PSI - Issue 7

Benoît Bracquart et al. / Procedia Structural Integrity 7 (2017) 242–247 B. Bracquart et al. / Structural Integrity Procedia 00 (2017) 000–000

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in cyclic behavior (the cyclic behavior of pure aluminium being composed of three stages: hardening, softening, and secondary hardening [20, 21]), denoted ∆ ε p s − h , that has been determined from previous tests on smooth specimens. Table 2 illustrates the change in plastic activity between the 4 configurations d − φ . When increasing the defect size, the change in plastic activity is more significant for the small grain microstructure. Table 2: Plasticity levels corresponding to 50% probability of crack initiation at the defect, in the 4 configurations d − φ (plasticity criterion: ∆ ε p s − h , for the corresponding smooth specimen).

d s = 100 µ m d l = 1000 µ m

φ s = 100 µ m φ l = 1000 µ m

0.079% 0.056% 0.098% 0.087%

Figure 3 shows that, for a constant relative defect size d /φ but di ff erent absolute sizes, the plastic activity corresponding to a 50% crack initiation probability is about the same. This is consistent with the results of Lorenzino et al. [18] and Vincent et al. [19] who observed that the relative defect size d /φ is more relevant than the actual physical defect size d . Also, on a macroscopic point of view, it seems that crack initiation is essentially governed by plasticity, regardless of the local microstructural features (e.g. crystallographic orientation, grain shape) that can influence dispersion.

5. Conclusion

In this work, the impact of geometrical defects on the HCF behavior of pure aluminium (AA1050) has been investigated. More specifically, experimental e ff orts have been made to better understand the role of local grain size on fatigue crack initiation in the presence of defects. First, di ff erent thermomechanical treatments have been applied to aluminium samples to obtain microstructures with either a small ( φ s = 100 µ m) or a large ( φ l = 1000 µ m) average grain size. These samples have been used to prepare fatigue test specimens with hemispherical defects. Two di ff erent defect sizes have been considered: small ( d s = 100 µ m) and large ( d l = 1000 µ m). Stress-controlled fully reversed uniaxial HCF tests have then been carried out. According to fatigue test results, crack initiation is delayed when the grain size is reduced.

φ s , d s

φ s , d l

φ l , d s

φ l , d l

0 . 1

0 . 09

0 . 05 Plastic strain range ∆ ε p s − h ( P 50) (%) 0 . 06 0 . 07 0 . 08

0 . 1

1

10

Relative defect size d/φ

Fig. 3: Plastic indicator for a probability of crack initiation to a defect of 50%, ∆ ε p s − h

( P 50), as a function of the relative defect size d /φ , for the 4

configurations.