PSI - Issue 54

Jenny Köckritz et al. / Procedia Structural Integrity 54 (2024) 423–430 J. Köckritz/ Structural Integrity Procedia 00 (2019) 000 – 000

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(a)

(b)

Fig. 5. Experimental and numerical results for (a) multiaxial load case 1 and (b) multiaxial load case 2

(a)

(b)

Fig. 6. Results of numerical and experimental investigations of tube-plate specimen with load collective (a), result SWF analysis of the cargo bike frame under the pedalling load case (b)

Based on the comparison of experimental and numerical results, the SWF Mat2 is selected for the evaluation of the cargo bike frame. The expected lifetimes with the SWF method under the constant amplitude frame load cases are shown in Table 3. The pedalling load case proves to be the most critical one for safety relevant welds of the frame, where the seam weld at the bottom bracket reaches only 20.080 cycles. This is possibly an underestimation of the actual life because the critical area at the bottom bracket consists of two overlapping welds, which hinders correct modelling. The resulting open weld corners lead to unrealistic stress peaks. At such areas, the SWF method is limited and is, just like HSS method or EN method, unlikely to deliver reliable results.

Table 3. Expected minimal lifetimes and critical weld location for considered constant amplitude load cases and SWF Mat2

Horizontal forces

Vertical forces

Pedaling

Rear brake moment

Min. life in cycles Position of critical weld

22740

12880

20080

171400

Steering tube suspension adapters

Rear deflection roller dome of chain guide

Bottom bracket

Left dropout, upper weld

Tests on one frame for all required load cases by an external test service show that the actual frame also withstands all load cases, even when applied sequentially. It should be noted that testing on only one frame does not provide the necessary statistical certainty. Through the careful numerical assessment of the frame, critical areas were adjusted before serial production, while areas like the bottom bracket and the steering tube suspension adapters are checked for cracks at regular intervals. Additionally, measured service loads under intended use were directly applied to the model. The numerical assessment showed no finite life under these loads, possibly because driver weight and driving style did not represent misuse or maximum load. To reach representative results with measured loads, measurements with maximum driver and cargo weight should be conducted and extreme cycling situations should be included.