*3.2. E*ff*ects of Prebending Radii on HCF*

As can be seen from the data in Figure 3, the S-N curves of 7075 aluminum alloy under different prebending radii have a significant influence on the fatigue life of the formed alloy. When the prebending radius ρ was 1500 mm, the fatigue life was the longest under the same stress level. Under the higher stress levels (>210 MPa), the fatigue life of the alloy formed with prebending radius 500 mm was longer than that with prebending radius 1000 mm, while under the lower stress levels (<210 MPa), the fatigue life of the alloy formed with the prebending radius 500 mm was the shortest. The results show that increasing the prebending radius contributes to reducing the damage of the alloy under low stress (<210 MPa) and to improving the HCF performance of 7075 aluminum alloy.

**Figure 3.** S-N curves of 7075 aluminum alloy at different prebending radii.

As aluminum and aluminum alloys are not commonly considered to have a fatigue limit, the conditioned fatigue limit is usually used in practical engineering applications. According to the method in literature [22], the conditioned fatigue limit is defined as the fatigue life when the number of cycles is 107. The conditional fatigue limits of 7075 aluminum alloy with different prebending radii are calculated and obtained from SN curves, as shown in Table 3, where the data also indicate that the prebending radius has a significant influence on the conditional fatigue limit of the alloy. The conditional fatigue limit of the alloy increased as the preload radius increased. When the prebending radius was 1500 mm, the conditional fatigue limit reaches the maximum value: 188.91 MPa.

**Table 3.** The conditional fatigue limits of 7075 aluminum alloy at different prebending radii.

