2.3 Fatigue behavior

In this section, the behavior of the plate subjected to dynamic cyclical loads will be studied. In addition, the influence on this behavior of external parameters, such as the moisture content of the plate or the value of the tightening torque applied to the clamping assembly will be assessed. For the fatigue characterization, the Locati test technique [3–5] was used; this consists in applying blocks (composed of a constant number of cycles) of increasing maximum load, starting from a value lower than the fatigue limit. At a certain level of maximum load, the maximum strain grows rapidly, preceding the failure of the plate.

### 2.3.1 Influence of the moisture content on the fatigue behavior of the plate

For this analysis, the Locati method was used applying loading blocks of 20,000 square wave cycles with a frequency of 5 Hz; the load for the initial block ranged between 5 and 45 kN. The minimum value was kept constant throughout the whole test, while the maximum load value was increased by 2 kN in each block. Table 1 and Figure 8 show, as a summary, the results obtained in the Locati tests carried out


Table 1. Influence of the moisture content on the fatigue behavior of the plates.

Figure 8. Influence of moisture content on fatigue limit and critical temperature.

on plates with different moisture contents, verifying how the fatigue limit, Δσe, increases with the decrease in the moisture content of the plate. In the same figure, the evolution of the critical temperature, Tc, for the different moisture contents is represented, obtaining the same decreasing relation.

The dry plates under static loading failed in a brittle manner, as opposed to the ductile failure of the wet plates. In contrast, under impact conditions, both types of plates exhibit a brittle response. Finally, the fracture of the plates under fatigue loading is ductile in all cases, regardless of the level of moisture content.

## 2.3.2 Influence of the tightening torque on the fatigue behavior of the plate

As in the static and impact cases, the tightening torque applied in the fastening system may modify the fatigue behavior of the plate. To verify the influence of this parameter, Locati tests were carried out on plates with the same moisture content (0.6%) varying the tightening torque according to the following values: 100, 250, and 350 N˜m. The parameters chosen in the Locati tests were the same as those described before. The increase in the tightening torque improves the fatigue behavior of the plate. Table 2 and Figure 9 show, as a summary, the results obtained in the Locati tests carried out on plates with different tightening torques and moisture contents.

As can be seen, as far as fatigue is concerned, increasing the tightening torque has similar consequences as reducing the moisture content. It can be concluded that the critical temperature depends not only on the material but also on the certain boundary conditions, in this case, on the tightening torque of the assembly.


Table 2. Influence of tightening torque on the fatigue behavior. Optimization of Components of Superstructure of High-Speed Rail: The Spanish Experience DOI: http://dx.doi.org/10.5772/intechopen.80013

Figure 9. Influence of the tightening torque on the fatigue behavior of the plate.
