Page 133 - Handbook of Materials Failure Analysis

3 Failure Mechanisms 129

3 FAILURE MECHANISMS
In recent years, there have been several incidents of cracking in concrete bearers in
turnouts within Australian railway networks. Such problem was in relation to signif-
icant degradation of turnout components due to large impact force such as that from
the Clyde 700 diamond (RailCorp’s western main rail network). White ballast (bal-
last pulverization) could be observed near the crossings. The failure mechanism is
illustrated in Figure 6.4.

3.1 STRUCTURAL INTEGRITY
Initially, the early warning sign was the deviated geometry profile of the diamond, as
seen in Figure 6.4a. White ballast (ballast pulverization) could then be observed near
v-crossings and k-crossing, as shown in Figure 6.4b and c. It is believed that the bal-
last was pulverized due to the impact vibration of crossing structures and the bearers
excited by the rail dip-like irregularity. It was also found that due to the large vibra-
tion, the wheelsets tended to spread the lubricant (as the wheel collected from friction
management trackside device) over the location slightly further from the nearby
crossing (such as in the crossing panel as shown in Figure 6.4b).
Figure 6.4d also shows the collapse of the bearer underneath the stockrail at
Country-end v-crossing. This negative (hogging) bending failure is due to a single
wheel impact at v-crossing. The bearer failure had led to the discrepancy of track
surface (top) between running rails and the wide track gauge irregularity. The shiny
steel at wing rail could be noticed due to the contact of wheelback and wing rails.
Similar mechanism can also be observed at k-crossing area.
It is noted that the concrete bearer cracked under a rail pad, the part which sup-
ports the rail, where the train wheels travel over the diamond section. The concrete
bearers experienced a different type of dynamic load, compared to standard railway
sleepers. This is due to their inherent stiffness nature of the diamond, which could be
much higher than that of a plain track. When trains move over changing track stiff-
ness and geometry, wheel-rail interaction induces high-magnitude impact forces. As
a result, it is recommended that such forces be accounted for in the design standards
to prevent further failure, as well as financial loss and possibly loss of life.
During such time, train speed limit restrictions were put in place around the
cracked bearer to prevent further damage to adjacent components and to minimize
any risk of derailment. The speed limit at this track location was limited to 50 km/h
due to the turnout degradation. Apart from that, it was also found that the condition of
the thick SA-47 pads is still in very good condition. No severe abrasion on either
bearers or pads can be observed.

3.2 GEOTECHNICAL INSPECTION
Geotechnical Services carried out an inspection and investigation by test pits at the
area during a track possession. Figure 6.4e and f shows the cross profile of a test
pit at the country-end v-crossing. From the geotechnical investigation, the ballast,