Development and Evaluation of a Test Specimen to Determine the Influence of Welding Parameters on the Fatigue Strength of Aluminum Frames for Lightweight Vehicle Designs

Abstract

In the bicycle and e-bike industry, lightweight designs often incorporate aluminum tube constructions that are joined through inert gas welding. These constructions are susceptible to fatigue damage. A standardized test specimen (two tubes joined to a T shape) was designed to determine how different welding parameters and surface and heat treatments affect the fatigue strength of the structure. In particular, it was evaluated how the gap between two tubes at the beginning of the welding process affects the fatigue strength. Therefore, test specimens without a gap, with a gap of 2 mm, and with a gap of 4 mm at the beginning of the welding process were manufactured in a production environment typical for the bicycle industry. To obtain standardized S-N curves and their slope b for each type and to check whether the results varied significantly, the tests were planned and evaluated according to DIN 50100, 8.2.2. A total number of 55 samples were tested until failure using force-controlled fatigue tests. The result of the evaluation shows that a larger gap between the tubes does correlate with a significant reduction in fatigue strength. A 4-mm gap results in approximately half the fatigue strength. After the fatigue testing, a metallographic analysis was done. No irregularities in the material could be found, which suggests that the reason for the reduced fatigue strength was a higher notch effect of the weld due to the bigger gaps. The test results were compared to a finite element analysis (FEA)–based simulation. The test specimen and test setup were published under a creative commons license, which enables others to do additional tests and compare the results. Based on further tests, additional fatigue classes (FATclasses), especially for thin-walled aluminum tube designs, could be defined.