The effect of microstructure on fatigue crack propagation in iron-carbon alloys
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The dependence of fatigue c r a c k growth rate on the cyclic s t r e s s intensity factor was determined for six iron-carbon alloys r a n g i n g in carbon content from 0.23 t o 1.08 wt pct carbon. Both ferrite/pearlite and f e r r i t e / f r e e iron carbide microstructures were studied. Scanning electron microscope fractography studies correlated the fatigue mechanism with microstructure. It was found that when the predominant mode of c r a c k growth was ductile, the c r a c k growth rate d a / d N could be r e l a t e d to the cyclic s t r e s s intensity f a c t o r by an equation of the form d a / d N = C (AK) m w h e r e C and m are constants. The constant m was approximately e q u a l t o four when the c r a c k growth mechanism presumably was the blunting and resharpening of the c r a c k tip by slip processes. The constant rn was g r e a t e r than four when the c r a c k growth mechanism was void coalescence in the int e r l a m e l l a f e r r i t e of pearlite colonies. The preferred fatigue c r a c k p a t h through the pearlitic alloys was through the free f e r r i t e phase.
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p u r p o s e of this study is to determine the effect of microstructure on fatigue c r a c k growth rate in binary iron-carbon alloys. We wish to know how the functional dependence of c r a c k growth rate on the cyclic s t r e s s intensity factor v a r i e s with the morphology, amount, and distribution of each microconstituent. The rate of growth of fatigue c r a c k s in many materials over a l a r g e r a n g e of growth r a t e s is knownI-S t o be g i v e n by the P a r i s equation: da/dN
=
C (AK) rn
[1]
where da/dN = the a v e r a g e fatigue c r a c k growth increment d u r i n g each s t r e s s cycle, = the r a n g e of the s t r e s s intensity factor during each s t r e s s cycle = Kmax - K m i n , and C = a constant dependent upon the properties of of the material. We wish t o e x a m i n e the validity of Eq. [1] for describing fatigue c r a c k growth rate in various iron-carbon alloys. Furthermore, if c r a c k growth rate in these alloys is r e l a t e d to the cyclic s t r e s s intensity factor by Eq. [1], w e wish to determine how the value of m is related to microstructure. The iron-carbon system was chosen for this study because grossly different microstructures could be obtained for materials with the same c h e m i c a l composition by s i m p l e variations in the heat treatment. The effect of each microconstituent on the rate at which a fatigue c r a c k grows was determined by comp a r i n g the functional dependence of c r a c k growth rate on the cyclic s t r e s s intensity factor for alloys with the same chemical composition but containing different microconstituents and for alloys containing the
C.R. AITA, formerly Research Assistant at Materials Science and Engineering Department and Materials ResearchCenter, Northwestern University, is now Senior Scientist at Gould Laboratories,Gould Inc., Rolling Meadows, Ill 60008. J. WEERTMAN is WalterP. Murphy Professor of Materials Scie
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