Issue 78

T. Nasri et alii, Frattura ed Integrità Strutturale, 78 (2026) 275-297; DOI: 10.3221/IGF-ESIS.78.17

variability ( n = 2.56 vs. 3.91), with both values remaining within the physically plausible long-crack range ( n = 2.53–3.82, FAA database [32]).

Parameter

Type A – Specimen 1

Type A- Specimen 2

Type B – Specimen 1

N

2.56

3.91

1.01

C (µm·cycle ⁻ ¹·(MPa √ m) ⁻ ⁿ )

2.32×10 ⁻⁵

6.64×10 ⁻⁷

1.32×10 ⁻ ³

N g (grains crossed)

3.72

4.84

1.55

Table 8: Level-1 parameter reproducibility across two specimens per orientation.

R ESULTS AND DISCUSSION

T

his section presents the results obtained with the enriched model for type A and type B holed specimens, together with the long-crack reference behaviour. The discussion focuses on crack growth kinetics, crack-length evolution, comparative d /dN– Δ K behaviour, modulation factor μ ( ξ ), propagation anisotropy, and the quantitative Fig. 6 shows the evolution of crack length a (N) for the type A specimen (  ∥ R D , crack ⊥ R D , d  = 154 µm, max  = 200 MPa, K t = 2.3, R = 0.1, 0 a = 35.4 µm), together with the five grain boundary positions GB ₁ -GB ₅ identified experimentally along the crack path. Fig. 7 shows the corresponding evolution of crack growth rate d a /dN as a function of crack length a . performance of the proposed formulation. Crack growth rate da/dN - Type A specimen

Figure 6: Crack length a vs. number of cycles N - type A (119 pts). Savitzky-Golay smoothing (red). Dashed lines: grain boundary positions GB ₁ -GB ₅ . Fig. 7 presents a composite view correlating three complementary observations of short crack propagation in the type A specimen (  ∥ R D , crack ⊥ R D , d  = 154 µm, max  = 200 MPa, K t = 2.3, R = 0.1, 0 a = 35.4 µm). Panel (a) presents the EBSD grain orientation map obtained in the TL plane (270 × 50 µm², step size = 1 µm). The map highlights the partially recrystallised microstructure of AA2024-T351, composed of randomly oriented recrystallised grains (blue) and elongated non-recrystallised grains (red). The crack propagation direction is indicated by the red line. Three grain boundaries, GB ₁ , GB ₂ and GB ₃ , are directly observed within the EBSD scan range ( a = 0–270 µm). Grain boundaries GB ₄ and GB ₅ , located at a = 308 µm and a = 523 µm respectively, are located beyond the EBSD scan range and were identified from SEM observations (panel b).

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