PSI - Issue 19

Jan Presse et al. / Procedia Structural Integrity 19 (2019) 423–432 Author am / Structural Integrity Procedia 00 (2019) 000 – 000 ( ) = ²

428 6

(2)

( , ) = , ²

(3)

One crucial advantage of this approach is the fact that it considers in the calculation of radial stress the most relevant parameters such as thickness, diameter and load distributions that affect the fatigue strength of SPRs. To get a valid database about the fatigue performance of riveted connections, in total 30 combinations with a wide range of relevant parameters have been tested. Although spot welds and self-piercing rivets are circular connections, the joints differ in stress contribution and the range of materials as assembly parts. Therefore, a regression through the calculated structural stress values for all tested riveted configurations show a significant scatter, caused by the difference of mechanical properties for several multi-material connections. Instead of modifying the master SN reference curve, by splitting several combinations into certain classes and perform a regression through the calculated stress values, this method modifies the stress values in the first place. This modification compensates the different stress contribution of SPR compared to RSW. In the next step, the calculated stress values are compared to a fixed material SN-curve with a notch factor depending on the relevant rivet diameter. Fig 4 shows that the scatter of stress values calculated with the modified formulas is negligible while the material SN-curves fit in the clusters of related stresses.

10000

LS-DP600 LS-CR3

LS-Al6xxx CP-DP600 CP-CR3 CP-Al6xxx Mat-SN DP600 Mat-SN CR3

1000

Stress  e,0 [MPa]

Mat-SN EN AW-6016 Mat-SN EN AW-6111

100

10

1,0E+04

1,0E+05

1,0E+06

1,0E+07

Cycles to failure N [-]

Fig 4. SPR fatigue parameters and material SN-curves.

This new method called FESPRi (Fatigue Estimation Self-Piercing Rivets) enables an accurate and performable estimation of fatigue life of thin sheet multi-material connections joined by SPRs within the observed parameter ranges. The major benefit of this method is the reduction of tests and calculations. Already determined material SN curves can be used as a reference curve in the calculation each time the material is assembled as one of the assembly parts. Besides, the universal parameter set simplified the calculation of the structural stress values.