Page 64 - Biaxial Multiaxial Fatigue and Fracture

Multiaxial Fatigue Assessment of Welded Structures by Local Approach 49

torsion and random uniaxial tension for 3 different random spectrums. In the case of random
loading, the life is expressed as the number of blocks up to crack initiation (Nf). The results are
plotted with two straight lines indicating a deviation interval band between N$3 and 3 Nf. This
comparison makes it possible for the prediction to be assessed within an interval between Nf/4
and 4 Nf.

1E+06
Random tension, specmi Tens
1 E+05 Random tension, spectrum Bend
. Random tensmn, spectrum Tors
.cI 1E+04 - 0 Proporhonal tensiodtorsion
e!
.- X Non-proportional tensiodto
1E+03 -
a
;E 1E+02 -
E-
l E+O l

1 E+OO
1E+00 1E+01 1E+02 1E+03 1E+04 1 E+05 1 E+06
N, experimental

Fig. 6. Comparison of predicted lives against experimental ones

The highest deviations between predictions and experiments are obtained for non-
proportional loading and random amplitude. In the case of non-proportional loading, the most
important part of the error comes from the fatigue criterion used. In fact, several authors [9,10]
showed that, for non-proportional loading, the critical plane criterion taking account of the
maximum of the shear stress amplitude encountered during one cycle give inaccurate results.
The average of the shear stress on the facet over the cycle or the integration of shear stresses
over all planes may give better results. This concept developed in some multiaxial fatigue
criteria is the so-called integral approach. Considerations on how to integrate such a criterion
for random loading are necessary. In the case of random tension loading, the calculation is
always optimistic, because the accelerating influence of previous small amplitude cycles onto
the damage rate due to large amplitude cycles is not taken into account. To improve the
prediction, a more physical damage parameter such as cumulative micro-plasticity on each
facet or micro-cracks nucleation on each facet should be used.

APPLICATION OF THE LOCAL APPROACH TO WELDED COMPONENTS

This section details all the steps of the local approach procedure which are successively run on
in order to assess the fatigue lifetime of a welded mechanical assembly.
Welded joints are particular structural details characterised by:
- Stress concentrations induced by local shape of joint and manufacturing geometrical defects
produced by the welding process (angular deviation and misalignment of the connected metal
components),