Evaluation of influences through materials processing on the fatigue life of quenched and tempered SAE 4140H (#46)
R. Acosta1, 4, T. Hielscher2, 5, M. Magin3, 5, P. Starke1, 5
1 University of Applied Sciences, Materials Science & Materials Testing, Kaiserslautern, Rhineland-Palatinate, Germany
The demands placed on highly stressed components are constantly increasing. For this reason, components with a higher degree of efficiency are required for the competitiveness of metallic components under operational conditions which may have fluctuating loads with variable or constant amplitudes. Therefore, a comprehensive understanding of the fatigue behavior of metallic materials and components as well as the development of new methods for the reliable calculation of the service life are of great importance. The fatigue properties of components are a multi-parameter property and are strongly influenced by surface integrity, as the surface and near-surface zones are the most stressed areas of the component. Surface integrity is influenced by each individual manufacturing process, its parameters, the process order and the process combination. Due to their high complexity, the influence-cause-effect relationships of process chains have not yet been sufficiently understood to determine process related durability performance completely.
In order to analyze the interactions between the manufacturing process chains, their design, variations of the material properties, in particular the microstructure and the resulting component properties, specimens were processed with specifically different machining parameters with and without polished surface. The specimens were characterized by different analyzing methods, as x-ray measurements for residual stress measurements and phase analysis as well as surface measurements by mean of a confocal microscope. The results from analytics are used as an input for the FEM analysis providing a corrected stress amplitude, which includes factors as stress concentration and residual stresses in order to predict their influence on the fatigue life.
This additional material information can be used in the currently existing short-time procedures StressLife and SteBLife and opens by this the possibility of a multi parameter SN curve as a fundamental basis for the evaluation of the structural durability performance.
The investigations presented in the scope of this paper were performed on quenched and tempered SAE 4140H specimens, which were machined with various processing parameters.
Cyclic deformation curves based on the change in temperature of 8 constant amplitude tests at a stress amplitude of 665 MPa are shown in Fig. 1a. The range of the received lifetimes was between 12,000 and 39,000 cycles, which is a large scatter just depending on the machining process parameters as well as the surface topography.
In addition, Figure 1b depicts graphs of surface topographies from a confocal microscope for two specimens exemplarily.
The methods, which were developed in the first step for constant amplitude experiments, have also a significant relevance for random loading, which is also an issue for the forthcoming investigations. Even on the basis of temperature and electrical resistance measurements it is possible to characterize the cyclic deformation behavior during random loading and to feed this back into the proposed methods. This paper then gives a brief outlook on the basis of selected random load increase as well as random load tests. Already in  it was shown that life-limiting factors equally have an influence under constant amplitude as well as random or even service loading. Hence the data for SAE 4140H equivalent steel grades from Gaßner-experiments can be seen as a VAL (variable amplitude loading) reference for this concept. The investigations carried out so far are only single experiments but are to be statistically verified in a proposed research project.
 E. Haibach, Betriebsfestigkeit: Verfahren und Daten zur Bauteilberechnung, (VDI-Buch) 3. Auflage, Springer-Verlag Berlin Heidelberg (2006).
Keywords: Fatigue life calculation, short time procedures, materials processing, process chain
About the cyclic material behavior of aluminum wrought alloys under variable amplitude loading conditions and its consideration during the numerical fatigue approach (#65)
A. Maciolek1, J. Bernhard1, T. Melz1
1 Fraunhofer Institute for Structural Durability and System Reliability, Darmstadt, Germany
Lightweight design using high strength aluminum wrought alloys is getting more and more important in transportation industries. Especially the latest developments in e-mobility requires exploiting the potential of aluminum alloys more than ever before. In order to design reliable components the cyclic material behavior under service loads is of great interest.
The investigation includes the determination of the fatigue lives of aluminum wrought alloys subjected to uniaxial cyclic loads under variable amplitudes. By comparing the results to the experimental fatigue lives, the estimation quality of calculated fatigue lives by local strain-life approach using different damage parameters and linear damage accumulation is shown. The influence of the strain life curve is discussed by applying the equations according to Coffin-Manson and the Trilinear Strain Life Curve in order to compare the calculated fatigue lives. To overcome inaccuracies by assuming cyclic stable material behavior according to the Ramberg-Osgood a description of the cyclic stress-strain behavior is also used to incorporate cyclic transient effects such as cyclic hardening or softening and mean stress relaxation or ratcheting.
Keywords: cyclic material behavior, aluminum wrought alloy, strain-life approach
Fatigue performance and modeling of a cast aluminum alloy under variable amplitude loading (#128)
A. Nourian-Avval1, A. Fatemi1
1 University of Memphis, Mechanical Engineering, Memphis, Tennessee, United States of America
The excellent castability, relatively low production costs, and high strength to weight ratios make Al-Si-Mg cast alloys an attractive choice for use in many industrial applications, including automotive and aerospace industries. However, it is well known that presence of casting defects can significantly affect the fatigue performance of cast components due to stress concentration around the defects, which can considerably reduce the crack nucleation period. On the other hand, most of the research, reported in the literature on cast aluminum alloys is related to constant amplitude loading, while in real applications cast aluminum components are typically subjected to variable amplitude loading conditions. Therefore, in this research, the fatigue performance of high-pressure-die-cast A356-T6 aluminum alloy is studied and modeled under variable amplitude loading conditions. To achieve this goal, variable amplitude axial loading tests, using a real industrial load spectrum, were performed. Mean stress effect was also studied by conducting variable amplitude fatigue tests with significant mean stress. The fatigue life of the specimens under different loading conditions was estimated based on damage tolerant approach, using modified version of Nisitani model to calculate fatigue damage. Rainflow counting method was used to separate the cycles with their ranges and mean stresses. In fatigue life calculations, extreme value statistics was also used to estimate the largest defect within the specimens based on 2D and 3D defect evaluation, obtained by metallography and X-ray computed tomography, respectively.
Keywords: variable amplitude loading, fatigue life estimation, casting defects, mean stress effect
Probabilistic Simulation of the fatigue strength of case hardened components under special consideration of residual stresses (#144)
J. A. Meis1, V. Iss2, A. Rajaei1, C. Broeckmann2
1 Flender GmbH, Bocholt, Germany
Keywords: residual stress, Probabilistic Simulation, hardened components