Issue 33

D. Shiozawa et alii, Frattura ed Integrità Strutturale, 33 (2015) 56-60; DOI: 10.3221/IGF-ESIS.33.07 57 characterisation by synchrotron X-ray tomography), providing a chronology of crack development with this loading mode both at the surface and in the bulk of the material. The observed crack paths are analysed in light of the mode I, II and II stress intensity factors obtained from finite element simulations. E XPERIMENTAL SET UP he material used in this study is a cast Aluminium alloy A357. The fatigue samples were cut from the central part of cylindrical rods (30 mm diameter) obtained by permanent mold casting. After a T6 treatment (solution treatment at 540°C for 10h followed by water quench and 8h at 160°C) the average Secondary Dendrite Arm Spacing (SDAS) was found equal to 38 μm and the average grain size to 501 μm; the value of the 0.2% yield stress obtained from tensile tests was 275 MPa. Macroscopic Torsion fatigue specimens were machined according to ISO-1352- 2011 Standard [5] and reversed torque-controlled fatigue tests were carried out using a MTS 809 Axial/Torsional servohydraulic machine at a frequency of 10 Hz. A polished flat surface was produced along the cylindrical sample gage length in order to monitor crack development by in situ optical microscopy. Electron Back Scattered Diffraction observations of this surface were performed post mortem . In situ reversed torque controlled torsion tests were also carried out at the European Synchrotron Radiation Facility (ESRF) on beamline ID19 on smaller samples (3mm diameter) in order to monitor crack initiation and growth in the bulk using X-ray tomography. The samples were cycled in situ with a dedicated rig and observed under load at the two extreme values of torque (+  max and –  max ). The sample shown here had been submitted to 195 kcycles at |  max |=120 MPa before a first imaging scan was performed. Then a scan was recorded every 10 or 20 kcycles until 305 kcycles. E XPERIMENTAL RESULTS AND DISCUSSION the first scan at 195 000 cycles at  a =120 MPa, vertical cracks ( ie cracks which intersection with the sample surface is aligned with the cylindrical sample axis) and horizontal cracks ( ie crack perpendicular or nearly perpendicular to the sample axis) were observed. An optical image of one horizontal crack is shown on Fig. 1. Figure 1 : Optical micrograph of a mode II intragranular crack fatigue crack observed after 100 000 kcycle (  max =95 MPa). The sample axis is vertical on the figure. The plane of the crack correspond to a {111} plane the white lines show the grain boundaries with two neighbouring grains. Figure 2 : 3D rendering of a crack perpendicular to the sample axis similar to the one shown in Fig. 1 at two different numbers of cycles.(195k cycles and 215k cycles). T O