PSI - Issue 57

Cheng Huang et al. / Procedia Structural Integrity 57 (2024) 42–52 Cheng Huang et al./ Structural Integrity Procedia 00 (2023) 000 – 000

49

8

250

Survival Failure

200

150

50 Alternating stress σ a,nom (MPa) 100

85 MPa

0

0

50

100

150

200

250

Mean stress σ m (MPa)

Fig. 5. Assessment of fatigue test results in a CLD.

4. Fatigue analysis using S - N diagrams S - N (stress-life) curves are widely used to represent the relationship between the stress range and fatigue life of conventional structural steels (Huang et al. 2019, 2022d). The basic S - N curve is expressed as:

(1)

-m N CS =

which, in double logarithmic coordinates, can be expressed as:

log log

log N C m S = −

(2)

where S is the stress range (i.e. Δ σ = σ max - σ min ), N is the number of cycles to failure, and C and m are material constants. In this section, the fatigue behaviour of the studied WAAM steel is assessed using S - N diagrams. The nominal stress ranges Δ σ nom of the tested as-built coupons are plotted against the number of cycles to failure N in log log coordinates in Fig. 16, where test data on the same WAAM material collected from Bartsch et al. (2021) are also depicted. Eq. (2) was fitted to all test data with finite fatigue lives ( N ≤ 2 × 10 6 ) in Fig. 6 by means of linear regression analysis; an inverse slope exponent of m = 2.9 was determined, which is very close to the value of m = 3 specified in EN 1993-1-9 (CEN 2005) and the IIW Recommendations (Bathias 2006; Sonsino 2007; Hobbacher 2016). Hence, an inverse slope of m = 3 was also adopted for the S - N curves for WAAM steel fitted herein. The fitted S - N curve is presented in Fig. 6, together with a scatter band corresponding to probabilities of survival P s = 2.5% and 97.5%, determined by assuming a log-normal distribution of the fatigue life N for each stress level. As can be seen from Fig. 6, the lower bound of the scatter band (P s = 97.5%) is very close to the fatigue class (FAT) of 80 MPa (i.e. a stress range at N = 2 × 10 6 ), which is a common FAT for steel welded joints (IIW 2016). A fatigue endurance limit of 170 MPa was also derived from the S - N diagram, in line with that predicted by the CLD method described in Section 4.