PSI - Issue 39

Jesús Toribio et al. / Procedia Structural Integrity 39 (2022) 479–483 Author name / Procedia Structural Integrity 00 (2021) 000–000

482

4

25

∆ K = 18.75 MPam 1/2 ∆ K = 25 MPam 1/2 ∆ K = 31.25 MPam 1/2

20

15

∆ a ( µ m)

10

5

0

0

5

10

15

20

N (cycles)

(a)

10 -6

(da/dN) 0 (m/cycle)

10 -7

20

30

∆ K (MPam 1/2 ) (b)

Fig. 3. Fatigue crack growth for a straight crack: (a) crack growth vs . number of cycles; (b) crack growth rate vs . SIF range (Paris curve).

3.2. Bifurcated crack tip A retardation effect appears in the matter of crack propagation rate. This phenomenon is characterized by the so called retardation factor (d a /d N )/(d a /d N ) 0 , where (d a /d N ) the growth rate when the crack has its bifurcated tip and (d a /d N ) 0 is the growth rate when the crack is fully straight (reference situation). Both crack growth rates are obtained as the average value after applying 20 cycles loading. A small retardation factor implies a smaller crack growth rate and, therefore, a greater retardation effect. Such a factor (d a /d N )/(d a /d N ) 0 is represented in Fig. 4. The results show that the retardation effect increases (i.e., the previously defined retardation factor decreases) with the initial branch length and with the initial branch angle, and decreases with the SIF range. 4. Conclusions The following conclusions may be drawn about the effect produced by the near-tip fatigue crack path bifurcation on the plasticity-induced fatigue crack growth: Cracks with bifurcated tip in a plate subjected to remote tensile loading exhibit plastic crack advance in mode I (they show a trend to growth in the direction of the original crack), with retardation in the fatigue crack growth when compared with a fully straight crack (with no branching). The retardation effect increases with the initial branch length and with the initial branch angle, and decreases with the SIF range.