Influence of Variable Amplitude Loading on Cyclic Material Behavior of AM Structures (#74)
M. Scurria1, R. Wagener1, T. Melz1
1 Fraunhofer Institute for Structural Durability and System Reliability LBF, Group Component-Related Material Behavior , Darmstadt, Germany
The flexibility in design offered by advanced additive manufacturing technologies makes this process more and more attractive for the automotive as well as the aircraft industry, especially for the production of safety-critical metal components. Nevertheless, while, on the one hand, additive manufacturing paved the way for new design solutions which were not possible before, on the other hand, it represents a new process, which has still not been standardized and, therefore, made exploitable. The challenge, together with improving the process, is now to build the path that will bring AM technologies from rapid prototyping to series production. Several studies have already demonstrated how the mechanical properties under quasi-static loading of metals produced by additive manufacturing and using conventional technologies can be comparable. However, the structural failures of safety-critical components are mainly caused by the effect of variable amplitude cyclic loadings, and the current state of the art about the fatigue behaviour of AM metals and its influencing factors is still limited. The few investigations available in literature aim to describe the effects on the fatigue strength of one single particular feature of AM structures, as, for example, the presence of pores, the anisotropy of the material, or the scan strategy, and are conducted on specimens manufactured from the bulk material, which are not representative of the component-related material behaviour. Moreover, some characteristics like the microstructure and the position of the pores are strictly connected to the scan strategy, or to the position of the part on the build platform, as well as to the atmosphere composition inside of the build chamber or to the quality of the powder. Assuming that the decoupling of all these factors is possible, the experimental campaigns necessary for investigating their effects on the fatigue strength of the material result extremely time consuming, also due to the large amount of homogenous specimens that have to be produced. This aspect represents a limit for the new additive manufacturing processes, which are not stable yet and for which the variation of the process parameters are often subjected to optimization. Due to the complexity of additive manufactured materials, for the formulation of a fatigue assessment, a local strain approach is to be preferred. In this work, the cyclic material behaviour of different metals used for additive manufacturing technologies is evaluated. Small-scale specimens, produced by Laser Powder Bed Fusion (LPBF) of AlSi10Mg and Inconel®718 powder, and from Scalmalloy®, were subjected to variable amplitude strain controlled tests, called Incremental Step Tests (IST), in order to evaluate the cyclic stress-strain behavior of the material. The tests have been performed at three different maximum strain amplitudes, in order to remark the effects of a different load history. Finally, the effects of anisotropy, different heat treatments and surface finishes on the cyclic stress-strain behaviour of the materials are discussed and compared.
Keywords: Variable Amplitude Loading, Cyclic Material Behavior, AM Structures
Fatigue strength values for components manufactured in the Wire Arc Additive Manufacturing process (#105)
M. Wächter1, M. Leicher2, C. Leistner3, M. Hupka1, L. Masendorf1, K. Treutler2, S. Kamper2, A. Esderts1, V. Wesling2, S. Hartmann3
1 Clausthal University of Technology, Institut für Maschinelle Anlagentechnik und Betriebsfestigkeit, Clausthal-Zellerfeld, Lower Saxony, Germany
Generative processes for the manufacturing of solid bodies (additive manufacturing) have been state of the art for several years. Process optimization and the increase and description of strength properties are in main focus of science.
One of these additive manufacturing processes is Wire Arc Additive Manufacturing (WAAM). Here, a variant of arc welding is used and the material is supplied in the form of a wire.
Characteristic values of fatigue strength are generally not available for such components, so that an operational component design is not accessible and the use in safety-relevant components can virtually not be realized economically. It is assumed that the fatigue strength of WAAM components is influenced by various process parameters, but that the working temperature and/or the cooling rate is of particular importance.
The aim of the investigations from which this article will report is to determine the local fatigue strength of components manufactured in the WAAM process. The dependency of these characteristic values on the working temperature used in the manufacturing process shall be demonstrated and described both quantitatively and qualitatively.
In this contribution, the challenges of the fabrication of suitable specimen material for fatigue testing, specimen extraction as well as the conduct of the tests shall be reported. Furthermore and even more important, the relationships between cooling rate and fatigue strength in different service life ranges determined in the tests will be reported. WAAM-material from two different working temperatures has been investigated each both in direction to the manufacturing process as well as perpendicular to the working direction.
The application of the data obtained in this way in a lifetime estimation for WAAM components is outlined.
