PSI - Issue 38

3

Jinchao Zhu et al. / Procedia Structural Integrity 38 (2022) 621–630 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

623

a

b

Fig. 1. Sketch of (a) Non-load carrying fillet cruciform joint; (b) Geometrical parameters.

2.2. Notch stress method with r ref = 1 mm and FAT 225 For welded joints with sharp notches, a fictitious notch radius r ref is introduced to replace the sharp notches (Radaj (1990)). The fatigue notch factor K f is assumed to be given by the stress concentration factor (SCF), K t at the notch computed using this fictitious notch radius (Rohani et al. (2021)), i.e. ( ) f t ref K K r = (1)

This fictious notch radius r ref can be written as

*

ref actual r r sr = +

(2)

where r actual is the actual sharp notch radius, s is a multiaxiality factor and r * is a material parameter named substitute microstructural length. An appropriate choice of r * for low strength steel can be derived based on experimental investigations and Neuber’s micro -support theory, resulting in r * = 0.4 mm (Sonsino et al. (2012)). For plane strain condition at the root of sharp notches, the multiaxiality parameter is set to s = 2.5 (Sonsino et al. (2012)), yielding sr * = 1 mm. A worst-case scenario is achieved by setting the actual notch radius to r actual = 0 resulting in a fictitious notch radius of r ref = 1 mm from Eq. (2) (Sonsino et al. (2012)). The notch stress range is therefore obtained from the applied nominal stress range S as ( ) r notch ef t 1mm r S S K = = (3) by computing the stress concentration factor due to the local notch K t ( r ref = 1 mm) at the fictitious notch radius using FEA. The fatigue life is determined according to (Hobbacher (2016))

m FAT 

 

6

2 10

N

= 

 

(4)

 

S

notch

A slope exponent of m = 3 is generally assumed for as-welded joints (Hobbacher (2016), Fricke (2012)). The FAT is described as lognormal distribution (Sonsino et al. (2012), Sonsino (2009)). The experimentally observed scatter of is used in the derivation of FAT 97.7% = 225 MPa (Sonsino et al. (2012)) and mean value and standard deviation of the distribution according to 10% 90% 1:[( / ] 1:1.5 T S S  = =