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The paper describes a method for optimal wheel design and shows a systematic procedure for determining acting and allowable stresses in the wheel. The traditional wheel fatigue test procedures are criticized because no definite relationship to customer service exists in most cases. The basic design concept, that dimensions should be based on service stresses and allowable stresses is strictly adhered to in the approach and is accomplished by providing a method for simulating service loads in the laboratory and by determining allowable stresses by fatigue testing of representative wheel areas. The operational wheel loads as well as frequency distribution of service stresses are discussed. A Flat Base Wheel Roll Test Facility for determining wheel stresses is presented. Also described is a new Biaxial Wheel Durability Test Machine which provides improved simulation of wheel loading conditions and makes it possible to test the total wheel in a single procedure.

The paper explains the most important criteria for the design of cast aluminum wheels on examples of recently developed European wheels. The basic differences in material behavior between steel and aluminum, such as fatigue strength, cyclic stress-strain properties and crack behavior are treated. The optimization of the wheel design is based on the fatigue strength of individual wheel sections which is determined through special test fixtures. The final proofout test for adequate fatigue life of the total wheel is established through a programmed test carried out in a new biaxial wheel test machine whereby the load program for testing European wheels represents European service conditions.

The durability and the life of spot welded sheet metal structures are mainly controlled by the strength of the joints. Designers are aiming at reliable information, already in early design phase, that the weld spots will indeed perform satisfactorily throughout the required life cycles of a complex structure under the operational loading conditions. Consequently, a theoretical approach for the determination of the structural durability and the reliable design and optimization of spot welded components has been developed and extensively verified for serial components by theoretical analysis and tests. The method starts with Finite-Element-computations of the structures in the design stage and strength data obtained from component-like specimens; both are commonly available in the design phase, but are not sufficient for the determination of the structural integrity.

The methodology “Service Strength Analysis” is discussed. The method allows design optimization of lightweight vehicle components whereby not only the required service life but also adequate reliability and safety are taken into account. While the service life depends primarily on operational loading, design as such, material and manufacturing processes, reliability and safety requirements depend on the function of the component. The methodology comprises stress analysis under service loading, derivation of stress spectra, strength evaluation and proof-out testing under simulated service load spectra. The method allows to determine not only areas critical to fatigue strength but also areas that are overdimensioned, where weight and cost may be saved. The methodology reduces development time, saves material and may improve manufacturing. As the method is being applied, data regarding product liability are collected automatically.

As part of an extensive investigation on automotive aluminum wheels, a study was carried out regarding the validity of theoretical fatigue life calculations by means of the local strain approach. Tests to substantiate the theoretical prediction were carried out on a test facility which allows for service-like variable amplitude biaxial loading in addition to rotation of the wheel. Under these simulated service conditions the strain-time histories from strain gages in critical areas of wheels were recorded on magnetic tape. These stress sequences were used twofold, first as test-control-signal for testing of unnotched test coupons taken from production wheels and secondly as input for theoretical fatigue life predictions. The specimen on which the material data for the theoretical prediction were determined were also taken from wheels; the data consisted of mono-tonic and cyclic stress-strain curves as well as strain controlled ε-N curves.

The method of fatigue life evaluation and validation testing under operational conditions is a prerequisite to achieve optimal designs with respect to weight and long-term durability of the structure considered. Criteria like driving behavior and braking performance, influenced by global stiffness as well as fatigue life evaluation were taken into account when optimizing a forged 6.5 front axle beam of a heavy truck. The procedure for weight optimization includes the following steps: Stress analysis e.g. using strain gage techniques and/or Finite Element Method, Road load data acquisition and derivation of design spectrum describing customer usage, Fatigue testing under constant amplitude and/or variable amplitude loading to establish component related SN-curves and/or fatigue life curves, Fatigue life evaluation using damage accumulation hypothesis (Miner's Rule), Optimization and weight reduction (in this case 14% weight savings were achieved) based on fatigue life evaluation.

For the development of new components, design engineers today have access to a broad amount of fatigue data, which were obtained from unnotched and notched specimens. These data can be transformed when the conditions of material, strength, geometry, surface and surface layer and loading mode in the fatigue critical areas are taken into account for constant and variable amplitude loading. The procedure of data transferability is discussed for the example of a randomly loaded truck stub axle where the failure criterion is the first detectable crack, and the local equivalent stress/strain and the maximum stressed/strained material volume are considered. In addition, several problems associated with fatigue life assessment under variable amplitude loading are discussed.