WELDING RESEARCH PO = S where PO is the weld rotation angle when joint experiences PO failure. The rotation models of the four types of joints are schematically shown in Fig. 17. It is obvious that the above equations can be applied to the joint Types I and II directly. For joint Type III, although both the two interfaces bear the tensile-shear load during the tensile-shear test, each interface bears the total tensile-shear load not the half of it. Accordingly, the above equations are also suitable for the Type III joint. Joint IV experiences pure shear during the tensile-shear test, i.e., the joint will not rotate during the test. The above model is not suitable for joint Type IV. When the maximum shear stress exceeds the shear strength of the weld nugget, a crack will form at the weld interface. This moment corresponds to where the tensile-shear force reached its peak, as shown in Figs. 5, 6. Letting the maximum shear stress equal to the shear strength of the weld nugget, and then the failure load at the IF mode FIF can be expressed as where d is the weld nugget, and WN is the shear strength of the weld nugget. For a three-sheet RSW, d should be replaced by dIN, which is the weld nugget diameter at the failure interface. Considering that the aluminum spot welds are more sensitive to porosity or voids, a porosity factor P can be introduced into Equation 5 (Ref. 19) where P = (Atotal - Aporosity)/Atotal. Atotal is the area of the fusion zone on the fracture surface and Aporosity is the area of the porosity on the fracture surface of the weld. Letting the shear stress equal the tensile strength of the pullout failure location, then the peak load for a weld to fail in the pullout mode under the tensile-shear test can be approximated A B C D E F G H as where tID is the local sheet thickness around the nugget accounting for indentation, 486-s WELDING JOURNAL / DECEMBER 2016, VOL. 95 and σFL is the shear strength of the failure location. In order to ensure pullout failure for a spot weld, the failure load for a PO failure should be less than that for IF failure, i.e., FPO < FIF. Thus, the critical nugget diameter DC can be obtained from Equations 6 and 7. Applying the linear relationship between the strength and hardness, and the linear approximate between shear strength and tensile strength, Equation 8 can be rewritten as where HFL is the hardness of the failure location, HWN is the hardness of the weld nugget, and f is a constant coefficient. For aluminum alloys, f is about 0.6 (Ref. 25). In this study, HWN should be replaced by HLCGZ because the failure location of the IF mode occurred in the LCGZ. Therefore, Equation 9 can be rewritten as a more widely applicable form FIF = πd2 6cosθIF τWN (5) FIF = P πdIN ( 2 ) 6cosθIF τWN (6) FPO = ��dINtID 2cos��PO ��FL (7) DC = 3tID P ��FL ��WN cos��IF cos��PO (8) DC = 3tID Pf HFL HWN cosθIF cosθPO (9) σS A = S πdt / 2 = 2FcosθPO πdt (4) Fig. 14 —Macrostructures of weld joints in 1.5/1.0/2.0 mm stack: A — IF failure in Type I joint (18 kA); B — PO failure in Type I joint (32 kA); C — IF failure in Type II joint (18 kA); D — IF failure in Type II joint (34 kA); E — IF failure in Type III joint (18 kA); F — PO failure in Type III joint (26 kA); G — DIF failure in Type IV joint (18 kA); H — BMF in Type IV joint (22 kA).
Welding Journal | December 2016
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