WELDING RESEARCH DC = 3tID P ��FL ��WN cos��IF cos��PO (8) DECEMBER 2016 / WELDING JOURNAL 483-s crostructures of the fracture surface cross section of welds that failed in IF mode and PO mode. From Fig. 8B, it can be seen that the crack began to form at the interior of LCGZ and then propagated through the interface between SCGZ and LCGZ. A crack was also found on the other workpiece/ workpiece interface —Fig. 8C. This inferred that there is a competition between the two interfaces in a threesheet spot weld joint, and that failure will occur on the weaker one. As can be seen with Fig. 8B, C, the weld failed at the interface where LCGZ formed (Fig. 8B) while the crack propagation was restrained at the interface where no LCGZ formed — Fig. 8C. This again verifies that the LCGZ is the weak area in a spot weld. In the PO mode, the weld joint failed in the PMZ — Fig. 8E. There is also a crack on the other workpiece/workpiece interface (Fig. 8F), indicating competition between the two interfaces during the tensileshear test. Figure 9 shows the effect of button size on the peak load and energy absorption of joint design III. The minimum button size that guarantees a PO mode was 5.1 mm. The results are similar to the case of joint Types I and II, i.e., a linear relation and two-orderpolynomial relation exists between the peak load and button size, and energy absorption and button size, respectively. Failure of Joint Type IV Since the Type IV joint experienced a pure shear during the tensileshear test, the failure modes of this type of joint are a little different from those of joint Types I, II, and III. When the nugget size was small, both of the two interfaces failed through the interfacial mechanism — Fig. 10A. This is called a double interfacial failure (DIF) in this paper. Note that the nugget deviated from its original position. The detailed process will be discussed in the following text. When the nugget size grew larger, one interface failed through the interfacial mechanism while the other one failed through the pullout mechanism (Fig. 10B), and this is called a one interfacial/ one pullout (IF/PO) failure. When the nugget size was large enough, the base metal fractured during the tensile process (Fig. 10C), and this is called a base metal fracture (BMF) failure. Figure 11A shows the typical loaddisplacement curves of the Type IV weld joint that interfacially failed. The load-displacement curve has two peaks. The first drop of the load corresponds to the initial formation of a crack, as shown in Fig. 11, B1. Figure 11C corresponds to the second peak. It can be seen that the middle sheet was pulled out along the tensile direction — Fig. 11, C1. The nugget was squeezed (Fig. 11, C2) by the middle sheet because of the movement of middle sheet. At the same time, cracks Fig. 7 — Effect of button size on the peak load and energy of joint designs I and II in the 1.0/1.0/1.0 mm stack. Fig. 8 — Macro/microstructures of Type III weld joints in the 1.0/1.0/1.0 mm stack that failed in A, B, C — IF mode; D, E, F — PO mode. Fig. 9 — Effect of button size on the peak load and energy absorption of the Type III joint in the 1.0/1.0/1.0 mm stack. A B C D E F
Welding Journal | December 2016
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