Fourth International Conference on Material and Component Performance under Variable Amplitude Loading
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Fatigue Assessment

Session chair: Rennert, Roland, Dr. (IMA Dresden, Dresden, Germany)
Shortcut: F
Date: Monday, 30. March 2020, 15:50
Room: Hall B
Session type: Oral


15:50 F-01

Definition of simplified fatigue tests using numerical fatigue simulation methods (#14)

R. Schrank1

1 IAMT Engineering GmbH & Co. KG, Simulation, Ludwigsfelde, Brandenburg, Germany

“Virtual prototyping” has been established as an important part of the development process for
technical components and technical systems. Fatigue simulation is a main discipline in virtual
prototyping, but it is a quite complex field, especially for structures being exposed to arbitrary timevarying
multi-channel loads. Thus, at the end of the development process, the series approval for
technical components is often based on extensive (“full”) fatigue tests, particularly with regard to
automotive chassis parts.

Currently, test-based fatigue qualification gathers interest in other process phases, for instance
- to assure fatigue safety just for non-series / prototype parts (before the end of the conventional
development process)
- to make quality checks during series parts production on the supplier’s site (after the end of the
conventional development process)
- to validate fatigue simulation methods just within the first phases of the development process

Main requirements concerning these fatigue tests are a simplified test set-up (cost reduction) as well as
a short test duration (time reduction). These requirements are interpretable as an objective function of a
kind of “fatigue test optimization”. Reproduction of the results of the “full” fatigue test (reference test) is
the main constraint of this optimization task.

A typical process of simplified fatigue definition will be clarified using the main structural part of a
double wishbone wheel suspension as a typical example. First, basics of simulation-based fatigue analysis
will be summarized. Next, some strategies of fatigue test simplification will be discussed. Results are
presented in terms of a comparison of the “full” fatigue test (reference test) and the simplified fatigue
test. Finally, the utilization of an in-house developed optimization tool for similar applications in future
will be outlined.

Keywords: automotive, test, simulation, optimization, simplification
16:10 F-02

Fatigue assessment of cast iron components under variable amplitude loading considering overloads (#31)

J. Hesseler1, C. Bleicher1, J. Baumgartner1, K. Schnabel1, T. Melz1

1 Fraunhofer LBF, Department of Materials and Components, Darmstadt, Germany

For a component development considering lightweight design as well as reliability the use of realistic load assumptions in the fatigue assessment is important. Especially in wind energy applications the structural components can be exposed beside service loads to overloads which can have a major influence on the life of the components. The overloads lead to an additional damage and to local yielding that generates residual stresses.

In strain controlled fatigue test, unnotched cast iron (EN-GJS-400-18U-LT, EN-GJS-700-2 and ADI-900) specimens were tested using two different load spectra with a length of LS = 500.000 and an R-ratio of R = -1. To investigate the influence of overloads on lifetime, single overloads with a maximum strain of 0.5% and 1% were added at the beginning of the load spectrum or after LS cycles. These overloads lead to tensile residual stresses (local mean stresses).

Fatigue assessment for the specimen used in fatigue test was carried out using stress life and strain life approaches. The used strain life approach takes sequence effects and elastic-plastic material behavior into account. No cyclic relaxation (transient material behavior) was considered. Nevertheless, the calculated relaxation of mean stress are in good agreement with the measured ones. Allowable damage sums are derived according to linear damage hypothesis using damage parameters.

In contrast to the strain life approach, the stress life approach takes neither the load sequence nor the elastic plastic material behavior into account. Due to the fact that the overload leads to high tensile residual stresses, the stress life approach was modified to take these residual tensile stresses additionally into account. The modification of the stress life approach allows a reliable assessment of high strength cast iron considering overloads.

Both fatigue approaches were compared and evaluated regarding their suitability for assessing the influence of overloads.

Keywords: Overload, Cast iron, strain life approach, stress life approach
16:30 F-03

Response of the structural materials to fatigue loads at bending block load with an mean value (#32)

R. Pawliczek1

1 Opole University of Technology, Department of Mechanics and Machine Design, Opole, Poland

The paper contains the results of fatigue tests of structural materials subjected to block loads, which take into account the effect of mean load.

The testing procedure included fatigue tests under conditions of bending loads performed in blocks with different mean load values. The length of block Nb expressed in cycles was assumed, divided into three equal sections (n = 1/3Nb). For each segment, the amplitude and mean value of the load were determined based on the results of fatigue tests under constant amplitude loads with the cycle asymmetry coefficients R = Mmin/Mmax = -1; -0.5 and 0. Two "paths" were used to change the average load value in blocks: increasing the mean load and decreasing the mean load, while maintaining the maximum load level. A schematic view of load cycles in individual blocks is shown in Fig. 1.

The experiment verified the method of loading in material samples with the mean load value and the influence of load sequence on fatigue life of material.

Samples made of two types of alloy steels (S355J0 and 10HNAP) and alumnium alloy AlCu4MgSi were tested, where differences in fatigue strength for steel were demonstrated, while aluminum showed no sensitivity to the load sequence. This can be seen in Fig. 2. and Fig. 3 showing exemplary relationships between strain and load.

The phenomenon in steel was presented in more detail on the example of SJ355J0 steel - the results of strains measurements and the observed differences in material properties after testing the samples under the conditions of the selected load cycle were presented. The test results for SJ355J0 steel indicate that the order of mean loads in the block influences the durability of the body, while the intensity of the phenomenon depends primarily on the value of the maximum load stresses during the test. For higher values ​​of load applied during tests the effect of the load sequence is smaller.

Keywords: block loading, mean load, load sequence
16:50 F-04

Simulation studies on the influence of cyclic stress-strain curve parameters on the degree of fatigue failure of material under block load conditions with the mean value (#33)

T. Lagoda1, R. Pawliczek1

1 Opole University of Technology, Department of Mechanics and Machine Design, Opole, Poland

The effect of changing the parameters of the stress-strain curve is commonly known. It is also indicated that this should be taken into consideration when analyzing the state of stress and strain under conditions of fatigue load. The results of simulation research presented in the article relate to an algorithm for estimating fatigue life under block load conditions with an mean value. The authors present the problem of block loads and propose a calculation model, which takes into account the change of stress-strain curve parameters caused by mean strain. Simulation tests were performed for generated strains, where load blocks with changing mean strain values ​​were distinguished. The comparative value is the degree of fatigue damage. The results of calculations obtained for standard values ​​of stress-strain parameters (for symmetric loads) and those determined taking into account changes in the curve parameters were compared and presented in this paper. It has been found that for smaller values ​​of mean strain, the results of the calculations do not change significantly.

Keywords: block loading, mean strain, stress-strain curve