PSI - Issue 37

Zijah Burzić et al. / Procedia Structural Integrity 37 (2022) 269 – 273 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

272

4

da/dN [m/cyc] for ΔK [ MPa∙ √ m] 10 20 30 1.2 4∙10 -9 1.53∙10 -8 6.63∙10 -8 2.13 ∙ 10 -9 2.99∙10 -8 2.13∙10 -7

Specimen ΔK th [ MPa∙ √ m]

C

m

1 2

6.8 5.8

2.98 ∙ 10 -13 3.30 ∙ 10 -13

3.62 3.81

10 4

10 4

Gazela - 1 L-T

Gazela - 2 T-L

10 3

10 3

10 2

10 2

10 1

10 1

10 0 da/dN,  m/ciklus

10 0 da/dN,  m/ciklus

10 -1

10 -1

10 -2

10 -2

10 -3

10 -3

10 0

10 1

10 2

10 0

10 1

10 2

 K, MPa m 1/2

 K, MPa m 1/2

Fig. 6. Fatigue crack growth rate vs. stress intensity factor range

3. Structural life assessment To assess structural life here we use Paris law:

dN da =  

(1)

( ) m C K

The initial crack length is taken as 620 mm, Fig. 1, whereas the critical crack length is calculated, according to the case of an edge crack, using common linear elastic fracture mechanics equation: (2) where Y=1.12. If one takes into account position and direction of defects, Fig. 1, LT direction should be taken as relevant, so the fracture toughness value is 114.8 MPa∙ √ m (Table 1), and one gets a c =924 mm, for σ max =120 MPa, [15]. Now one should calculate number of cycles needed for crack to grow from 620 mm to 924 mm, under the amplitude loading estimated to Δσ max =20 MPa, producing ΔK=31.4 MPa∙√m for the initial crack length. Since the crack length does not change much from the initial to the critical value, one can take this value as a constant. In this case, it is simple matter to calculate the number of cycles representing the life of bridge steel construction using the following equation: a c =(K Ic /Yσ max ) 2