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Fatigue Principles Many castings are subjected to


cyclic loading and consequently fatigue strength is the most critical design parameter. Most laboratory testing programs


measure cyclic properties and the fa- tigue strength in fully reversed loading. Te testing is performed on machined and, smooth-walled specimens, not castings. For many wrought materials the fatigue strength or endurance limit generally equals one- third to one-half of the tensile strength. In castings, due to the presence of discontinuities, the fatigue strength can be lower, approach- ing a quarter of the tensile strength. Until recently, the fatigue proper- ties of the various cast metals have been unavailable. AFS recognized this and several years ago began to build a database on the graphitic irons. Te database includes the strain-life prop- erties that the Finite Element Analysis (FEA) models require to predict the fatigue strength of a part. Te database is being expanded to include aluminum


Fig. 5. Stress–strain curves of four ductile iron test bars removed from various locations in a large cylindrical ductile iron casting are shown.


and steel casting alloys. Designers are using the database to design castings for structural applications.


Discussion Many defect indications can be


reduced or eliminated. Te facility


engineer can use chills to refine or eliminate porosity in critical areas of the casting. Addition agents can be utilized for grain refinement. Extra care can be employed to improve surface finish. Controlled cooling can help reduce residual stresses. Shot- peening, fillet rolling and other surface treatments can be applied to promote beneficial compressive stresses to resist fatigue failure. All these activities con- tribute to increased fatigue strength. While porosity and other discon-


tinuities reduce the monotonic and cyclic strength of castings, the tremen- dous freedom of design can make cast parts competitive with wrought parts and weld assemblies. Due to limitations in the design of


the part, wrought parts often contain local regions with stress concentra- tions. Welded joints produce elevated residual stresses that further reduce the fatigue strength of a part. By flexibility in design, both types of features can be substantially reduced or removed by redesigning of the part as a casting. Investigators have shown the


Fig. 6. This schematic shows the relationship between shape and imposed stress. (a) Maxi- mum fiber stress (at notch in location 1) due to 14,000 lb. load is 66,000 psi and (b) after redesign the maximum fiber stress due to 14,000 lb. load is 25,000 psi.


design stresses can be substantially re- duced by redesigning a part, thus mak- ing castings quite competitive with wrought parts. A good understanding of the power of design will make a casting stronger and more tolerant of the imperfections in castings. 


Tis paper was adapted from the Hoyt Lecture delivered at the 2016 CastExpo in Minneapolis.


Nov/Dec 2016 | METAL CASTING DESIGN & PURCHASING | 37


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