Page 408 - Biaxial Multiaxial Fatigue and Fracture

392 M. FILIPPINI ET AL.

negligible (see Foletti and Passerini [ 15 I). This small difference may be appreciated only when
a full set of strain controlled experimental results is available, as in the case of the Inconel 71 8
alloy data [16,17].

EXPERIMENTAL RESULTS
Specimen, material and input loading histories

Different sets of proportional and non-proportional multiaxial strain controlled low-cycle
fatigue test results taken from the literature have been analyzed by the new proposal and by the
Sonsino-Grubisic approach. The results of multiaxial fatigue experiments camed out on
un-notched tubular specimens submitted to in- and out-of-phase axial and torsional loading,
conducted on the SAE 1045 steel [18], on a low carbon (MILD) steel [19] and on the Inconel
718 alloy [16,17], have been checked against the life predictions given by the criteria. All tests
on SAE 1045 steel and on MILD STEEL have been performed by imposing sinusoidal
proportional and non-proportional axial/torsional load paths without mean strains (cases A, B,
C, M and N in Fig. 4). In the case of the Inconel 718 alloy a fill set of data, including loading
paths with superimposed axial or torsional mean strains, is also available (all cases in Fig. 4).

Fig. 4 Axial-torsional loading paths

All biaxial tests have been conducted on a servo-controlled closed-loop system with
computer control and data acquisition using tubular specimens. The failure was defined as a
10% axial load drop from the previous logarithmic interval of data acquisition for any axial-
torsional test with a cyclic axial loading. For torsion-only histories or torsional histories with
static axial stress or strain, a torque drop was applied.

Comparison with experimental results

The life predictions given by the new criterion are presented in Fig. 5 for Inconel 718 alloy, in
Fig. 6 for MILD STEEL and in Fig. 7 for SAE 1045 steel (bottom part of the figures).
Predictions obtained by the Sonsino-Grubisic approach are also shown in the same figures
(upper Part).