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Laboratory based durability testing - Methods and background

Automotive engineering is continuously moving towards the integration of different processes for accelerating vehicle development. Since the products become more and more a system, the methods to assist this transformation process become even more complicated.

Experimental proof is an essential measure for reliability, robustness and quality. Since many vehicle components are relevant for operational safety, these parts must not fail causing an accident. This has to be ensured by experimental methods such as rig based durability tests. Hence, service load simulation & evaluation becomes an instrument to actively guide the product development process, rather than to release newly developed parts only. If both, loading as well as assembly interaction are incorporated in a realistic manner, cost and time are saved due to the elimination of specialized on-track measurements.

Hence, lab based fatigue evaluation is presented as an integrated part of the overall vehicle development process. For wheel & hub assemblies as well as complete axle systems, the LBF durability test procedures are introduced, covering rotating and load transfer components.

Baugruppenerprobung für straßen- und schienengebundene Räder in der integrativen Simulationsumgebung ZWARP (German)

Since the beginning of the 1980s the methods of biaxial wheel testing have developed to become a widely-acknowledged standard in laboratory simulation. Compared with other methods, the biaxial wheel test facility (ZWARP) combines the benefits of complete assembly interaction and realistic time-stress functions. This article describes the main characteristics of ZWARP tests on wheels and/or wheel sets for road or rail vehicles, as well as the design of the testing facilities. The use of a standardised load ranges as a relevant concept is explained and evaluated. Finally the properties of a state-of-the art fully-kinematic simulation environment for wheel testing, which is currently in development at Fraunhofer LBF, are presented.

Zeitverkürzende Lastprogramme für die Qualitätssicherung bei Rädern (German)

The shorter operational load simulation of rotating chassis components is often presented alongside the methodology of biaxial wheel and wheel hub testing, with corresponding load ranges. These load ranges are normally derived from load spectrums and the damage content of these is rendered over an experimental distance of approximately 10,000 km for passenger car components.

On the basis of a load spectrum derived for a passenger car steel disc wheel, a component-specific load range was developed for the laboratory testing of complete samples and its overall runtime was optimised. The lecture discusses the time and cost-effective utilisation of load ranges optimised in this way, as well as the results obtained as regards quality assurance in the manufacture of vehicle wheels. Finally the technical and economic perspectives of using significantly shorter load ranges, as well as the implications for using simplified testing methods, are presented.

Typspezifische Bemessungslasten für die Raderprobung mit Hilfe von GPS-basierten Datenmodellen (German)

The structural durability of wheels and wheel hubs is increasingly being tested and verified using the biaxial wheel test (ZWARP). The method-specific features are:

  • Direction of action of external forces analogous to stress in the wheel contact point
  • Time-varying loads in a block range to illustrate quasi-stochastic excitation mechanisms

  • Particularly as regards the second bullet point, in the past different load ranges were developed to represent equivalent stress spectrums for corresponding reference distances. For example, the standardised “Europe Cycle” load ranges were partly based on test driving conditions on the northern loop of the Nürburgring and transferred to a normal form with the help of static wheel loads.

    The general tendency in recent years towards developing platform concepts and thus a growing number of bodywork derivatives increases the demand for type-specific design loads, even for components such as wheels, wheel hubs and wheel bearings. The “Europe Cycle for Passenger Cars” load range, which is adapted for a wheel load of 425kg in normal form can be calibrated for specific types using GPS-based data models and thus plays an important role in further improving the simulation quality in biaxial wheel testing. The GPS vectors recorded in driving mode are used to map a virtual journey in the CarSim® simulation software. The vehicle models selected for this test in CarSim® are oriented towards the features of the target vehicles for the development of the rotating components. Due to the complexity of the vehicle models, which have been optimised in the interest of driving dynamics, the expected or estimated tolerance ranges can be considered in CarSim® as part of the statistical design of experiments (DoE) and thus type-specific design loads and spectrums can be developed.

    The method described here allows improved adaptation of the relevant control parameters of standardised load ranges and thus plays an important role in the development of type-specific experimental ZWARP test environments.

    Extended Mobility Tires and their Effct on Loads to the Wheel Structure

    State-of-the-art developments in the field of tire technology do show a clear trend towards extended-mobility-tires. These tires - also known as runflat tires - have reinforced side walls to guarantee basic mobility properties even in the case of air pressure loss. Since the individual layers as well as the complete build up are different to traditional tire technology, the enhanced layer stiffness generates higher loads to the rim flanges, wheel well and wheel disc. Although these tires seem to be perfectly compatible to conventional wheels with regard to design and mounting process, the stress loading as well as the structural durability of the wheel may be an issue, which has to be examined carefully: Are the design and cross-sectional properties of the wheels appropriate for the complete life cycle of the vehicle?

    Since automotive wheels are relevant for operational safety, these parts must not fail causing an accident. This has to be ensured by experimental methods such as rig based durability tests. While simplified test procedures such as cornering fatigue tests or radial fatigue tests do not cover the complete load mechanics by the tire, the unique Biaxial Wheel Fatigue Test facilities (ZWARP) were used to generate technical expertise in the field of runflat tires and their relation to the loading of the wheels. Since both loading as well as assembly interaction in the ZWARP are incorporated in a realistic manner, costs and time are saved due to an appropriate accelerated life testing and the elimination of specialized on-road testing.