Page 73 - Biaxial Multiaxial Fatigue and Fracture

58 l? LABESSE-JIED ET AL.

Influence of angular distortion defects

The effect of geometrical defects is more difficult to analyse and must be looked at on a case-
by-case basis. Generally speaking, an axial misalignment or an angular distortion creates a
supplementary bending moment, which induces some compressive stresses on one side of the
joint and some tensile stresses on the other side. The effect of these static stresses on the
fatigue strength of the welded joint depends on the loading and on the micro-geometry of the
bead:
- generally, when the geometrical defect induces stresses opposite to those induced by the
service loading, some improvement in fatigue strength can be expected,
- when the micro-geometry is symmetrical with respect to the mean fibre, the geometrical
defect causes a reduction in the fatigue strength of the welded joint,
- when the transition radii are smaller on the side on which the geometrical defect induces
compressive stresses, then the defect makes improve the fatigue strength of the welded joint.
On the contrary, i.e. with tensile static stresses applied on the smallest transition radii side, the
fatigue strength of the welded joint is reduced.
To sum up, the effect of geometrical defects may be positive or negative. Well-controlled
defects could in principle contribute to improve the fatigue strength of welded joints.
The effect of angular distortion is quantified by calculation of the fatigue strength of butt-
welded joints subjected to alternate tensile loading. The direction of the modelled distortion, in
relation to the geometry of the joint, corresponds to the direction that was measured, namely a
closing of the welded joint on the root side.
The S-N curves of Fig. 15 show the importance of angular distortion upon the predicted
fatigue strength of welds. All the calculations are realised with the average geometry and the
mean residual stresses. The highest fatigue strength is obtained with a zero angular distortion.
The general trend is that the fatigue strength decreases as the angular distortion increases. Most
of the experimental test results are close to the predicted fatigue strengths obtained with
considering the average measured angular distortion.

-angular distorsion = 3", axial = 0.5 mm
--angular distorsion = 1.4', axial = 0.5 mm
-angular distorsion = 0.7", axial = 0.5 mm
*jt -angular distorsion = O", axial = 0.5 mm
" ,
1 E+3 1 E+4 1 E+5 1 E+6 1 E+7
Number of cycles

Fig. 15. Influence of angular distortion on the predicted alternate tensile S-N curve (R = -1)
of butt welds