Aluminium Design and Construction John Dwight

Figure 11.5 Weld failure planes: (A) weld metal; (B) fusion boundary; (C) HAZ.
For each of these there are three possible planes on which failure can
occur (Figure 11.5):
A. Joint failure in the weld metal;
B. Joint failure in the HAZ at the edge of the weld deposit, known as
fusion boundary failure;
C. Failure in the HAZ at a small distance from the actual weld. This
mainly applies to heat-treated 7xxx-series material for which there is
a dip in the HAZ properties at the position concerned (Figure 6.2).
The resistance must be checked on all three planes, except with a full
penetration butt under transverse compression for which it is only
necessary to consider plane C. Note that failure on plane C has already
been covered under member design in Chapters 8 and 9. Here we just
consider A and B.
11.3.2 Basic checking procedure
Looking first at the loading cases (a) and (b), the basic procedure for
checking a weld against failure on plane A or B is as follows:
1. Find the force P – (transverse or longitudinal) transmitted per unit length
of weld, when factored loading acts on the structure (Section 11.3.3).
2. Obtain values of the calculated resistance P – c per unit length of weld
(transverse or longitudinal as relevant), corresponding to weld metal
failure (Section 11.3.4) and fusion boundary failure (Section 11.3.5).
3. The weld is acceptable if, at any position along its length, the following
is satisfied for both the possible failure planes:
(11.11)
where � m is the material factor (Section 5.1.3). Weld strength is notoriously
difficult to predict, and a designer should resist the temptation to use
too low a value for � m . British Standard BS.8118 requires it to be taken
in the range 1.3–1.6 for welded joints, depending on the level of control
exercized in fabrication. The value � m =1.3 is permitted when the procedure
meets normal quality welding, as laid down in Part 2 of the BS.8118
(Section 3.3.5).
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