## 3 Experimental Determination of Mechanical Properties  
In order to determine the mechanical properties of the HDPE pipes, mechanical tests were carried out on specimens made for this purpose from the pipes used in the water supply networks. The mechanical properties resulting from the experimental tests are of great practical importance, as they are necessary for the designers of the power supply networks and must be known for a numerical calculation of the stress field appearing in such pipes. In this study, tensile-compression tests, three- and four-point bending tests were made. The hysteretic behavior of the polyethylene pipes was studied at tensile-compression load. The values obtained from the tests were used in the finite element analysis of the pipes. We mention that the products made by the same manufacturer can have, in the case of HDPE, large variations of the mechanical properties and, for this reason, attempts to see which properties of each lot are required. In the case of the HDPE pipes used in water supply networks, the samples are cut from the pipes, these having different diameters and thicknesses of the walls. For final distribution to customers, pipes with external diameters between 20-110 mm are used. For transport the arteries pipes with external diameters between 110-400 mm. For the main pipelines, pipes with external diameters between 400-1000 mm are used. The specimens are cut from the pipes studied at a width of 10 mm and a length of 150 mm, their thickness being given by the thickness of the wall of the respective pipe. For each specific characteristic, at least 12-15 samples are required. If a higher accuracy of the results is sought, the number of tested specimens is increased. The test speed is chosen so as to ensure a rate increase of the extension of approximately 1%-2% per minute. For the samples presented above, a test speed of 2 mm/min was adopted. The following sizes were determined: stress; deformation; Young's modulus. The test speed must be maintained with a deviation of \(\pm 10\%\). The material tests were performed on the Lloyd LS 100, for traction/compression and on the Lloyd LR5K, for bending. The pipes \(\Phi 20\times 2\), \(\Phi 63\times 4\) and \(\Phi 90\times 6\) (diameter x wall thickness in mm) were tested for traction. For pipes \(\Phi 20\times 2\) specimens had dimensions (in mm) \(150\times 10\times 2\). The diagram stress-strain obtained experimentally for a specimen of this type of specimen is shown, for example, in Figure 3.  
For HDPE pipes \(\Phi 63\times 4\), specimens were \(150\times 10\times 4\) (Figure 4). The specimens were subjected to a test speed of 3 mm/min, using an extensometer. The following characteristics were determined: rigidity; Young's module; normal stress at a maximum load; strain at a maximum load; deformation at maximum load. Figure 5 shows some of the samples that were subjected to the tensile test and Figure 6 shows the extensometer used for this test. Figure 4: Diagram stress-strain for traction test, pipe \(\Phi 63\times 4\). Figure 3: Diagram stress-strain for traction, tub \(\Phi 20\times 2\). Further tests were performed on pipes \(\Phi 90\times 6\) where the specimens were cut to dimensions \(150\times 10\times 6\). The diagram stress-strain, mediated, is shown in Figure 7.  
Figure 5: Specimens after traction test. Figure 6: Extensometer used in the experiment. Figure 7: Diagram stress-strain for traction test, pipe \(\Phi 90\times 6\). Figure 8 presents some samples of HDPE pipe \(\Phi 90\times 6\) broken at traction. The following mechanical characteristics have been determined on traction: Young's modulus; maximum stress at traction; strain to maximum load. The dispersion of elasticity modules for the set tested is quite large, which proves that the material is manufactured in a non-homogeneous manner. The average of these values, used later in the paper, is 1300 MPa. Compression tests were performed for the HDPE pipe \(\Phi 90\times 6\).