Page 74 - Biaxial Multiaxial Fatigue and Fracture

Multiaxial Fatigue Assessment of Welded Structures by Local Approach 59

Influence of the transition radius

The preponderant parameter in welding quality is the transition radius at the weld toe and at the
weld root. Indeed this parameter primarily determines the maximum stress concentration factor
of the welded joint. The other micro-geometrical parameters, namely the width of the bead and
the height of the weld dome, are secondary.
Figure 16 shows the influence of the transition radius on the predicted alternate tensile
fatigue strength of butt welds, assuming mean geometrical parameters and also intermediate
level of residual stresses between the maximum and minimum measured values.

100

80
n
z.
60
u)
e
c
!! 40
E .-
5
20
-r = 5.9 mm
-r = 10 mm
0
1 E+3 1 E+4 1 E+5 1 E+6 1 E+7
Number of cycles

Fig. 16. Influence of the transition radius (uniform radius)
on alternate tensile fatigue curves ( R = -1) of butt welds

The transition radius is clearly shown to be a strongly influent parameter regarding to the
fatigue strength of the butt weld. On the contrary, a treatment allowing to obtain large
transition radii, as grinding for instance, makes increase the fatigue life by a factor of 3
approximately with respect to the average quality level.
It can be noted that, within the assumptions stated in this study (mean level of residual
stresses and average geometry), the tests results fairly fit with the predicted lives obtained with
the mean transition radii minus the standard deviation.

CONCLUSIONS
A multiscale approach has been developed to design against fatigue structures subjected to
multiaxial constant or variable amplitude loading. The evaluation of the lifetime starts from the
knowledge of the service conditions of the whole structure (Le. operating loading spectrum)
and requires to assess the stress states in its critical zones. Stresses are actually induced both by
the external loading and the local geometry. They may be severely increased because of stress
concentration generated by geometrical discontinuities and angular or axial misalignment.
The main step of the proposed fatigue evaluation procedure is the determination of the local
effective stresses as these govern the fatigue phenomenon, Le. the crack initiation. An elastic-