Issue 41

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 41 (2017) 191-196; DOI: 10.3221/IGF-ESIS.41.26 193 Figure 1 : Miniature dog bone fatigue samples (cross section 0.8x0.8 mm 2 ) extracted at the surface of the forged materials from the cold forged bars (materials I and II) (a) or from the hot forged connecting rods (materials II and IV) (b). Artificial defects that act as the initiation point of the fatigue cracks were introduced at the sample surface by laser machining as described elsewhere in the case of cast iron [4]. In this study the notch obtained has the shape of a narrow wedge of dimensions (depth/width/opening): 50x100-150x5 µm 3 . It is shown schematically on Fig. 2. In-situ fatigue tests were performed on ID19 beamline at the European Synchrotron Radiation Facility (ESRF). A ”pink” X ray beam [5] with a photon energy of 60 keV is used with a Pco Edge CCD camera (2160 x 2560 pixels). The samples were cycled in situ using a dedicated fatigue machine mounted onto the rotation stage of the beamline [6]. Once crack initiation was detected (by inspection of the radiographs of the sample under load) tomographic scans (2000 projections, exposure time of 0.07s duration 3.68 min) were recorded regularly after a given number of cycles. The samples were scanned under maximum load in order to improve crack visibility. Uniaxial fatigue tests were carried out in pull-pull loading conditions with R=0.1. Reconstruction of the tomographic data was performed with a standard filtered back- projection algorithm. A 0.65 µm voxel size was obtained. Fiji and ParaView open softwares were used for post-processing the 3D images. Figure 2 : Left: Projected view (along the loading direction) of the crack shape for material IV (hot forged + shot blasted) after 66kcycles fatigue cycles at σ max = 450 MPa R=0.1. The blue rectangle highlights the shape of the laser notch. Right: reconstructed slice showing the crack shape a few micrometers below the surface (loading direction horizontal).