ISSUE 1/2015

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Stroppe, H.; Bähr, R. Calculating the fatigue strength of ductile iron from tensile tests

The growing use of ductile cast iron materials in safety-relevant components and structures increases the importance of methods and concepts for a stress-related assessment of the strength behavior of these materials. Particular importance is given to spheroidal graphite cast iron (SGI) since it incorporates both the advantages of lamellar graphite iron and the good properties of steel, such as high strength, good hardenability along with high levels of ductility. This also involves the cyclic strength properties of ductile iron, of which the relevant “Wöhler” curve (or s-n curve) provides information. The Wöhler curve describes the relationship between the tolerable strain amplitude of the material or component and the number of cycles to failure N at fatigue failure. Some references describe an analytical model based on fracture-mechanical considerations for the calculation of the Wöhler curve for aluminum casting alloys from parameters of the uniaxial tensile test (tensile strength Rm, 0.2 % yield limit Rp0,2  and modulus of elasticity E). This article seeks to examine the extent to which this calculation model – if necessary, with modifications – can also be applied to the calculation of the fatigue strength of SGI materials. For this purpose, the fatigue strengths and life span determined from tensile test values of various SGI materials in different structural conditions, whose Wöhler curves were determined and thoroughly documented in a further publication, are calculated and compared to the test values derived from this reference.  

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