Page 103 - Biaxial Multiaxial Fatigue and Fracture
P. 103

88                         L. SUSMEL AND N. PETRONE


            EXPERIMENTAL DETAILS
            Fatigue  tests  were  carried  out  on  solid  cylindrical  specimens  subjected  to  both  in-phase  and
            out-of-phase bendingkorsion  loadings. The  material  used  in  this  investigation was  aluminium
            6082 T6,  supplied in  30-mm-diameter bars.  The material  chemical  composition is  reported  in
            Table 1. The bars were produced by means of  an extrusion process, which introduced a certain
            degree of anisotropy by orientating grains mainly along the bar axis.
              The specimen (Figure 3) was machined in a CNC lathe and its 50mm long gauge length surface
            was successively polished down to a 6-pm diamond compound to obtain a mirror-like finish. The
            average values of the experimentally determined mechanical properties of the used material, along
            the longitudinal extrusion axis, were: tensile strength 343 MPa, yield stress 301 MPa and Young’s
            modulus 69400 MPa. In Figure 4 it has been plotted the lowest axial stress-strain curve recorded
            from a test conducted to evaluate the material static properties. Strains reported on this diagram
            were directly measured by means of an axial/torsional extensometer.
              The  fully  reversed  bending  and  torsion  moments  were  generated  by  using  two  hydraulic
            actuators, which loaded the specimen by means of  a friction clamped loading arm (Figure 5). The
            contemporary use  of  a  LabVIEW  software  and  two  MTS  407  digital  controllers  allowed  to
            generate and control the applied forces during each test; axial and shear strains were monitored by
            means of  strain gages (Figures 3 and 5) applied at known positions with respect to the nominal
            maximum bending stress point. Signals were gathered by a National Instruments SCXI-1000DC
            data acquisition system. Length and orientation of the macro-cracks were measured by means of a
            Leika stereoscope. Finally, the fatigue failure was defined as 2% bending or torsion stiffness drop
            and the frequency of each test was equal to 4 Hz.

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            Fig. 3.  Specimen geometry (a) and picture of a polished specimen with rosette (b).
            Table 1. Chemical composition of the tested 6082-T6 aluminium alloy [ %].

                 Si      ME       Mn        Fe       Cr       Zn       cu       Ti
                                       ~    ~~
               0.9;l.l   0.8~1.0  OSi-0.9   0.5     0.25     0.20      0.1     0.10
   98   99   100   101   102   103   104   105   106   107   108