PSI - Issue 5

M.H. Hebdon et al. / Procedia Structural Integrity 5 (2017) 1027–1034 Liu et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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5

limit is about 40% of Δ σ C . For a constant amplitude stress range equal to or below the CAFL, the fatigue life is assumed to be infinite. For the variable amplitude loading, the CAFL does not exist and the slope coefficient m is specified to be 5 between the CAFL and cut-off limit. A ll cycles with stress ranges equal to or below Δ σ L can be neglected when performing a damage sum. Hence, the fatigue strength for nominal stress ranges is represented by a series of S-N curves, as follows:         3 3 6 5 5 6 2 10 , 5 10 , C D C L D N if N if                                (1)

a

b

500 1000

Detail category   C (MPa): 160, 140, 125, 112, 100, 90, 80, 71, 63, 56, 50, 45,40, 36   D =0.74   C

50 100

50 100

  L =0.4   C

5 10

Stress range (ksi)

Stress range (MPa)

10

10 4

10 5

10 6

10 7

10 8

10 9

Number of cycles

Fig. 5. FLM-3 in Eurocode (CEN, 2003).

Fig. 6. (a) S-N curves specified in Eurocode.

3.4. Detail category

For stringer-to-floor-beam connections, the fatigue damage accumulation initiate as a crack from the surface discontinuity at the toe or at the weld root, and then extend into the base metal or along the fusion boundary (Fricke, 2006). As each stringer-to-floor-beam connection has two stringers symmetrically connected to both sides of the web of a floor beam, the distortion-induced fatigue can be greatly prevented. Consequently, the connection depicted in Fig. 3 is respectively classified as category 80 MPa, according to Eurocode and reference (Kuehn et al., 2008).

3.5. Effective stress range and fatigue life

For the variable-amplitude loading, the constant stress ranges and its corresponding number of cycles can be extracted from stress histories. According to Eurocode, the damage factor D i of particular stress range Δ σ i can be derived from the Eq. (1), as shown in Eq. (2). Failure occurs when the damage factor D i reaches 1.0 according to the Miner’ s rule (Guo et al., 2013). Hence, the effective stress range can be derived, as shown in Eq. (3). Hence, the fatigue life of detail can be calculated as using Eq. (4), according to Eurocode.

3

5

  

  

  

  

n

n





i 

i 

D

if

if

,

;

,





  



    

 

  

i

i

(2)

i

i

D

L

i

D

6  

6  

5 10

5 10





D

D

1 2 3

     

    

  j 

3

    3 i i  n

j

     

j n n n 

      

D    



e    

(3)

i  

j