Page 130 - Handbook of Materials Failure Analysis

126 CHAPTER 6 Failure analysis of concrete sleepers/bearers

has been developed to determine if the turnout diamond systems were sufficient for
the impact loading experienced by that bearer. The emphasis of this study will be
placed on the current design and analysis of turnout bearers and the application of
nonlinear finite-element modeling to aid failure analysis of railway structural
components.
Table 6.1 summarizes recent research on failure analysis of turnout components.
It can be found that most research has paid special attention on traditional turnouts,
while information on such special trackworks as diamond, single, and double slips
are very limited. As a result, this study is imperative for complex turnout design,
maintenance, and renewal.

2 INTERACTION OF TRAIN AND TURNOUT STRUCTURE

The wheel/rail contact over the crossing transfer zone has a dip-like shape where the
wheel trajectory is not smooth. The accurate shape of the wheel trajectory (running
top) and dip angle will depend on the wheel and running rail profiles. The associated
dip angles, which are the acute angles between the tangents to the wheel trajectory at
the point where it abruptly changes direction, can then be estimated from the wheel
trajectories, as illustrated in Figure 6.3.

2.1 TURNOUT FORCES
It is generally assumed that the high-frequency impact force (P 1 ) that occurs either at
a nose or at a wing rail has little effect on the rail foot [14]. On the other hand, the
dynamic P 2 force (the second peak in the impact force history) has significant influ-
ence on the crossing components. The distance from the point of impact to the point
of the peak impact force depends on a number of factors including train speed. The
common damage zones to be considered are the rail foot within 0.75 m of any joints
to plain rail; the base of the crossing in transfer zone (extending 0.75 m on both sides
of the pick up point); and other components in vicinity of the crossings.
In a calculation of P 2 force, the track damping C t is normally negligible. For plain
tracks, it is commonly found that the track mass is relatively low in comparison with
the wheel set mass and is then neglected. In contrast, for a turnout crossing, the track
mass tends to be of significance and it cannot be neglected. Jenkins et al. [15] has
proposed a formula for estimating a dynamic P 2 force as follows:
1=2 " #
M u π C t 1=2
½
P 2 ¼ P 0 +2α v 1 p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi K t M u Š ; (6.1)
ð
M u + M t 4 K t M u + M t Þ
where P 2 is the dynamic vertical force (kN); P 0 is the vehicle static wheel load (kN);
M u is the vehicle unstrung mass per wheel (kg); 2α is the total joint angle or equiv-
alent dip angle (rad); v is the vehicle velocity (m/s); K t is the equivalent track stiffness
(MN/m); C t is the equivalent track damping (kNs/m); and M t is the equivalent track
mass (kg).