4.1 Dowel

The dowel consists of two components, namely, the main body made of polyamide 6.6 reinforced with glass fiber (30% wt.) and the metal sheet that surrounds the core peripherally. Due to the hygroscopic nature of the polyamide, the following treatments were applied to obtain a series of dowels with different moisture contents in order to know the influence of this parameter on the mechanical behavior:


These types of treatments were aimed at simulating the worst conditions that may take place in the preparations of the sleepers.

The moisture contents obtained with the different treatments were determined in two different ways. On the one hand, the overall content was measured weighing the dowel before and after the treatment and, on the other hand, the moisture content of the thread was calculated once it had been removed from the dowel in the mechanical tests and weighing the same before and after placing it in an oven for 7 days at 100°C. The latter procedure provides more representative values, since it is being measured in the resistant zone of the component. In addition, the humidity of the thread is, in all cases, higher than the overall humidity, since it is an area with greater external surface, and the water is absorbed more easily, being lighter in the last two treatments, since the interior area is the one that was in direct contact with the water that contained the dowel. The results obtained from the different moisture contents are shown in Table 3.

The critical stresses that the anchor must support are, mainly, parallel to the axis of the dowel-screw assembly, that is to say, forces that pull the screw out of the dowel. Therefore, the tool designed for the mechanical characterization of the dowels


#### Table 3. Humidity of the dowel according to the treatment.

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

Figure 14. Static test on dowels.

simulated this type of loading conditions. The characterization was based on both static and dynamic regimes, where the last one includes impact and fatigue efforts.

### 4.1.1 Characterization of the dowels under static loading

For the static characterization of the dowels, the load screw was driven to a depth of 64 mm, similar to the in-service position; then, an extraction force was applied until fracture at a loading rate of 1 kN/s. The results obtained on the dowels subjected to the different moisture treatments are represented in Figure 14. In view of the results, it is verified that the breaking load, as well as the initial stiffness of the component, increases as humidity decreases, while the deformation at fracture remains approximately constant. Therefore, the energy at failure decreases with increasing humidity. The constant value of the displacement at failure is explained because the applied stress is distributed uniformly along 14 thread passages, provoking the fracture of all of them simultaneously by shearing. As can also be seen, even for the highest moisture content, the limit value imposed by the technical specification (60 kN) is exceeded.

### 4.1.2 Characterization of the dowel under impact loading

In this case, the screw was driven to a depth of 64 mm and then a square wave cycle was applied to provoke the fracture of the component. The results obtained for the dowels subjected to the different moisture treatments appear in the graph of Figure 15, and these values are higher than those obtained in the static tests, while deformation increases with humidity, until reaching values similar to those obtained under static conditions. In the case of the two dowels with the lowest moisture content, fracture occurred at the depth where the screw thread reaches, this being the reason that explains why the fracture deformations of the dry samples were slightly lower than those of the wet ones.

The results allow it to be verified that the fracture load increases as the humidity decreases, characterization of the dowels under fatigue loading.

The Locati methodology was employed for fatigue characterization. Blocks of 20,000 square wave cycles were applied at a frequency of 5 Hz, with an initial loading range between 5 and 50 kN. The minimum load was constant during the

Figure 15. Impact test on dowels.

test, while the maximum value was increased by 2 kN in each block. The results obtained in this experiment cannot be used to be compared with different tests because a nonquantified relative displacement between the dowel and the screw took place. The analysis of the displacement variation versus the number of cycles was used to determine the critical block. The results summarized in Table 4 are obtained from dowels with different moisture contents in order to determine the critical parameters of fatigue.

The strong influence of moisture content on the fatigue behavior of this material is verified; note that the fatigue resistance of the dry dowel changes from 74 to 52 kN in the case of the wet dowel. The requirement of the technical specification regarding the extraction of fasteners is 60 kN in static regime; therefore, the fatigue limit exceeds this condition in all cases but for the case of maximum humidity.

### 4.2 Screw

The mechanical characterization was carried out following the requirements indicated by the internal control standard of the supplier [11]. The properties characterized were tensile, bending, and impact strength. Screws were used in the as-received state and were subjected to accelerated corrosion for 300 hours in a salt spray chamber to evaluate the influence of the exposure to a corrosive environment on the mechanical properties.


Table 4. Summary of the results obtained in the Locati tests.

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