Page 71 - Biaxial Multiaxial Fatigue and Fracture

56 E LABESSEJIED ET AL.

Mean residual stresses. The mean residual stresses were measured on the surface of the toe of
the bead with means of X-Rays diffraction. They reach 25 MPa and 37 MPa in the transversal
and longitudinal directions respectively.

Mean geometricalparameters. Experimental comparisons are made on the basis of the average
geometrical parameters measured on welded test specimens. Table 6 summarises these
geometrical parameters.

Table 6. Geometrical parameters of the aluminium alloy butt welds

Tor, bead Root bead
bin ~pl: S Rmin Lrc S
Mean value (mm) 2.67 34.03 2.94 3.69 28.40 3.64
Standard deviation (mm) 1.55 3.00 0.59 2.20 2.43 0.68

Moreover, the average axial misalignment of 0.5 mm and angular distortion of 0.7" are
taken into account for the fatigue life assessment.
The following sections describe the sensitivity of the butt welds, regarding their predicted
fatigue life under constant amplitude loading, to residual stresses, angular distortion and
transition radius.

Imjluence of residual stresses

Residual stresses are known for their determinant role on the fatigue strength of materials,
including of course welding joints. This influence depends on the local sensitivity of the
material to hydrostatic stress. Fatigue properties, determined from results of fatigue tests
performed on the overheated metallurgical state corresponding to the weld toe, indicate an
important sensitivity to hydrostatic stress.
This sensitivity can also be quantified by the ratio of fatigue strengths obtained under
alternate tensile test and alternate torsion test respectively, for a given number of cycles. The
smaller this ratio is, the more the material is sensitive to the residual stresses. A material
insensitive to the hydrostatic stress effect has the following strength ratio:

The present overheated material located at the weld toe provides the following fatigue strength
ratio, at 1 o6 cycles:

It is important to note that this ratio is about 1.6 for high-strength low alloy steels
hardendtempered at high temperature. This indicates a lower sensitivity of these steels to
residual stresses.