Prediction of the Fatigue Life of Cast Steel Containing Shrinkage Porosity
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UCTION
INHOMOGENEITIES due to porosity are currently not considered in the design of structural components made from metal castings. Instead, ad hoc safety factors are used to address a designer’s uncertainty in how the casting will perform in service. These safety factors are based on the assumption that castings perform unpredictably, if not poorly. Applying such safety factors to the entire cast material might do little for the robustness of the design other than increase the casting weight. Many part designers become frustrated by castings designed with very large safety factors that fail in service; they are hesitant to use castings. Such frustrations could be avoided if the quality of the cast metal throughout the casting could be known ahead of time and incorporated into the design. The present study extends our recently developed method of modeling the effects of porosity on stiffness and stress redistribution[1] to the prediction of the fatigue life of steel castings containing porosity. For ease of use in standard design practice, commonly used commercial software is employed in the present stress RICHARD A. HARDIN, Research Engineer, and CHRISTOPH BECKERMANN, Professor, are with the Department of Mechanical and Industrial Engineering, University of Iowa, Iowa City, IA 52242. Contact e-mail: [email protected] Manuscript submitted January 30, 2008. Article published online January 21, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
and fatigue life simulations. The part design can be made safe by assuring that the casting process results in the best possible quality (i.e., lowest porosity) steel at highly stressed locations. Should porosity form, the casting rigging or process parameters can be changed so that the porosity will not affect the service performance of the part. If designers wish to use lighter-weight and more thinly walled steel castings, understanding the effects of porosity becomes especially critical. An integrated design process is emerging in which a casting process simulation that predicts the location, amount, and size of the porosity is directly coupled with the mechanical simulation of the part performance that takes into account the effects of porosity.[2] It is anticipated that such a design process will also help guide and improve casting inspection procedures, by linking acceptance criteria with expected performance. Fatigue life analysis can be divided into two parts:[3] (1) the stage of life of the component up to the initiation of a crack on the order of 1 mm in size and (2) the stage of life of the component undergoing the growth of a crack and its propagation to failure. The combination of the two gives the total life of a component. Perhaps the most often used approach to predicting crack initiation is the strain-life method,[3–6] in which specimen fatigue life test data to failure is fitted to the strain-life curve and the curve is used in life estimation. Because fatigue test specimens are small compared to most components, once cracks initiate in them, propagation i
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