Design with magnesium alloy AZ31 under variable thermomechanical conditions (#18)
D. Šeruga1, M. Nagode1, J. Klemenc1
1 University of Ljubljana, Faculty of Mechanical Engineering, Ljubljana, Slovenia
New environmental demands and better fuel efficiency encourage the use of new light-weight load-bearing materials especially in the automotive industry [1,2,3]. Magnesium alloys are a promising choice as magnesium is one of the lightest metals with a convenient strength to weight ratio as compared to more traditional load-bearing materials such as steel and aluminium [1,2,3]. As the materials used in the vital load-bearing components of the chassis, brake system or exhaust system are subjected to thermomechanical fatigue during the operation due to variable mechanical and thermal loads, it is crucial for automotive design engineers to understand the cyclic behaviour of these new materials under such conditions. Due to a complex exchange of deformation mechanisms in the crystal structure of magnesium alloys, both asymmetric and anisotropic stress-strain behaviours occur during the mechanical loading [1,3,4]. In this paper we study how the stress-strain behaviour of magnesium alloy AZ31 changes during variable thermomechanical loading. Uniaxially loaded flat specimens are first mechanically loaded at room temperature and their stress-strain behaviour is followed until rupture. For the next set of the specimens, the temperature is increased whilst the variable mechanical loading remains unchanged. The stress-strain behaviour is again followed until rupture. Finally for the last set of the specimens, the temperature is decreased below the room temperature with unchanged mechanical loading. Stress-strain behaviour of the magnesium alloy AZ31 is then compared considering both variable temperature and variable mechanical load. Design guidelines are given to consider the specific properties of magnesium alloy AZ31.
 Dallmeier J, Huber O, Saage H, Eigenfeld K. Uniaxial cyclic deformation and fatigue behavior of AM50 magnesium alloy sheet metals under symmetric and asymmetric loadings. Materials & Design, 2015; 70: 10-30.
 Luo A, Magnesium: current and potential automotive applications. JOM, 2002; 54 (2): 42-48.
 Roostaei AA, Jahed H., A cyclic small-strain plasticity model for wrought Mg alloys under multiaxial loading: Numerical implementation and validation. International Journal of Mechanical Sciences, 2018; 145: 318-329.
 Wu PD, Guo XG, Qiao H, Agnew SR, Lloyd DJ, Embury JD. On the rapid hardening and exhaustion of twinning in magnesium alloy. Acta Materialia, 2017; 122: 369-377.
Keywords: durability, magnesium alloy AZ31, thermomechanical fatigue
Cut edge influence on the fatigue behavior of sheet metals under constant and variable amplitude loading (#54)
M. Thum1, P. Haefele1
1 Hochschule Esslingen, Faculty of Automotive Engineering / Laboratory of Materials and Joining Technology, Esslingen, Baden-Württemberg, Germany
In the different development stages of car body and chassis components made of aluminum or steel sheets, the components are produced with varying cutting processes. The prototype components are often produced with laser cutting, whereas the series components are mainly manufactured by shear cutting. Each cutting process results in specific properties for the cut-edge like surface finish, hardness distribution and residual stresses in the near surface region. Since the fatigue crack in such components usually starts from a cut-edge, the fatigue lifetime will depend on different conditions at cut-edge. The research work on the influence of the cut-edge conditions on the fatigue behavior so far mainly focuses on constant amplitude loading. The objective of this paper is the influence of variable amplitude loading on the cut edge effect. The results of stress-controlled fatigue test with constant and variable amplitude loading, using a Gaussian spectrum, are compared for steel and aluminum sheets with different cut-edge conditions.
Keywords: Fatigue testing, sheet metal, cut-edge influence, variable amplitude loading, Miner modifications
Evaluation of Spectrum Loaded Components by the Damage Equivalent Method of Required Fatigue Strength (RFS) through the Example of Passenger Car Wheels (#114)
C. M. Sonsino1, M. Breitenberger1, S. Schröder2
1 Fraunhofer LBF, Darmstadt, Germany
The usual way for the evaluation of components or structures submitted to variable amplitude (spectrum) loadings is the application of the Palmgren-Miner law. For this, the course of the Woehler-line after its knee point is modified according to Haibach and an allowable damage sum Dal < Dth differing from the theoretical value Dth = 1.0 is assumed. The starting information for the cumulative calculation are the local spectrum and the local Woehler-line for any individual point of the component. The position of the Woehler-line regarding the knee-point, the slopes before and after it, the probability of survival and the stress ratio must be determined by experiments or must be estimated basing on experiences. If then the calculated damage sum of the spectrum Dspec is lower than the allowable Dal, the durability requirement is fulfilled.
