Page 70 - Biaxial Multiaxial Fatigue and Fracture

Multiaxial Fatigue Assessment of Welded Structures by Local Approach 55

The precise measure of the characteristic geometry of these welded components makes it
possible to accurately determine the local stress states existing within them. This is of major
importance as these stresses are responsible for the local fatigue behaviour of the welds.

ANALYSIS OF THE EFFECT OF THE WELD QUALITY ON THE FATIGUE STRENGTH

The local approach method was applied to welded joints in aluminium alloy to analyse the
effect of the welds quality upon the fatigue strength. The examples presented here concern
butt-welded joints in 7020 aluminium alloy of 20 mm thickness. The welds were obtained by
using multi-pass MIG process with a 5183 filler metal. Figure 13 shows a macrograph of the
weld.

Fig. 13. Aluminium alloy butt welds with the fatigue crack occuring in the vicinity
of the weld toe

Material properties and geometrical parameters

Mechanical properties. Fatigue cracks always start at the weld toe or at the weld root. The
material parameters of these zones must therefore be determined and used for the local
approach. In order to determine these parameters, the metallurgical state at the toe of the bead
is reproduced by an overheating thermal treatment on test samples. The material parameters
that were obtained by this way are given in Table 5.

Table 5. Mechanical properties of heat affected zones of aluminium samples
Elasticity parameters Cyclic plasticity parameters Fatigue parameters
E V R3 ca c Q b &f b P
(MPa) (MPa) (MPa) (MPa) (ma)
7.3.104 0.3 200 lo4 150 100 50 670 -0.095 2.3.10-3