Resistivity-Based Evaluation of the Fatigue Behavior of Cast Irons
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INTRODUCTION
CAST irons are used for various applications in the automotive and commercial vehicle industry. Recently, there has been an increasing interest in cast irons as structural components in the wind power industry, e.g., rotor hubs or nacelles.[1–4] For all these components, a detailed knowledge of the cyclic deformation behavior is of major importance. Generally, the plastic strain amplitude ea,p is used to describe the material response to cyclic loading.[5–7] In recent years, it has become more and more common to complete mechanical hysteresis data by additional electrical[8–11] measurements. In particular, for material and loading conditions leading to very small plastic deformation, complementary changes in electrical resistance data provide a high potential to describe cyclic deformation processes. The physical quantities ea,p and DR are directly related to deformation-induced changes of the microstructure and represent the actual fatigue state.[12–15] Besides the geometry, the change in electrical resistance DR depends on the resistivity q*, which is very sensitive in respect to load- and cycle-dependent defects, e.g., dislocation density and arrangement, vacancies, micro-pinholes, or micro-cracks. In the literature, cast-iron-related fatigue investigations are available primarily for nodular cast iron grades.[16–20] Lifetime-oriented investigations were done to evaluate the geometrical size and mean stress effect[16]
HOLGER GERMANN and PETER STARKE, Scientific Assistants, and DIETMAR EIFLER, Head, are with the Institute of Materials Science and Engineering, University of Kaiserslautern, 67653 Kaiserslautern, Germany. Contact e-mail: [email protected] Manuscript submitted April 5, 2011. Article published online August 12, 2011 2792—VOLUME 43A, AUGUST 2012
as well as the influence of the sampling position[17] on the fatigue behavior. In References 18 and 19, fatigue tests with mechanical stress-strain hysteresis measurements are shown. There are only a few contributions dealing with the fatigue behavior of lamellar[21] and compacted cast irons.[15] In the scope of this article, mechanical stress-strain (r-e) hysteresis and change in electrical resistance (DR) measurements were applied to describe the fatigue behavior of the lamellar cast iron EN-GJL-250 (ASTM A48 35B), the compacted cast iron EN-GJV-400, and the nodular cast iron EN-GJS-600 (ASTM 80-55-06). To evaluate the relation between cyclic deformation processes and DR, in defined fatigue states, the changes of DR and q*, respectively, were correlated with SEM micrographs. In addition, the electrical resistance was measured in the virgin material state (R0) to get detailed information about the defect density of each individual specimen.
II.
MATERIALS
The investigated materials were provided by the Daimler AG in round bars with a diameter of 36 mm and a length of 300 mm. Figure 1 shows characteristic light (LM) and scanning electron micrographs (SEM) of EN-GJL-250 (a), EN-GJV-400 (b), and EN-GJS-600 (c). The Brinell hardness, the ferrite fr
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