PSI - Issue 68

Erik Calvo-García et al. / Procedia Structural Integrity 68 (2025) 809–814 Erik Calvo-García et al. / Structural Integrity Procedia 00 (2025) 000–000

812

4

During the precrack stage, crack lengths were monitored to control their propagation up to 10 mm approximately, which corresponds to 0.45-0.55 times the specimen width. Next, the J-R curves were obtained using the resistance curve procedure. Specimens were loaded under displacement control, applying sequences of unloading and reloading. J-integral values for each unloading/reloading sequence were plotted as a function of the crack extension in order to construct the J-R curves, which allowed to calculate a value of fracture toughness J Ic . These fracture toughness values have proven to be size-independent as indicated in the ASTM E1820 standard. 3. Results and Discussion The surface plastic deformation attained by the shot peening treatments can be observed in Fig. 3. It is observed that both shot peening treatments affected the totality of the treated surfaces. Both treatments resulted in a significant increase of surface roughness, as indicated in Table 3. Alumina particles led to a much rougher surface than silica microspheres, as the latter presented smaller size and lower hardness.

Table 3. Roughness values of different surface treatments. No treatment Silica microspheres

Alumina particles

Ra (µm)

0.35 ± 0.02

0.49 ± 0.06

1.3 ± 0.3

a

b

c

Fig. 3. Surface topography of 6060 T6 specimens (a) with no surface treatment and shot peened with (b) silica and (c) alumina.

The results of the crack growth stage of the tests are shown in Fig. 4. It is observed that the shot peening treatments had little effect in the number of cycles required to reach a certain crack extension. However, shot peened specimens presented lower crack growth rates than the non-treated ones for the same stress intensity range values. No differences were observed between both shot peening treatments. The Paris law constants for each curve are presented in Table 4. The slopes of the shot peened specimens were slightly lower than that of the non-treated one. Fracture toughness results are represented in Fig. 5 and Table 4. The J Ic values characterize the toughness of the material near the onset of crack extension. These results show that surface treatments did not have a significant effect on the fracture toughness of the material.

Table 4. Crack propagation constants and fracture toughness for each surface treatment. Shot medium C m J Ic (kJ/m 2 ) No treatment 2.8 × 10 -8 3.8 80.1 ± 2.4 Silica microspheres 6.2 × 10 -8 3.4 78.4 ± 9.9 Alumina particles 7.9 × 10 -8 3.3 82.9 ± 8.6

Shot peening has the potential to delay crack propagation due to the compressive residual stresses in the surface of the treated materials. However, the reduction in crack growth rates of the aluminium alloy was not too significant.