PSI - Issue 42

Miloslav Kepka et al. / Procedia Structural Integrity 42 (2022) 687–693 Author name / Structural Integrity Procedia 00 (2019) 000 – 000 3 The critical cross-section of tested welded nodes was subjected to reverse bending load (the cycle stress ratio was = −1 ). The limit state was defined by the formation of a macroscopic fatigue crack (1 to 2 mm). All results are collected in two research reports, Minich and Kepka jr. (2020) and Minich et al. (2021). Fig. 3 shows an example of the FEM analysis of the welded node of the window and roof profile. Based on the FEM analysis, strain gauges were installed for the laboratory fatigue test so that it was possible to determine both the nominal stress and the local stress near the welds and to evaluate the corresponding S-N curves. 689

Fig. 2. FEM analysis and laboratory fatigue test including detailed strain gauge measurement.

3. Service stress spectra 3.1. Past approach

It has been confirmed that the main factors affecting the fatigue life of city bus bodies are the vertical dynamic characteristics of the vehicle and the stress on critical structural body details when the vehicle is driven on an uneven road surface. The effects of significant unevenness in the road surface can be effectively simulated by driving the vehicle over a precisely defined unevenness. An artificial obstacle in the form of a cylindrical section with a base of 500 mm and a height of 60 mm has proven itself for this purpose, see Fig. 3. This experiment is also very suitable for dynamic simulations and comparing the quality of the computational model with reality, in contrast to the simulation of driving a vehicle over random road irregularities. It has been empirically confirmed that the largest stress amplitudes (stress ranges) when driving a vehicle on a poor-quality road usually do not exceed the stress amplitudes when crossing an artificial obstacle. Based on this knowledge (and other information from the computational modal analysis), it is possible to approximately estimate the future stress spectra and assess the fatigue life of the main structural nodes of the vehicle already at the stage of its design, Kepka and Kepka jr. (2021). The final measurement is made with a prototype vehicle while driving on real tracks. The measured stress-time histories (random processes) are converted into histograms of the frequency (number) of harmonic cycles, the so called service stress spectra. These spectra together with the S-N curves of the investigated structural nodes serve as inputs for the calculations of their fatigue damage and the prediction of their operational fatigue life.