13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Experimental investigations on the crack growth behavior in graded ferritic martensitic steel Tobias Stein1,*, Frank Zeismann1, Angelika Brückner-Foit1 1 Institute of Materials Engineering, Quality and Reliability Group, University of Kassel, Moenchebergstrasse 3, D-34109 Kassel, Germany * Corresponding author: stein@uni-kassel.de Abstract An experimental approach is presented for characterizing the fatigue behavior of a functionally graded flange shaft. Micro-specimens are defined based on the inhomogeneous phase distribution and microstructure of the flange. Subsequently the experimental setup is described for the investigation of the phase dependent fatigue behavior, and some results are presented. It was found that there is a significant amount of crack retardation in the transition zone in the flange which is located between the soft ferritic phase and the hard martensitic phase. This is verified using a two-sided observation of the propagating crack with two travelling microscopes. Keywords Graded material, crack growth, fatigue, edge crack 1. Introduction Functionally graded steel components can play a key-role in the on-going challenges about increase of productivity and saving of resources as they can be tailor-made to the specific requirements in a given application case. However, standard testing and design procedures based on results obtained with homogenous materials may not be sufficient to ensure reliable operation of such components as the interaction of the different regions in the graded component can induce additional damage mechanisms. This is especially true for microstructure-based failure processes such as fatigue failure. A simple example for a functional gradation is the surface hardening process of a steel component in order to achieve a soft, ductile core and a hard, wear resistant surface. The fatigue behavior of case or surface hardened steel component has investigated (e.g. [1, 2]) recently, and it was found that the failure behavior could not be predicted by just superimposing the results obtained with the constituents in standard tests. Another, more complex way to produce a functionally graded component is an integrated thermo- mechanical treatment on a steel shaft to form a flange shaft. [3] Such a flange shaft is characterized by a defined local distribution of at least three phases – the soft ferritic-pearlitic base material in an untreated and a deformed state, a hard martensitic phase and a small, mainly bainitic transition zone in between. Under fatigue loading, crack may initiate in the soft phase and propagate towards the hard phase leading to catastrophic failure. In this case, crack initiation and propagation depends not only on the fatigue properties of each phase, but also on their spatial distribution. Obviously, conventional fatigue testing with homogenous specimens will not capture these specific features of the damage accumulation process in a graded component, and may therefore lead to erroneous results. Instead, specimens have to be defined which can directly be cut from the component and contain the phases of interest together with the component specific grading [4, 5]. This paper deals with obtaining valid crack growth data in the flange-shaft mentioned above [3].
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