PSI - Issue 68

Mihaela Iordachescu et al. / Procedia Structural Integrity 68 (2025) 1147–1152 Iordachescu M. et al. / Structural Integrity Procedia 00 (2025) 000–000

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3. Fatigue resistance and damage tolerance of the bar steel 3.1. Fatigue resistance

Fig. 2 shows the load – CMOD plots resulting from the post-fatigue fracture tests of FC1-FC5 cylindrical specimens, with the elliptical fronts attained by the crack along their fatigue tests being given in Fig. 3. These were revealed on the fractured surface of specimens by the heat tinting applied at the end of each fatigue step. The optically measured major and minor semi-axes, a and b, of each fatigue front were plotted in Fig. 4a and Fig. 4b as a function of the number of the applied fatigue cycles. These measurements allow the mean crack growth rate da/ D N at the deepest point of the crack front and the mean value of the stress intensity factor range D K to be computed for each fatigue step. Fig. 4c shows that both quantities are related according to the Paris-Erdogan law:

da dN

= C Δ K ( ) m

(1)

with constants m = 3 and C = 4.45 ・ 10 –12 MPa -3 m -1/2 /cycle, which reflect a typical fatigue resistance of the ferrite pearlite structural steels and significantly higher than that of martensitic steels (Barsom and Rolfe, 1999). In Eq (1) da/dN and ∆K are respectively, the crack growth per fatigue cycle and the stress intensity range per same cycle; then, if the load range is ∆F and the applied cycle correspond to the semi-axes a and b of the crack front, ∆K is:

⎛ ⎝⎜

⎞ ⎠⎟

Δ F π R 2

a 2 R

a b

(2)

Δ K = Y A

,

π a

Fig. 3. Fracture surfaces and crack fronts of the fatigue tested specimens: a) FC3; b) FC1; c) FC2; d) FC5; e) FC4.

Fig. 4. (a) a – minor semi-axis of the elliptical cracking fronts vs. number of fatigue cycles of tested specimens; (b) b – major semi-axis vs number of fatigue cycles; (c) Experimental Paris law obtained by testing FC1-FC5 specimens.