Higher than Expected Strengths

This article summarizes a paper entitled, “Higher than Expected Strengths from Dissimilar Configuration Advanced High-Strength Steel Spot Welds”, by E. Biro, et al.^B-6

This study shows that the cross tension strength (CTS) is always higher than the strength expected from the lower strength material in the joint. Figure 1 verifies the assumption that the load bearing capacity of a heterogeneous configuration is supposed to equal the minimum strength of both homogenous assemblies.  Material used in this study was a 1 mm low carbon equivalent Dual Phase 980 (DP980 LCE) steel.

Figure 1: Example of dissimilar configuration with CTS matching the “minimum rule”.^B-6

The materials chosen for this study are in Table 1.

Table 1: Steel sheet samples.^B-6

The material thickness combinations for all of the two sheet joints are shown in Table 2.

Table 2: Welded 2-sheet configurations.^B-6

The three-sheet stackups all were made using the 1 mm DP980 LCE. These configurations were designed to understand what happens in such cases, knowing that three-sheet welding is very common in car body manufacturing. The three-sheet stackup configurations are shown in Figure 2 and are as were follows:

1. a square DP980 coupon (patch) is inserted between the two classical cross-tension coupons for welding (1+patch+1 mm);
2. two coupons oriented the same way welded with one coupon oriented in the transverse direction to form a cross-tension sample (1+[1+1] mm);
3. same configuration as a) but the external coupon is removed by manual torsion before cross-tension testing (1+1+0 mm);
4. same configuration as a), but the two coupons oriented the same way are first spot welded together strongly (with several spots) in the extremities, before the actual 3-sheet spot weld is done ([1++++1]+1 mm).

Figure 2: Three-sheet configurations based on 1mm DP980 LCE sample.^B-6

The welding parameters for each configuration are listed in Table 3.

Table 3 : Welding parameters.^B-6

CTS is strongly dependent on weld diameter (Figure 3).

Figure 3: Cross-tension Strength for TRIP800 configurations.^B-6

CTS for the main DP980 configurations are shown as a function of weld diameter in Figures 4 and 5.

Figure 4: Cross-tension Strength for DP980 1+1, 1+2 and 2+2 configurations.^B-6

Figure 5: Cross-tension Strength for DP980 1.25+1.25, 1.25+2 and 2+2 configurations.^B-6

The three-sheet configurations based on 1mm DP980 LC results are shown in Figures 6 and 7. These results again verify that dissimilar configuration performances appear above the “minimum rule” assumption described in Figure 1.

Figure 6: Cross-tension Strength for DP980 1+1, 1+1+0 and 1+patch+1 configurations.^B-6

Figure 7: Cross-tension Strength for DP980 1+1, 1+2, 1+[1+1]and [1+++1]+1 configurations.^B-6

The observation that CTS is greater than predicted by the “minimum rule” has been called a “positive deviation” from the expected strengths.

This work concluded that while material qualification tests are frequently based on similar welding configurations, real car body applications are quite systematically dissimilar configurations. For spot welds failing in plug mode, the strength of the assembly only depends on the weakest material strength. In case of AHSS+AHSS welded combinations, however, things turn out to be different. Similar grade but dissimilar thickness High-Strength Steel configurations have been spot welded and tested in Cross-Tension. The following main conclusions can be highlighted:

1. For dissimilar thickness configurations, the cross-tensile strength is above the standard “minimum rule” assumptions, this phenomenon being called a “positive deviation”;
2. Limited thermal and notch location effects can explain part of this positive deviation, but the main reason is mechanical;
3. As evidenced through several analytic and numerical studies, this mechanical effect is due to the less severe local stresses at the notch in case of uneven thickness, and improves the positive deviation when the thickness ratio increases. Although widely used for material qualification and scientific purposes, similar configurations appear as the worst case in terms of cross-tension performance for high strength steels. Actual vehicle design should consider positive deviation in dissimilar configurations to maximize the potential strength of spot welds in High-Strength steels.