3.4.2. Three thicknesses stack
The capability of the NDT technique for three thicknesses (3t)
stacks, which commonly occur throughout vehicle structures, was
also evaluated through the floor assembly. As a 3t stack has two
interfaces, different gates with a tolerance range correlating with
the total thickness of each interface need to be set. The C-scan
image for each interface is obtained simultaneously and stored in
a separate channel. Fig. 19 shows the C-scan and peeled results
for a 3T stack with three 1.5 mm sheets. The C-scan images,
scanned from the top ground surface, suggested different sized
weld nuggets for the two interfaces; which were confirmed by the
peeled plugs. The difference in weld nugget size is attributed to
the MFDC process; where more heat is generated at the anode
compared to the cathode electrode due to directional current flow
and the Peltier effect. This additional heating can have the effect
of drawing molten metal towards the anode electrode as the weld
nugget develops, in particular for 3t stacks. The C-scan images
obtained are accurate in showing different weld nugget diametersat the two interfaces, which provides confidence that the technique
can be used for more complex 3t stacks.
3.4.2. Three thicknesses stack
The capability of the NDT technique for three thicknesses (3t)
stacks, which commonly occur throughout vehicle structures, was
also evaluated through the floor assembly. As a 3t stack has two
interfaces, different gates with a tolerance range correlating with
the total thickness of each interface need to be set. The C-scan
image for each interface is obtained simultaneously and stored in
a separate channel. Fig. 19 shows the C-scan and peeled results
for a 3T stack with three 1.5 mm sheets. The C-scan images,
scanned from the top ground surface, suggested different sized
weld nuggets for the two interfaces; which were confirmed by the
peeled plugs. The difference in weld nugget size is attributed to
the MFDC process; where more heat is generated at the anode
compared to the cathode electrode due to directional current flow
and the Peltier effect. This additional heating can have the effect
of drawing molten metal towards the anode electrode as the weld
nugget develops, in particular for 3t stacks. The C-scan images
obtained are accurate in showing different weld nugget diametersat the two interfaces, which provides confidence that the technique
can be used for more complex 3t stacks.
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