PSI - Issue 19

Thorsten Voigt et al. / Procedia Structural Integrity 19 (2019) 4–11 Dr.-Ing. Thorsten Voigt / Structural Integrity Procedia 00 (2019) 000 – 000

10

7

10 12 14 16

1068 1246 1424

Woehler curve AlSi7Mg (R=-1) design spectra (w corrosion) design spectra (w/o corrosion)

test spectra 1 (#2) test spectra 2 (#3)

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178 356 534 712 890

ሺ͓͵ •’‡…–”ƒሻ ൌ Ͳǡ͸͵ ሺ͓ʹ •’‡…–”ƒሻ ൌ Ͳǡ͹ͷ

0 2 4 6 8

Stress (double Amplitude), [MPa]

Force (double amplitude), [kN]

1,0E+08 0

1,0E+00

1,0E+01

1,0E+02

1,0E+03

1,0E+04

1,0E+05

1,0E+06

1,0E+07

cycles (log)

sensitivity of the material of = 0.45 for variable amplitude test #2 and #3 theoretical damage sums of #2 = 0.75 and #3 = 0.63 were calculated respectively by linear Palmgren-Miner damage accumulation calculation. Again these are typical result for these kind of specimens. The damage sum again indicates a good accordance of the local material cyclic fatigue strength to the chosen parameters of the Woehler curve. Figure 15: half axle tests, max. principle stress at hot spot and max. amplitude (range pair) analysis

4.4. Conclusion and resources for optimization

The half rear axle module was subject to endurance tests under variable amplitudes in a two-channel test rig. The crack of the control arm of test specimen #1 under static load showed, that the subassembly would fail under typical load cases for this vehicle with high vertical forces (pothole/curbstone). For test specimens #2 and #3 the load program was mitigated in a way, that special load cases were removed from the load signal and the static wheel load was reduced to 1.4 kN and 1.0 kN respectively. Neverthe less, in both test runs component failure occurred before the required scope of 100 laps of the load signal was reached. The fact that all specimen showed a similar failure mode lead to the assumption that a high torsional moment which arises in the control arm under vertical loads is the common cause of damage in all three cases. FEM calculations at the control arm support this assumption. Measurements with strain gauges at test specimen #3 were used to confirm the calculation results and to validate the endurance tests. With regard to the vertical loads, a redesign of the rear half axle was recommended for a later serial production of the URBAN EV vehicle. Out of different options to reduce the local stress or to reduce the arising torsional loading, it was decided to change the kinematic of vertical load introduction. In the final version of the vehicle, which was used for driving tests and crash evaluation, in the region of vertical load introduction an additional bar was introduced to intercept the vertical loads (Figure 16).

5. Summary

This paper describes the concept of experimental operational durability testing for a new resource and space-saving electric powered city vehicle. The vehicle was developed in a European joint project. In addition to a hinged rear axle, which should

old design

new solution

253MPa

95MPa

16.5mm

3.7mm

Figure 16: Half axle tests, redesign of control arm suspension