PSI - Issue 7

Dalila Dimaggio et al. / Procedia Structural Integrity 7 (2017) 198–205 D. Dimaggio et al. / Structural Integrity Procedia 00 (2017) 000–000

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5

The solid black line with k=3.94 in Figure 3 represents the results obtained by constant amplitude loading at a stress ratio R=0 (already mentioned in Figure 2), while the dotted line shows a fictive line for damage calculation and is described in more detail in [5]. The orange line represents the Gaßner line obtained through application of the Palmgren-Miner rule on validation tests spectrum (more details are reported in §4). A detailed description of the Gaßner line can be found in [5] and [6]. 3.3. Influence of the mean stress In Figure4 the Haigh diagram for notched specimens is shown (stresses are normalized to the knee-point of R=0 axial test results). The endurable stress amplitude for P s =50% is plotted as a function of the mean stress for selected number of cycles. In the considered case of application only the lines for N>10 6 are of interest. The means stress sensitivity factors M 1 and M 2 could be calculated to M 1 and M 2 by using equation (1).

( ) ( ) 2 1 R

( ) ( ) 1 2 R R

σ

−

σ

with R 1

M R a =

a

σ

−

σ

m

m

All lines in the Haigh diagram for N≥ 10 6 are parallel since the slopes are constant to be k*=44.9 as shown in Figure 2. Since there is no possibility to perform tests in the range 0.70.7. The static strength of a notched specimen, σ SK , is reached when either the fracture elongation is exceeded on the notch surface or the cross section in the notch is fully plasticized [9].

Figure 4: Haigh diagram at T=120°C for notched specimens

4. Fatigue life assessment method To determine the fatigue life of a component the state of elastic local stress has to be compared with the local endurable stress that depends mainly on the local mean stress and the stress distribution (related to HSV). The HSV 90% approach according to [7] is defined as the volume in which the local stress is within 90% of the maximum stress. In order to get a more generalized dependency (geometry and load type independent) between fatigue strength and highly stressed volume for the investigated steel, the same exponent and reference volume HSV 0 =30mm 3 evaluated in [8] for a similar material on a larger basis of test data (different geometries, notch factors and applied load) has been used. Therefore the HSV 90% -line from [8] is shifted parallel to fit the actual test results at different numbers of cycles as described in Figure 5(a) . The supportable stress amplitude σ va can be described with Equation (2): ν σ σ  = 90% ,0 HSV va va for HSV 90%

  

 

HSV

0

for HSV 90% ≥ HSV 0

va σ =

σ

(2)

,0 va