Page 65 - Biaxial Multiaxial Fatigue and Fracture

50 F LABESSE-JIED ET AL.

- Metallurgical transformation resulting in local changes of the mechanical static and fatigue
properties,
- Residual stresses induced by a non-homogeneous cooling of the welded components.
The purpose of the procedure included in the local approach software is to take these
peculiarities into account in the fatigue calculation. The approach is local as the fatigue life
prediction is made from the local stress states in the most critical welded area. Studies
concerning the influence of these three peculiarities showed that the stress concentration factor
is the most important parameter to be considered. Figure 7 shows the stress concentration on
the weld toe or on the weld root of a butt joint when it is subjected to axial loading. The stress
concentration can easily exceed a factor equal to 2.

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Fig. 7. Stress distributions within a butt-welded joint: (a) with angular distortion; (b) without
angular distortion.

Geometrical defects modify also the stress distributions and have consequently to be taken
into account. Frequently geometrical defects are the angular distortion of a butt-welded joint
and the misalignment of a cruciform welded joint as shown in Fig. 8. The importance of the
angular distortion is illustrated on Fig. 7; as it shows that the location of the weakest point of
the weld moves from the weld root (b) to the weld toe (a) when the angular distortion is taken
into account for the stress calculation. The strong modifications regarding the stress
distributions are explained by a significantly different change of the applied loading. For
example, an angular distortion often generates a static bending moment when the welded
sample is installed and clamped in the jaws of the fatigue testing machine. This bending
moment is due to the fact that the sample is straightened because of jaws' alignment. This load
is induced in fact by boundary conditions and is superimposed to the fatigue loading.
The stress concentration is represented by the matrix [K] associated with the stress
concentration located at the critical area. This critical zone may be the weld toe or the weld
root. It depends in fact on the particular conditions as loading and local geometry of the case
encountered. The partial penetration at the weld root can be interpreted as a geometrical defect
too. The corresponding stress concentration factors are calculated from Finite Element
Analysis of the real geometry of the weld subjected to the given particular loading. Figure 9
shows as an example the calculation results of the [K] stress concentration matrix.