Resistivity-Based Evaluation of the Fatigue Behaviorof Cast Irons

HOLGER GERMANN, PETER STARKE, and DIETMAR EIFLER

Cast irons are used in particular for highly stressed components in the automotive and com-mercial vehicle industry, e.g., for crankcases and in the wind power industry, e.g., for rotorhubs. The mechanical properties of cast irons are strongly influenced by parameters like phasecomposition of the matrix, graphite shape, micro-pinholes, and micro-cracks. The measurementof the electrical resistance in the unloaded state and its change during cyclic loading offers thepossibility to get detailed information about the actual defect density and the cyclic deformationbehavior. In the scope of the present work, stress-controlled load increase tests and constantamplitude tests were carried out at ambient temperature with specimens of the perlitic cast ironsEN-GJL-250 (ASTM A48 35B), EN-GJV-400, and EN-GJS-600 (ASTM 80-55-06). Besideelectrical measurements, scanning electron microscopy (SEM) was used to characterize themicrostructure and to correlate the change of microstructural details with cyclic properties.

DOI: 10.1007/s11661-011-0852-3� The Minerals, Metals & Materials Society and ASM International 2011

I. INTRODUCTION

CAST irons are used for various applications in theautomotive and commercial vehicle industry. Recently,there has been an increasing interest in cast irons asstructural components in the wind power industry, e.g.,rotor hubs or nacelles.[1–4] For all these components, adetailed knowledge of the cyclic deformation behavior isof major importance.

Generally, the plastic strain amplitude ea,p is used todescribe the material response to cyclic loading.[5–7] Inrecent years, it has become more and more common tocomplete mechanical hysteresis data by additionalelectrical[8–11] measurements. In particular, for materialand loading conditions leading to very small plasticdeformation, complementary changes in electrical resis-tance data provide a high potential to describe cyclicdeformation processes. The physical quantities ea,p andDR are directly related to deformation-induced changesof the microstructure and represent the actual fatiguestate.[12–15] Besides the geometry, the change in electricalresistance DR depends on the resistivity q*, which isvery sensitive in respect to load- and cycle-dependentdefects, e.g., dislocation density and arrangement,vacancies, micro-pinholes, or micro-cracks.

In the literature, cast-iron-related fatigue investiga-tions are available primarily for nodular cast irongrades.[16–20] Lifetime-oriented investigations were doneto evaluate the geometrical size and mean stress effect[16]

as well as the influence of the sampling position[17] onthe fatigue behavior. In References 18 and 19, fatiguetests with mechanical stress-strain hysteresis measure-ments are shown. There are only a few contributionsdealing with the fatigue behavior of lamellar[21] andcompacted cast irons.[15]

In the scopeof this article,mechanical stress-strain (r-e)hysteresis and change in electrical resistance (DR) mea-surementswere applied to describe the fatigue behavior ofthe lamellar cast iron EN-GJL-250 (ASTMA48 35B), thecompacted cast iron EN-GJV-400, and the nodular castiron EN-GJS-600 (ASTM 80-55-06). To evaluate therelation between cyclic deformation processes and DR, indefined fatigue states, the changes of DR and q*,respectively, were correlated with SEM micrographs. Inaddition, the electrical resistance was measured in thevirgin material state (R0) to get detailed informationabout the defect density of each individual specimen.

II. MATERIALS

The investigated materials were provided by theDaimler AG in round bars with a diameter of 36 mmand a length of 300 mm. Figure 1 shows characteristiclight (LM) and scanning electron micrographs (SEM) ofEN-GJL-250 (a), EN-GJV-400 (b), and EN-GJS-600(c). The Brinell hardness, the ferrite fraction, and thegraphite fraction are summarized in Table I.The microstructure consists predominantly of a pearl-

itic matrix with lamellar graphite (EN-GJL-250), nod-ular graphite (EN-GJS-600), and compacted graphite(EN-GJV-400). In contrast to the lamellar graphite(compare with Figure 1(a)), the nodular graphite (com-pare with Figure 1(c)) and partially the compactedgraphite (compare with Figure 1(b)) are surrounded bya ferrite zone.

