PSI - Issue 42

766 Miloslav Kepka jr. et al. / Procedia Structural Integrity 42 (2022) 762–768 Author name / Structural Integrity Procedia 00 (2019) 000 – 000 5 Since the FAT classes are for the R = 0.5 value, the testing is recommended for this stress ratio. However for well welded samples, this value of the stress ratio does not appear to be suitable, since can reach yield strength of the base material. For this reason, at our workplace we recommend testing for a value of = 0 and subsequently reducing the FAT values by 20 %. In order for the method to be applicable, there must be no initiation at the root of the weld as in Fig 6.

Fig. 6. Example of not fully penetrated weld.

3. SN lines To evaluate the SN lines, a linear model was chosen. The outputs of the regression analysis are the mean SN lines with a 50% probability of failure. The number of cycles until the occurrence of a macroscopic fatigue crack is expressed as a function of the range of the applied force during the test or, after conversion, as a function of the range of the hot-spot stress: log = − log ∆ ℎ − , Where is number of cycles to macro crack and ∆ ℎ − is range of hot spot structural stress. Table 1. SN lines of tested samples

Base material

treatment

Coefficient q

Coefficient m

so-called fatigue limit in 10 7 cycles [MPa]

S460 MC S460 MC S235 JR S235 JR

As welded

16.8 21.3 15.3 16.3

4.4 5.9 4.0 4.3

167 274 113 146

HFMI

As welded

HFMI

The requested life of the railway vehicles is usually 30-40 years according to the customer's request. Therefore, design to infinite life against so-called fatigue limit in 10 7 cycles can be used. These values can be directly compared to maximums stress range values of random operating stresses.