PSI - Issue 75

Said Allouch et al. / Procedia Structural Integrity 75 (2025) 299–310 S. Allouch / Structural Integrity Procedia (2025)

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In order to avoid failure of wheels with potentially serious consequences, reliable testing under loads as close as possible to the operating conditions should therefore be aimed for. For the rotating components of the wheel end assembly, LBF has a proven design concept that has been used for many years as part of the BiAx/ZWARP technology (Grubisic and Fischer, 1983), (Fischer & Grubisic , 1985). This was developed to overcome limits of simplified wheel tests such as radial fatigue test (rft) and cornering fatigue test (cft) which cannot fully represent the load condition on the road in different environmental conditions. Both are designed to test load case individually: rft covering straight driving and cft covering cornering maneuvers. Based on decades of experiences with correlations of these tests to real usage of cars, they are still used. But they cannot simulate the real usage where at least two loads, vertical and lateral, are acting concurrently, which leads to different local stresses. Since the introduction of BiAx technology a state-of the-art technology has been available for realistic service load simulation which enables reliable, and weight optimized wheel designs (Grubisic and Fischer, 1984). Since local stresses and related material properties are the root cause for failures the BiAx technology deals with local stress spectra describing the lifetime of variable amplitudes, caused by concurrently acting lateral and horizontal wheel forces. While the BiAx technology was formerly derived as an inner drum machine newer test environment also offer the possibility to make a wheel fatigue test on the outer side of a drum. Within a large research campaign, the difference in the load scenario and the fatigue result for inner and outer drum machines as well as the influence of the tire ratio, the tire width and the different wheel sizes of passenger car wheels have been investigated at Fraunhofer LBF. This information and the significances of the differences in the assessment procedures offer the potential for a further detailed lifetime analysis of wheels as mass product with highest safety requirements. 2. LBF design spectrum Alike to all other cyclically loaded components also for the fatigue assessment of wheels the design of a load spectrum is the first point to consider. A design spectrum is the expected collective of operating stresses at a system point. It is defined by the maximum stress values and their frequency distribution, which can be obtained, for example, from road load data acquisition. The maximum stress at a given system point on the wheel can be determined through stress analysis, where the wheel is gradually rotated under a constant load. For the LBF design spectrum the frequency distribution varies based on the load case: for straight-driving, it follows a linear distribution, whereas during cornering, it exhibits a normal distribution with a Lin-log representation. Depending on the vehicle type, individual case-dependent sub-collectives are combined to form an overall design spectrum. For passenger car wheels, the most relevant load cases typically include straight-driving and cornering, as these are usually sufficient to carry out the design of the wheel. The LBF design spectrum, as shown in Fig. 1, represents this expected distribution. The rated service life of a passenger car wheel is generally set at 300,000 km, which is a common value for many car manufacturers as a design point.

Fig. 1. LBF Design Spectrum