PSI - Issue 54

5

R.F. Fernandes et al. / Procedia Structural Integrity 54 (2024) 300–306 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

304

ĂͿ

ďͿ

175 150 125 200

175 150 125 200

AB

AB

SR

SR

100

100

Ds [MPa]

Ds [MPa]

75

75

10 4

10 5

10 6

10 7

10 4

10 5

10 6

10 7

Number of cycles to failure, N f

Number of cycles to failure, N f

Fig. 3. Fatigue results for both as built (AB) and stress relief (SR) with: (a) semicircular notch and (b) hole notch.

The fatigue behavior of the hole notched specimens is shown in Figure 3b). In this case, the SR series demonstrated a higher fatigue strength, especially for a shorter fatigue life. For a higher fatigue life, there is a convergence in the fatigue behavior for both the series. Considering the life 10 5 cycles, the SR series showed an improvement of 10%. To provide a better discussion of the results, it is crucial to investigate the effect of stress relief on residual stresses. This subject will be discussed in the following section. 3.3. Residual stresses A preliminary study of residual stresses was conducted to compare their behavior between the AB and SR series, within a thickness range of 0-500 µm. Both series exhibited the same level of residual stresses at the surface of the specimen, approximately 50 MPa. However, in the AB series this value increased throughout the depth reaching almost three times the surface value at a depth of 500 µm. On the other hand, the SR series effectively reduced the residual stress along the thickness of the specimen. This resulted in a value approximately three times lower at a depth of 500 µm. In order words, the residual stresses were homogenized and maintained a stable value throughout the thickness in the SR series. Therefore, the homogenization of the residual stress has been a key factor contributing to the increase in the fatigue strength observed in the SR series. With the reduction and stabilization of the residual stresses throughout the thickness of the specimen, the stress relief has contributed to the mitigation of detrimental effects of residual stresses on the fatigue behavior, leading to improved fatigue strength. 3.4. Fatigue life predictions Fatigue life predictions of notched specimens were calculated using a modified version of SWT parameter (Smith, 1970), expressed as: = × , (1) here, eff represents the effective stress, and ε is the deformation amplitude. The use of effective stress in this work improved the quality of the predicted values, and its value is determined based on the nominal stresses amplified by the fatigue stress concentration factor (K f ).