HOLGER GERMANN and PETER STARKE, Scientific Assis-tants, and DIETMAR EIFLER, Head, are with the Institute ofMaterials Science and Engineering, University of Kaiserslautern,67653 Kaiserslautern, Germany. Contact e-mail: germann@mv.uni-kl.de

Manuscript submitted April 5, 2011.Article published online August 12, 2011

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III. EXPERIMENTAL SETUP

Prior to the fatigue tests, the electrical resistance R0 inthe unloaded state was measured with the experimentalsetup shown in Figure 2 in a photographic (a) and in aschematic (b) manner. R0 is strongly influenced by theindividual microstructure of each specimen and mate-rial, e.g., defect density, ferrite and graphite fraction, orgraphite shape. Among others, by means of R0, it ispossible to detect differences between the specimensconcerning the individual density of typical castingdefects like micro-pinholes (compare with Figure 6).

Stress-controlled load increase tests (LITs) and con-stant amplitude tests (CATs) were carried out at ambienttemperature with a frequency of 5 Hz on servohydraulictesting systems using a triangular load-time function at aload ratio of R = –1. In the LITs, the stress amplitudera was increased from ra, start = 120 MPa continuouslywith the rate dra/dt = 11.1 9 10�3 MPa/s until speci-men failure. The CATs were performed until failure or toa maximum number of cycles Nmax of 2 9 106. InFigure 3, the experimental setup is shown in a photo-graph (a) and schematically (b).

During cyclic loading, the plastic strain amplitude ea,pand the change in electrical resistance DR were mea-sured to detect deformation-induced changes in themicrostructure. For the measurement of ea,p, an exten-someter was fixed in the middle of the gauge length. Tomeasure the electrical resistance, a DC-power supplywas fixed at both shafts and two wires were spot weldedat the transition of the gauge length and the shafts(compare with Figure 3). Besides the geometry, thechange in electrical resistance DR strongly depends onthe resistivity q*, which is directly related to the defectstructure and density. In the case of cast iron, q* isadditionally influenced by graphite-matrix debonding.

IV. RESULTS AND DISCUSSION

A. Load Increase Tests

LITs allow a reliable estimation of the endurance limitwith one single specimen. In Figure 4, the stress ampli-tude ra, the plastic strain amplitude ea,p, and the changein electrical resistance DR are plotted versus the number

Table I. Mechanical and Microstructural Parameters of the Investigated Cast Irons

EN-GJL-250 EN-GJV-400 EN-GJS-600

Brinell hardness [HBW30] 224 ± 7 227 ± 5 235 ± 6Ferrite fraction [area-pct] 2.7 ± 0.5 7.4 ± 0.8 14.6 ± 2Graphite fraction [area-pct] 12.1 ± 0.1 11.5 ± 1.2 9.8 ± 0.8

Fig. 1—LM and SEM micrographs of the microstructure of the cast irons EN-GJL-250 (a), EN-GJV-400 (b), and EN-GJS-600 (c).

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of cycles N for the cast iron EN-GJS-600. The DR-Ncurve indicates an initial decrease, among others causedby closing micro-cracks during the compression halfcycles (compare with Figure 10(b) and (c)). Then, thecourses of the change in electrical resistance DR arecharacterized by a saturation state between 6 9 104 and1 9 105 cycles, followed by an increase indicating cumu-lative graphite-matrix debonding (compare Figure 9 aswell as Figure 10(d) and (e)). A significant change in theslope of the ea,p-N and DR-N curves of the LIT isobserved at rRW, LIT = 220 MPa (indicated in Figure 4by the dashed lines). This stress amplitude rRW, LIT canbe used for the estimation of the endurance limit.[22]

B. Constant Amplitude Tests

In Figure 5(a), the electrical resistance R0 of theunloaded state is plotted versus the number of cycles tofailure Nf in the range 110 £ ra £ 140 MPa for the castiron EN-GJL-250. For the CATs performed with

Fig. 2—Experimental setup for electrical resistance measurements (R0) in the unloaded state in a photograph (a) and schematically (b).