But there is also another way for evaluating components without knowing the real position of the local Woehler-line. This is performed by the damage equivalent method of Required Fatigue Strength (RFS) founded by V. Grubisic in the early 70ies of past century, initially for the assessment of passenger car wheels; the method is applied since then with big success also for the assessment of other components. For this, the slope before the knee point, the position of the knee point regarding the number of cycles only, and the modified course after the knee point must be assumed. Then the fictive position of the local Woehler-line resulting the required allowable damage sum Dal is derived; the stress amplitude at the rendered knee-point is the RFS-value. As long as the production process delivers a material quality exceeding the RFS-value, not only the required durability is fulfilled, but also the production quality is evaluated. Should the manufacturing process deliver an inferior value than the RFS, then improvement means such as a change of geometry or a surface treatment have to be performed.
The fictive Woehler-line which is fixed by the RFS-value offers also the possibility for substituting the appertaining spectrum by a constant amplitude loading. For this, particular criteria for the selection of the constant amplitude must be respected: These are same site of failure as under spectrum loading, maximum amplitude not higher than the maximum spectrum value and sufficient fatigue life for considering effects like friction or environmental corrosion.
The paper will show by the example of a cast aluminium wheel the evaluation of critical areas through the RFS-values derived from the local spectra of the most critical areas of the component.
Keywords: wheels, cast aluminium, cumulative damage, design evaluation
Modelling load history effects on steel fatigue properties using self-heating measurements (#129)
J. Louge1, 2, S. Moyne1, S. Calloch1, C. Doudard1, B. Weber2, B. Levieil1
1 Ensta Bretagne, UMR CNRS 6027, IRDL, F-29200, Brest, France
In high-cycle fatigue, the experimental characterization of fatigue properties is often performed using specimens in a reference state (i.e., without residual stresses or plastic deformations) and under constant stress amplitude. However, manufacturing processes of real structures can modify the initial state of the material. In addition, the applied loads are often multiaxial and with variable amplitudes. Both these history effects are considered in this study but only the variable loading aspects will be presented in this paper.
Taking into account the load history effects with classic testing would be time-consuming. It is proposed here to characterize the load history effects on the high cycle fatigue properties by means of an alternative method: self-heating measurements under cyclic loadings. This method is based on the observation of the mean steady-state temperature evolution of a specimen under constant cyclic loading. The measured temperature rise is mostly related to micro-plasticity which is linked to the fatigue properties of the material. These measurements are repeated for increasing amplitude levels in order to obtain the complete self-heating curve (Figure 1). A probabilistic two-scale model has been previously developed to estimate the S-N-P (stress-number of cycles-probability) curve from the self-heating curve . The main objective of this study is to extend this methodology to take into account the load history effect on fatigue properties.
The effect of preliminary cycles on the self-heating curves is presented in figure 2. For that, a pre-cyclic loading of thousands of cycles with constant stress amplitude is applied to a specimen before to perform a self-heating test. A significant evolution of the self-heating curves can be noticed with the pre-cyclic loading. From these experimental results, a history variable representing the macroscopic plastic strains and the microscopic cumulative plastic strains in a representative elementary volume is added in the model. To validate this modified probabilistic two-scale model, classical uniaxial fatigue tests have been performed on the same pre-loaded specimen. The effect of the preliminary cycles on the dissipation and thus on the thermal measurements induces a modification of the predicted fatigue curve obtained with the new probabilistic model, that correlates well with the classic fatigue tests.
 R. Munier, C. Doudard, S. Calloch, B. Weber, Determination of high cycle fatigue properties of a wide range of steel sheet grades from self-heating measurements, Int J Fatigue, 63 (2014), pp. 46-61
Keywords: High-Cycle Fatigue, Load History Effects, Variable amplitude loadings, Self-Heating Measurements, Two-Scale Probabilistic Model