2.1.3. Influence of fatigue deterioration on the static behavior

The assessment of the influence of fatigue deterioration on the static behavior was carried out on plates with the same humidity, 0.6%, one of them in the original state and the other deteriorated after being subjected to fatigue loading with the following parameters: frequency: 5 Hz, maximum force: 37.7 kN, minimum force: 2.5 kN, and the number of cycles: 500,000. The results can be seen in Figure 4, which represents the evolution of these two static tests prior and after fatigue. The behavior of the two plates is similar; the most remarkable difference consists in the greater initial stiffness of the previously fatigued plate; besides, no differences in the values of the contact load can be seen.

### 2.2 Impact resistance

As in the previous section, the goal of the tests is to determine the influence of variables such as moisture content, tightening torque or fatigue deterioration on the impact resistance of the component. The impact tests were performed applying square wave cycles under control of displacement conditions with sufficient amplitude to provoke the failure of the plate.

#### 2.2.1 Influence of the moisture content on the impact resistance

To study the influence of the moisture content on the behavior under an impact loading, plates with the following humidity contents were used: dried in an oven, 0.5, 1.8, 2.2, and 3.6%. The results obtained after the impact tests of plates with different degrees of humidity are represented in Figure 5. The first remarkable difference with the static response is that the failure of the plates occurs in all cases before the contact with the screw. Only the plate with a moisture content of 3.6% may raise a doubt because its displacement slightly exceeds the limit of 4.4 mm established previously; nevertheless, since there is no change in the slope of the curve nor any type of mark on the plate after test, the contact with the screw can be discarded.

Figure 5. Results of the impact tests on plates with different moisture contents.

The above figure makes it possible to draw the following additional conclusions: the stiffness of the plates increases as the moisture content decreases; notice that this pattern was also observed for the static tests. Moreover, the deformation at fracture increases with humidity, whereas the maximum force decreases.

## 2.2.2 Influence of the tightening torque on the impact resistance

To study the influence of the tightening torque, the same tests were carried out as in the static study, that is, the three following tightening torques were applied on dry plates: 100, 250, and 350 N˜m, respectively. In Figure 6, the force-displacement curves of the three tests are shown. As can be seen, the plates show a stiffer behavior when the tightening torque is increased (as occurred for the static behavior); moreover, increasing the torque also increases the maximum force and decreases the deformation. This can be explained on the basis that, as the torque decreases, the contact force on the plate decreases too, facilitating the sliding of the plate and therefore its displacement.

## 2.2.3 Influence of fatigue on the impact resistance

To finish the study on the impact resistance, the influence of the deterioration due to fatigue was studied. The experiment was carried out on plates with a

Figure 6. Influence of the tightening torque on the impact behavior of the plate.

Optimization of Components of Superstructure of High-Speed Rail: The Spanish Experience DOI: http://dx.doi.org/10.5772/intechopen.80013

Figure 7. Influence of the fatigue aging on the impact resistance.

humidity of 0.6% that were subjected to 500,000 sinus-type cycles with the parameters described above. The results are represented in Figure 7, where the load-displacement curves of two impact tests are shown, one performed on an original plate and the other on a fatigue-aged plate. The graph shows that fatigue aging by fatigue tends to stiffen the plate, decreasing the deformability at fracture by more than 0.5 mm without affecting the fracture load.
