Issue 48

K. Kimakh et alii, Frattura ed Integrità Strutturale, 48 (2019) 429-441; DOI: 10.3221/IGF-ESIS.48.41 435 In our case, the crack propagation area presents ridges in the direction of crack propagation that diverge from the crack origin. In fact, the crack don’t initiate from a point, as seen regularly, but it occurs from a curvilinear line. These line can also be observed from the global observations of surface specimens, as shown in Fig. 5. They are in the form of bands depending on the roughness of the specimens. These are scratches generated by the cutting tool. They represent the streaks of roughness. Fig. 5 focus on crack propagation area and the crack origin. The macrographs on the left sides of the Fig. 5 display the morphologies of the crack propagation areas in three exemplified specimens, as well as the spots of crack initiation situated inside the black frames. These images were taken by using a SEM microscope at the magnification 100x. Mechanism of the steel specimen cracking and crack origin can be analyzed on the basis of the micrographs taken by SEM microscope at higher magnification (800x) and presented on the right sides of Fig. 5. In the case of Specimen Nr 6 its micrograph (a) indicates mainly plastic (ductile) mechanism of material cracking in the form of plastic striations. However, several places on the fracture surface show brittle mechanism of cracking along cleavage planes. This mechanism of cracking is associated with nonmetallic precipitates existed in the AISI 1045 steel. In Specimen Nr 6 crack has been initiated in the spot of a precipitate existed near the bottom of a specimen surface scratch (white frame). In the case of Specimen Nr 8 the micrograph (b) indicates that the crack developed from a deeper slot made by the tool lathe. This micrograph indicates as well more voids after cracking through the boundaries of the precipitates existed in the steel. More impurities in the steel material and impurities of greater sizes are observed on the micrograph belong to Specimen Nr 12. This state of specimen material was confirmed both by its macrograph and the micrograph (c). The crack initiated in a deeper slot made by the tool lathe. The streaks can be considered as notches generating stress concentrations. Depending on the level of these concentrations, the lifetime of the specimens can be reduced or lengthened [17]. To characterize the fatigue damage of specimens, we measured for the different surface roughness the width of the streak (l defect ) and the radius of the crack propagation area r ZPL (Fig. 6). Figure 6 : Fracture surface diagram. Figure 7 : Definition of depth a (a= Ra or a= Ry) and pitch 2b of the roughness profile, l defect represents the depth of the surface defect generating the fracture. Tab. 6 groups the values of the streaks width recorded of each surface roughness (Fig. 5). Their values are very closer to those of the total roughness Ry which represents the difference between the highest peak and the deepest valley of the profile. These widths always remain lower to Ry: it is unusual to find a roughness profile where the highest peak R Z2 and the deepest valley R Z1 correspond to the same streak. As shown in Fig. 7, the values of R Z1 and R Z2 are always smaller than Ry. Also, we measured the radius of the crack