Page 92 - Welding of Aluminium and its Alloys
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Welding design     81





                                                          Actual throat


                     Reduction in                               Expected throat
                     throat











                    5.13 Effect of an irregular cut edge on the fillet weld throat thickness.



             the two weld types (Fig. 5.5c). The T-joint is a simple, easily designed weld-
             ment which, except in the case of the T-butt, requires little or no edge prepa-
             ration. The accuracy of the fit-up depends to a great extent on the accuracy
             with which the edge of the upright of the T is prepared. An irregular cut
             will give a variable gap, which may result in an inadequate throat thickness
             as illustrated in Fig. 5.13.
               A fillet weld can present difficulties in achieving full penetration into the
             root,resulting in a void being formed in the corner.This is regarded as unde-
             sirable, particularly in critical applications, as this lack of fusion acts as a
             stress raiser in the root and also reduces the throat thickness. The welder
             needs to be made aware of this problem as the main cause is incorrect
             welder technique.
               If the T-butt is a flanged joint, sufficient metal must be left that the weld
             does not melt away the corner of the flange and to allow for variations in
             fit-up. As a rule of thumb, some 2.5 to 3 times the fillet weld leg length is
             regarded as being adequate (Fig. 5.14). This may not prevent the edge of
             the flange from buckling due to distortion from the heat of welding, par-
             ticularly where the fillet size is large in proportion to the flange thickness.
               The strength of a fillet welded T-joint is determined by the shear strength
             of the fillet weld or welds, the strength of a butt-welded T-joint by the
             strength of the weld metal or the HAZ. If the joint is subjected to trans-
             verse shear loads the bending stresses in the joint can lead to premature
             failure, particularly if the joint is fillet welded on one side only. Fillet welds
             on both sides or a full penetration T-butt joint will permit substantially
             increased loads before failure occurs (Fig. 5.15). Dynamic performance of
             T-joints is not good: the change in section from the horizontal to vertical
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