PSI - Issue 57

6

Author name / Structural Integrity Procedia 00 (2019) 000 – 000

Kalle Lipiäinen et al. / Procedia Structural Integrity 57 (2024) 785–792

790

Table 1. Material properties for 4R method application. Material Specimen Tensile strength [MPa]

Initial crack length [μm]

Residual stress [MPa]

Unnotched

825

50 50 50 50

0

CP

Shear cut hole 825

400

Unnotched

825

0

SPF

Shear cut hole 825

400

Fig. 7 gives a graphical illustration on the local cyclic behaviour, fatigue notch factors ( K f ) in the unnotched and notched specimens and the results of statistical analyses on the fatigue data. An intersection of the geometric notch surface obtained SCF and TCD-based (Point method, PM) SCF was found at the 0.05 (unnotched) and 0.09 mm (notched) de pths for the 50 μm flaw size (see Fig 7b). For reasonable quality-based fatigue strength assessment, the equivalent crack modelled SCF should be higher than with plain geometric notch (without modelling crack) or quality parameter is neglected from the analysis. The 0.04 mm stress averaging distance found the most suitable in (Lipiäinen, Ahola, Kaijalainen, et al., 2022), can be found suitable based on SCF plot as the effect of quality can be seen on both notched and unnotched specimens (Fig 7b vertical lines intersections). Scatter value of 4R reference stresses are given in Fig. 7c with fitting mean curve ( m = 3 and m = 5) on 4R reference stress data group. Slope of data group is indicated with black dashed line in Fig 7c. parameter is influenced by slope parameter and its direct comparison is compromised with different critical distances when slope parameter varies. However, analysis indicate that the most suitable critical distance for studied data group would be 0.01-0.04 mm.

a

b

c

8

1.5

0 1 2 3 4 5 6 7 8 9

Unnotched - Crack length 50 μm Notched - Crack length 50 μm Notched - Without crack

Stress

7

Slope parameter m

1.4

6

Right axis

5

1.3

Quality parameter exceeds geometry parameter

1.2 Scatter T σ [-]

Left axis T σ m = 5 T σ m =3

PM: K f [-]

4

3

Quality parameter non-existent

Slope parameter m [-]

2

Strain

1.1

Reasonable stress averaging distance

1

Reasonable stress averaging distance

1

0

0

0.02 0.04 0.06 0.08 0.1 Stress averaging distance a [mm]

0.00 0.02 0.04 0.06 0.08 0.10 Stress averaging distance a [mm]

Fig. 7. (a) Schematic local cyclic behaviour for unnotched base material and shear cut hole with similar loading, (b) fatigue notch factors with respect to the critical distances and (c) scatter of fatigue data evaluated with the 4R reference stresses as a function of critical distance. The fatigue life and loading from experimental test, fractography obtained equivalent crack length leading TCD based analysis are illustrated in Fig. 8 with non-filled symbols. Supplementing data with residual stresses and local stress ratio are used to plot 4R master curve (4R reference stresses) in Fig. 8 with solid symbols. Plain TCD and 4R are connected with R local parameter and when R local < 0, the 4R reference stress is reduced from non-corrected TCD value.