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AFS RESEARCH REPORT


COMPARISON OF MONOTONIC AND CYCLIC PROPERTIES OF DUCTILE IRONS IN THE AFS/DOE STRAIN-LIFE FATIGUE DATABASE FOR CAST IRON


J. Tartaglia Element Materials Technology, Wixom, MI, USA Copyright © 2012 American Foundry Society Abstract


A strain-life fatigue database for cast irons was developed utilizing American Foundry Society (AFS) and the United States Department of Energy (DOE) funding. The database contains monotonic and cyclic property data, as well as the associated chemical analysis and microstructural data for a variety of cast irons, including gray, ductile, compacted graphite, and white cast irons.


This paper first reviews the contents and format of the da- tabase. The database and its associated report contain no comparative analysis of any of the cast iron grades. The fresh approach of this paper is to analyze and compare the data across each section size and strength-ductility combi- nation for four ductile iron grades, i.e., ferritic 60-40-18, ferritic-pearlitic 65-45-12, pearlitic 100-70-03 and tem- pered martensitic (Q&T) 120-90-02.


The ductile cast iron results in the database generally exhib- ited the expected trends. Modulus and Poisson’s ratio were the same for all the conditions. Monotonic strength always decreased with increasing elongation and ferrite content.


Introduction


Description of Fatigue A standard definition1


of fatigue is that it is the process of


progressive, localized, and permanent structural change oc- curring in a material subjected to conditions


• that produce fluctuating stresses and strains at some point or points, and


• that may culminate in cracks or complete fracture after a sufficient number of fluctuations.


Another common definition2 introduces the concepts that


• fatigue fractures occur at fluctuating stresses with a maximum value less than the ultimate tensile strength of the material,


• fatigue failure generally occurs at loads which ap- plied statically would produce little perceptible ef- fect, and


International Journal of Metalcasting/Spring 2012


For over hundred years, the stress-life or S-N method was the first approach used in an attempt to quantify and design by fatigue. Although the S-N approach can be used in design applications where the applied stress is primarily within the elastic range of material response, the strain-life approach is required for low cycle fatigue life predictions between 10 and 100,000 cycles.3


The low cycle or strain-life approach also offers the advan- tage that both fatigue stress and strain are tested, analyzed, and modeled. Although high cycle fatigue only handles stress, low cycle or strain-life fatigue testing offers the op- portunity for stress and strain to be calculated from each oth- er. It is nearly impossible to measure stress with load cells in complex and moving structures, whereas strain can be read- ily measured with optical or strain gauge methods.


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The higher strength grades and conditions exhibited greater high cycle fatigue resistance and decreased low cycle fa- tigue resistance.


Heat treatments had a mixed effect on monotonic properties. Annealing decreased the monotonic strength of 60-40-18 whereas normalizing increased the strength and dramatically increased fatigue resistance of pearlitic 100-70-03. Monoton- ic ductility was only slightly affected by heat treatment.


However, in most conditions, the heat treatments produced greater low cycle fatigue lives for high ductility conditions. Although the high cycle fatigue resistance was affected less by heat treatment, slightly increased high cycle fatigue lives were obtained with higher strength conditions. The normal- ized 25 mm samples of grade 100-70-03 exhibited signifi- cantly greater monotonic strength as well as fatigue resis- tance in both the low and high cycle regime.


Keywords: strain-life fatigue, low cycle fatigue, cast iron, ductile cast iron, microstructure, heat treatment


• fatigue fractures begin as minute cracks that grow under the action of fluctuating stress.


High versus Low Cycle Fatigue


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