Page 72 - Biaxial Multiaxial Fatigue and Fracture

Multiaxial Fatigue Assessment of Welded Structures by Local Approach 51

The analysis of the influence of the residual stresses level on the fatigue strength of butt
welded joints subjected to alternate cyclic load has been carried out by the proposed local
approach. These calculations were done with the mean measured geometry of the welds.
Figure 14 shows the influence of the residual stresses on the fatigue strength of the butt
welds. The different curves are examined from the left side to the right side, Le. in the
increasing order of the fatigue strength.

experimental results -nil SIG-res
-maximis& SIG-res -minimum SIG-res measured
--maximum SIG-res measured -SIG-res shot peening
1 1 ! l!l!l 1 I I lI!/l/ i I I i I Ill! I I l l
1 E+4 1 E+5 1 E+6 1 E+7 1 E+8
Number of cycles

Fig. 14. Influence of residual stresses on predicted alternate tensile fatigue strength (R = -1)
of butt welds

The lowest S-N curve has been obtained with the maximised residual stresses, it is to say
=
=
qOng otrans 140 MPa. The second S-N curve was plotted by considering the maximum
measured values of residual stresses which correspond to: qOng = 70 MPa and om,,, = 60 MPa.
These two assessed S-N curves are found to be quite conservative. The minimum measured
residual stresses (along = 15 MPa and om,, = -20 ma) lead approximately to the same
simulation results as the calculation carried out accounting for no residual stresses. These
assumptions seem to be a little optimistic regarding to the experimental S-N curve. The last S-
N curve was obtained assuming that residual stresses are compressive and reach the maximum
absolute value, i.e. along = otran, = -100 MPa. This assumption brings quite non conservative
predictions. It is similar in fact to the effect of pre-stressed shot peening.
As a conclusion, there is clearly a very considerable effect of residual stresses on the fatigue
strength of aluminium alloy welded joints. The influence is beneficial in the case of
compressive residual stresses and detrimental in the other case. A pre-stress shot peening type
finishing treatment makes strongly increase the fatigue strength of welded joints.