PSI - Issue 31

M. Mlikota et al. / Procedia Structural Integrity 31 (2021) 3–7 Marijo Mlikota et al. / Structural Integrity Procedia 00 (2019) 000–000

5

3

specimen) by using the submodelling technique. Fig. 1 shows a zoomed view of the investigated notch region in the microstructural model, giving von Mises stresses for the two investigated cases. Aside of the reduction of the surface area containing crack nucleation-favorable grains, it could be observed that the residual stresses move to a certain degree the crack nucleation sites from the surface to the subsurface regions (Fig. 1b), when compared to the pure outer loading case (Fig. 1a). This coincides with the experimental observations of de los Rios et al. (1995) and Mirzazadeh et al. (2012).

(a) (b) Fig. 1. Damage in a microstructural model of AISI 1141 steel (101 nucleated micro-cracks): (a) Outer loading of 224 MPa; (b) Superimposed outer loading (224 MPa) and compressive residual stresses. The analysis has been performed for different stress amplitude (ranging from 65 to 112 MPa). Eventually, by plotting the S - N ini curve of shot-peened specimen in Fig. 2 and comparing it with the non-shot-peened specimen, one can clearly see the influence of compressive residual stresses. Aside of the endurance improvement in the slope region of the S - N ini curve, a 5% higher fatigue limit has been achieved with the residual stresses, too; 160 MPa with respect to 152 MPa in the case of the model without the residual stresses.

Fig. 2. S - N ini curves of AISI 1141 steel showing the influence of compressive residual stresses.

A more extended and detailed description of the implementation of the TM equation into FEM-based modelling and simulation of the crack initiation process has been reported in publications of Mlikota et al. (2017, 2018, 2019, 2020), Glodež et al. (2010) and Jezernik et al (2010). 3. Conclusions It is apparent that the analysis provides a qualitative agreement with the experimental observations, i.e. that compressive residual stresses enhance the fatigue life of the investigated material. Namely, the definite life portion of the fatigue life curve (the slope) of such a structure is shifted to the right in the S - N ini diagram, while the infinite portion of the curve (fatigue limit) is shifted upwards.