Issue 48

K. Kimakh et alii, Frattura ed Integrità Strutturale, 48 (2019) 429-441; DOI: 10.3221/IGF-ESIS.48.41 436 propagation area r ZPL from fracture surface morphologies Fig. 3. It decreases with the increase of the roughness, thus the crack propagation surface is larger for the low roughness test specimen. Indeed, with the increase of surface roughness, the width of the streak l defect increase. However the radius of the crack propagation region r ZPL decrease and the number of cycles to failure reduced (Tab. 6). Table 6 : Streaks width and radius of the crack propagation region for different surface roughness. F ATIGUE LIFETIME PREDICTION Prediction Model everal researchers have tried to estimate by different models the fatigue limit of materials in the case of inclusions or surface defects [4, 17,18]. In our case, we were interested to Yukitaka Murakami model that established the inclusion area parameter in the surface defect [19]. This model allows the evaluation of the equivalent defect size area that present the surface roughness defect, which will provide to predict the fatigue limit σ w . According to this study, the equivalent defect size was expressed by the following equation: 2 3 area 2.97 3.51 9.74 ,                     0.195  2 2 2 2 2 a a a a b b b b b                       (1) area 0.38,                                                         0.195  2 2 a b b   (2) The Geometric parameters a and 2b were determined from the roughness profile as shown in Fig. 7. Based on the equivalent defect size, Murakami proposed a model to predict the fatigue limit σ w [MPa] by the following relation:     1/6 1.43 120 1 . 2 w HV R area           with 4 0.226 *10 HV     min max S R S  HV: vickers hardness [kgf/mm 2 ]. Specimen Ra(µm) Ry(µm) l defect (µm) r ZPL (mm) N f (cycles) N12 3.12 21.62 17.62 1.34 3.63 10 5 N8 2.26 17.23 16.71 1.42 3.72 10 5 N6 1.24 8.65 7.05 1.54 6.22 10 5 S