Page 159 - Handbook of Materials Failure Analysis

6 Results and Discussions 155

6.2 COMPLETE DIAMOND ANALYSIS
The resultant model of the diamond crossover analysis can be found in Figure 6.28.
They are the loading results from the analysis. As can be seen from the numbers, the
results correlate well with the individual bearer forces experienced, sitting in
between the minimum and maximum resultant force figures, at 378N.

6.3 DISCUSSIONS
The reaction forces underneath the bearers show that as the load is increased, the
bearer will definitely experience a greater force. Figure 6.29 presents the sleeper/
ballast contact pressures obtained from the methods of determining the rail seat
load. It is found that the data is in very good agreement with those obtained from
themorecompletecrossovermodel. It is found that the maximum resultant forces
from the current methods of analysis are still far below the force envelope spec-
ified in the design standards. Even if it is to be increased substantially, the load
would still not cause a failure of the bearer, according to the finite-element
analyses.
This puts into question whether the diamond design contributed to the fail-
ure. One factor that was not taken into account for the analysis is the possibility
of high-amplitude dynamic forces experienced by the rail and bearer. The
dynamic impact forces would in turn apply a greater moment force upon the
bearer. It is clear from Figure 6.29 that if the rail seat loads are to increase only
about 20-30%, the contact pressure between ballast and sleeper will have
reached the allowable limit of 750 kPa [16,17]. Considering AS1085.14, if
the specified impact factor of 2.5 is adopted, the resultant dynamic wheel load
will gain 27% over the maximum allowable load (295 kN). It implies that
potentially the ballast would significantly and rapidly settle under the crossings
and diamonds. The large settlement can cause additional bending moment
actions and their consequences, resulting in a flexural failure mode of bearer
as observed in the field.
Table 6.6 illustrates an estimation of turnout renewal costs when failure of
diamond component incurs. It is important to note that criticality of turnout fail-
ure is relatively high as a turnout (only about 100-150 m in track length) tends to
connect two or more plain tracks together. As such, its consequences are progres-
sive and highly interconnected to the service values other connected tracks pro-
vide. This provides the fundamental rationale for higher priority of turnout
renewal and maintenance than that of a plain track. From the cost estimate list,
it can be seen that material costs cover around 6-7% of the total cost. It implies
that re-designing critical structural components such as concrete bearers for the
capability to resist higher impact loads could potentially save the associated pen-
alty costs of 77%.