Cetin Morris Sonsino

The experimental structural durability proof bases on a damage equivalent shortening of the design spectrum into a test spectrum with reduced length taking statistics founded safety factors into account. Historically, test spectra were first derived for steel parts using Woehler‐lines for steel; the damage equivalence was calculated by applying the Palmgren–Miner rule with the modification according to Haibach. When aluminium parts were introduced later, the question arose, whether test spectra developed for steel parts could be applied without modifications for their durability proof. Caused by different slopes of aluminium before and after the knee‐point, the test spectra derived for steel wheels result for aluminium components a lower cumulated damage in comparison to the design spectra. This fact requires a longer test duration for aluminium components when test spectra or proving grounds developed for steel components are used.By the example of steel and aluminium wheels for commercial vehicles with comparable highly stressed areas, the material dependent features regarding the required test life for the structural durability proof of aluminium parts are demonstrated.

This paper compares two methods for transforming a stress or load spectrum into a damage-equivalent constant amplitude loading, i.e., the required fatigue strength (RFS) and the consistent equivalent strength (CES) concepts. Because of their shared fundamentals, i.e., the modified Palmgren-Miner hypothesis for a given allowable damage sum, the resulting RFS- and CES-amplitudes are comparable to each other, but not equal, when the slopes after the knee-points of the Woehler-lines differ. The RFS-method delivers a damage-equivalent stress as the knee-point of a fictitious bi-linear or linear Woehler-line by positioning of the Woehler-line for a given allowable damage sum. In contrast to this, the CES-method also results in a damage-equivalent stress as the knee-point, which depends on the consistent fatigue strength reduction by increasing cumulating damage until the allowable damage sum is reached. Thus, the main difference to the RFS-method is that the CES-method retains the constant amplitude loading slope after the knee-point. Both methods indicate a lowest necessary strength quality, but, like the calculated fatigue lives, they differ according to the calculation methods, and depend on the slopes of the Woehler-lines after the knee-points.

Current experiences show that a non-proportional loading of ductile materials such as wrought steels, wrought aluminium or magnesium alloys, not welded or welded, causes a significant fatigue life reduction under an out-of-phase shear strain or shear stress superimposed on a normal strain or normal stress compared with proportional in-phase loading. However, when ductility, here characterised by tensile elongation, is reduced by a heat treatment or by another manufacturing technology such as casting or sintering, the afore-mentioned life reduction is compensated or even inversed, i. e. longer fatigue life results compared with proportional loading. Some actual results, determined with additive manufactured titanium, suggest that microstructural features such as manufacturing-dependent internal defects like microporosities should be considered in addition to the ductility level. This complex life behaviour under non-proportional loading cannot always be estimated. Therefore, in experimental proofs of multiaxial loaded parts, especially safety-critical components or structures, with real or service-like signals, emphasis must be placed on retaining non-proportionalities between loads and stresses/strains, respectively.