examination of worn cutter discs has been carried out. The goal was to map fracture mechanisms on TBM cutters with different reason of change, position on the cutter head and rock mass involved. Testing was performed on samples from a full scale TBM and from a novel test method for cutter wear assessments on hard rock tunnel boring recently developed by NTNU. This method is called the Rolling Indentation Abrasion Test (RIAT).

The mode of contact for TBM cutters was found to be rolling, while RIAT mini-cutters cutters also experienced a minor sliding component. Abrasive wear was dominant in both sample types. An increase in fatigue wear was seen in TBM cutters as rock abrasivity decreased. This was credited longer cutter life and larger fluctuation in load. The presence of nanocrystalline deformation layers of comparable thickness in both sample types indicate similar deformation in the two processes.

A furnace test on undeformed TBM cutter material was performed to correlate temperature and holding time with the sample hardness profiles. Comparison of hardness measurements and tempering results indicate a surface temperature in excess of 620 C in three TBM cutters. This temperature is given a holding time of one hour. The temperature gradient was seen to correlate with depth of deformation in the material. A high deformation rate is believed to account for steep hardness gradients found. No estimate for surface temperature was found for the RIAT mini-cutters.