Keywords: additive manufacturing, wire arc additive manufacturing, fatigue life, analytical strength assessment
Energy based method for fatigue damage prediction of rubber fibre composites and the influence of different modelling techniques on the method results (#25)
S. Oman1, J. Klemenc1, M. Nagode1
1 University of Ljubljana, Faculty of Mechanical Engineering, Chair for Machine Elements and Structure Evaluation, Ljubljana, Slovenia
Rubber fibre composites can be found in a broad range of consumer and industrial products such as tyres, power transmission belts, hydraulic hoses, conveyors, and air springs. All of these components are stressed with a variable amplitude (spectrum) loading and thus subjected to fatigue of the material. The content of this paper focuses on determining the fatigue life of a rubber fibre composite and, above all, the influence of different modelling techniques on the final result of the analysis. To determine the fatigue life, a recently developed energy based method [1, 2] is used to which the energy durability curve of the (critical) material and the stress-strain state of the component during operation is entered as an input. Since the structure of rubber composites is rather complex, FEM analysis is used to determine the stress-strain state over the entire structure volume during its operation. Depending on the purpose of the analysis, different modelling techniques for composite structures can be used:
Due to the different complexity of individual modelling techniques, the obtained stress strain states can vary. Similarly, critical locations can also be distinguished, as the most simplified modelling techniques neglect the real local connection between the matrix and the reinforcing fibres, but the most accurate ones take this into account.
In the article, the example of the selected rubber fibre structure show how individual modelling techniques affect the final fatigue life prediction and which of the techniques are at all suitable for such an analysis.
 Nagode M, Šeruga D. Fatigue life prediction using multiaxial energy calculations with the mean stress effect to predict failure of linear and nonlinear elastic solids. Results Phys. 2016;6:352‐364. https://doi.org/10.1016/j.rinp.2016.06.007
 Gosar A, Nagode M, Oman S. Continuous fatigue damage prediction of a rubber fibre composite structure using multiaxial energy based approach. Fatigue Fract Eng Mater Struct. 2018;1.13. https://doi.org/10.1111/ffe.12908
Keywords: rubber fibre composite, fatigue life, finite element analysis
Behaviour of a grain refined low carbon bainitic TRIP steel under cyclic loading (#13)
I. Burda1, R. Koller1, C. Affolter1, A. Arabi-Hashemi2, K. Zweiacker2, L. Oberli3, H. Roelofs4
1 Empa Dübendorf, Mechanical Systems Engineering, Dübendorf, Zürich, Switzerland
Applying a new industrial hot rolling technology called XTP to long products, finer austenite grain sizes (~5 microns) in comparison to conventional hot rolling (~25 microns) are obtained [1-2]. As a consequence the impact toughness can be significantly improved. Furthermore, it is expected that grain size refinement also affects dynamic properties, although the interplay is not obvious. It is well known that TRIP steels can undergo phase transformation under deformation or overload, as shown in . However, the potency of this effect will be a function of the mechanical stability of retained austenite, and thus, microstructure fineness.
In the present work low carbon bainitic TRIP steels were produced by conventional hot rolling with and without subsequent grain refinement by XTP. Thus, microstructures containing different grain sizes were obtained and investigated.
In order to analyse the influence of grain refinement on fatigue behaviour tensile-compression tests (R=-1) were performed using unnotched specimens. The cyclic deformation behaviour was monitored continuously by measuring mechanical stress-strain hysteresis loops alongside with material temperature [4,5]. Furthermore, the fatigue lifes under constant (Wöhler) and variable amplitude loading (Gassner) were investigated. In case of Gassner tests a Gaussian distribution of stress amplitudes was used.
FEG-Scanning Electron Microscopy, Electron Back Scattering Diffraction, X-Ray Diffraction and measurement of the magnetic saturation using a Vibrating-Sample Magnetometer (VSM) were applied for detailed investigations of the microstructure and its subsequent changes.
In addition, fatigue crack growth and microstructural changes at the crack tip were studied to get a better understanding about potential effects of the microstructure on crack propagation (e.g. grain size, TRIP effect, textures).
 A. Borowikow, H. Blei, “Integration of Screw Rolling in the Thermo-Mechanical Treatment of Steel Bars”, The Roll Pass Designer No. 72, p. 53-60 (2011)
 M. I. Lembke, L. Oberli, G. Olschewski, “Probing the limits of steel by producing an ultrafine microstructure in a single extreme deformation step“, 5th Int. Conf. on Steel in Cars and Trucks, Amsterdam-Schiphol, The Netherlands (2017)
 L. Elek, R. Wagener, H. Kaufmann, V. Wirths, T. Melz, “New Bainitic Steel for Cyclic Loaded Safety Parts with Improved Cyclic Material Behaviour”, 3rd International Conference on Material and Component Performance under Variable Amplitude Loading, VAL2015, Procedia Engineering 101, p.151 – 158 (2015)
 P. Starke, F. Walther, D. Eifler, “PHYBAL—A new method for lifetime prediction based on strain, temperature and electrical measurements”, International Journal of Fatigue, p.1020-1036 (2006)
 F. Walther, „Physikalisch basierte Messverfahren zur mikrostrukturbasierten Charakterisierung des Ermüdungsverhaltens metallischer Werkstoffe“, TU Kasierslautern: Professorial dissertation (2007)
Keywords: grain refinement, TRIP effect, bainitic steel, crack propagation, Gassner curves