# Experimental Evidence of Specimen-Size Effects on EN-AW6082 Aluminum Alloy in VHCF Regime
## 3 Results  
### Experimental Results and Fracture Surface Analysis  
Ultrasonic fatigue tests were conducted under fully reversed constant stress amplitude tension-compression conditions (\(R=-1\)) at room temperature. The specimens were tested up to failure or up to a run-out of cycles equal to \(10^{9}\). For the hourglass specimens of 3 mm and 6 mm in diameter it was possible to carry out the tests continuously, since the air-cooling system was able to keep the specimen temperature always below 50 \({}^{\circ}\)C, except for the very final stage of the total fatigue life, when the specimen temperature rose abruptly just before failure due to a crack propagation [27,30]. More in detail, the abrupt increase in the temperature due to the fatigue crack propagation lasts always less than 6-7% of the total lifetime of the specimens. For the specimen of 12 mm in diameter, to avoid heating of the samples, a pulse-pause sequence with a period of 1 s was applied for all loading levels. The pulse and pause lengths were set up at 500 ms and 500 ms, respectively. It is reasonable to suppose that the higher self-heating of the specimen of 12 mm in diameter is due to the larger sample dimensions, despite the investigated aluminum alloy is characterized by a higher thermal conductivity, compared to high-strength steel. In this way, the temperature of the specimens of 12 mm in diameter was kept below 50 \({}^{\circ}\)C for almost the duration of tests, except for the final stage of crack propagation in which the temperature reached in many cases a value higher than 100 \({}^{\circ}\)C. For all the three different sets of specimens, failures were obtained in the range between \(7\cdot 10^{6}\div 10^{9}\) cycles, i.e., in the VHCF regime. In particular, sixteen tests were carried out on an hourglass-specimen of 3 mm in diameter at a stress amplitude ranging between a minimum value of 160 MPa and a maximum value of 210 MPa; thirteen of these led to a failure, whereas three were runouts. Moreover, seventeen tests were carried out on specimens of 6 mm in diameter, reporting two runouts at 150 MPa. Finally, fifteen failures were obtained for the specimens of 12 mm in diameter between 140 MPa and 180 MPa, whereas two runouts were obtained at 130 MPa. The experimental results are reported in Figure 7.  
Figure 6: The three tested hourglass specimens: (**a**) 3 mm; (**b**) 6 mm; (**c**) 12 mm.  
Figure 8 shows the fracture surface morphology of a specimen of 12 mm in diameter, which has been obtained by using Field Emission SEM. From the figure, it is possible to distinguish three different zones, i.e. the crack initiation origin, the crack propagation zone, and the final failure region. It can be observed that the crack initiation is located close to the sample surface, which is in accordance with other experimental investigations regarding aluminum alloys. It is worth noting that the total rupture of the specimen in two distinct parts was obtained with a three-point bending test, since the crack propagation during the ultrasonic test was such that the specimen frequency fell below the lower limit of 19.5 kHz, without yet leading to unstable crack propagation.  
Figure 9 shows a magnification of the crack propagation zone, where prominent striation marks with a river-like appearance are visible. Eventually, similar fracture morphology is observed for the other specimens of 3 mm and 6 mm in diameter.  
Figure 8: SEM image of the fracture surface morphology of the specimen of 12 mm in diameter.  
Figure 7: S-N diagram of the experimental data.