PSI - Issue 54

Zahra Silvayeh et al. / Procedia Structural Integrity 54 (2024) 431–436 Z. Silvayeh et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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Figure 1 (b) shows the die force that was monitored during the SPR process using soft rivets (H0) and hard rivets (H4) for different offsets of e = 0.0 mm, 0.5 mm and 1.0 mm. The die force comprises the blankholder force (BHF) of 8 kN that is constant and the punch force that increases with progressing displacement of the punch. The punch displacement from the initial contact between rivet and upper sheet until completing the riveting operation was ≈ 7.1-7.2 mm. The die force increases from 8 kN to ≈ 52 kN and ≈ 56 kN for rivets of hardness class H0 and H4, respectively. A notable difference of the force-displacement curves that is related to the hardness of the rivet exists beyond the displacement of ≈ 3.5 mm; however, the difference of the curves that is related to the offset is negligible. Therefore, assessing the integrity of the joint only by analyzing the force-displacement curves is not recommended and the preparation of cross-sections of the joint is useful.

Figure 1. (a) SPR system used for preparing the samples (in the detail the blankholder is lowered onto the die), and (b) typical force-displacement curves monitored during the SPR process using soft (H0) and hard (H4) rivets at different offsets of 0.0 mm, 0.5 mm and 1.0 mm.

After preparing the SPR joints, a spindle-driven Zwick/Roell Z100 uniaxial testing machine equipped with a 100 kN-load cell and with mechanical clamping jaws was used for shear-tensile testing of the samples, as exemplarily shown in Figure 3 (a). The quasi-static testing speed was 5 mm/min. During testing of each sample the tensile force and the displacement of the moving jaw were monitored to evaluate the specific force-displacement curves. Moreover, representative SPR joints were sectioned using an automatic QATM Brillant 220 precision cutting machine. The cut-off joint segments were cold embedded using epoxy resin, before their cross-sections were ground with #600, #1200, #2500 and #4000 sandpapers using a QATM Saphir 550 grinding machine. Controlling the material removal during grinding was important to ensure that the prepared cross-sections were exactly located at the center plane of the joints. Finally, the cross-sections were captured using a Keyence VHX-6000 digital micro scope to assess the characteristic dimensions (protrusion height of the rivet head, horizontal undercut of the rivet and minimum bottom thickness of the lower sheet). 3. Results and discussion Figure 2 shows nine representative cross-sections of SPR joints produced with rivets of different hardness classes (H0, H2 and H4) and with different offsets between rivet and die (without offset 0.0 mm, 0.5 mm and 1.0 mm). Increasing the offset between rivet and die promoted asymmetric deformation of the rivet shank during the SPR process. It is evident that the deformation was most asymmetric if soft rivets of hardness class H0 were used, but the asymmetry was negligible if hard rivets of hardness class H4 were used. This demonstrates that asymmetric deformation of the rivet during the SPR process and, thus, the asymmetry of the final joint depends not only on the offset between rivet and die, but also on the hardness of the rivet. Nevertheless, the asymmetry of the joint does not necessarily affect its integrity. Even though the asymmetry is evident, the horizontal undercut at the “short” side of the rivet shank and the remaining thickness of the lower sheet at the “ long ” side of the shank can still be considered as sufficient. As the rivet head was almost flush with the edge of the upper sheet, the protrusion of the rivet head was also negligible.