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.
, 106
c
270
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