Fig. 3—Experimental setup for mechanical hysteresis and electrical resistance measurements during cyclic loading in a photograph (a) and sche-matically (b).

Fig. 4—Plastic strain amplitude and change in electrical resistance ina continuous load increase test of the cast iron EN-GJS-600.

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ra = 110 MPa, the Nf values were indicated by num-bers that correspond to the indices at ra = 110 MPa inthe Woehler curve (compare with Figure 5(b)). On thebasis of R0 a linear fit of R0-Nf for each stress amplitudeexists. The R0-Nf relations prove that specimens withhigher (lower) R0 values consequently show a shorter(longer) lifetime. This result underlines that R0 valuescan be used to identify differences between the speci-mens concerning casting defects like micro-pinholes inthe virgin state of the individual microstructure. TheSEM micrographs in Figure 6 document that the virginmicrostructure of EN-GJL-250 includes characteristicmicro-pinholes.

Figure 7 shows the comparison of the cyclic defor-mation (ea,p-N) (a) and cyclic resistance (DR-N) (b)curves taken from CATs with ra = 140 MPa (EN-GJL-250), ra = 210 MPa (EN-GJV-400), and ra = 280MPa (EN-GJS-600). The stress amplitudes used in theCATs were chosen to reach comparable lifetimes of theinvestigated materials of approximately 1.2 9 104.The stress amplitude for EN-GJL-250 is half of the

stress amplitude for EN-GJS-600. Under these loadingconditions, the highest values of ea,p as well as of DR aremeasured for EN-GJL-250 because of the relatively highlocal stress concentrations at the tips of the graphitelamellae. The ea,p-N curves of EN-GJL-250, EN-GJV-400, and EN-GJS-600 show plastic strain amplitudes ofabout 0.068 9 10�3, 0.051 9 10�3, and 0.043 9 10�3

during the first cycles, followed by cyclic hardeningprocesses. The DR-N values indicate a decrease with aminimum in DR of about –0.67 lX, –2.11 lX, and–3.46 lX, respectively. The courses of the measuredvalues are a function of the individual microstructure ofeach cast iron and strongly depend on the internal notchfactor of the different graphite shapes. Finally, pro-nounced graphite-matrix debonding (compare withFigure 9(a)–(d)) and macroscopic crack growth withsignificantly increasing DR values until specimen failureare measured. The effect was proved by SEM; e.g.,Figure 9(a) and (b) reveal a considerable increase ofgraphite-matrix debonding between the fatigue states 50pct Nf and 100 pct Nf for the lamellar cast iron

Fig. 5—R0-Nf fits for constant amplitude tests with stress amplitudes in the range of 110 £ ra £ 140 MPa (a) as well as a Woehler curve (b) forthe cast iron EN-GJL-250.

Fig. 6—SEM micrographs of micro-pinholes in the virgin microstructure of the cast iron EN-GJL-250.

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EN-GJL-250. These results prove that the plastic strainamplitude as well as the electrical resistance can be usedto describe the development of fatigue damages in castiron.

Cyclic resistance curves of the cast iron EN-GJS-600are plotted in Figure 8. In the range 240 £ ra £ 340MPa, with increasing stress amplitude, the minimumvalues of DR increase from –2.20 lX for ra = 240 MPato –1.32 lX for ra = 340 MPa, and the numbers ofcycles to failure consequently decrease from 1 9 106

cycles for ra = 240 MPa to 2 9 104 cycles forra = 340 MPa. Figure 9(c) and (d) show SEM micro-graphs of the microstructure in fatigue states after theDR minimum for constant amplitude loading withra = 280 MPa. In the early fatigue state at 25 pct Nf,

the initial matrix debonding of the graphite (comparewith Figure 9(c)) correlates very well with the increase ofDR (compare with Figure 8). Finally, a more pro-nounced graphite-matrix debonding which can beobserved in Figure 9(d) at 100 pct Nf and increasingmicro-crack formation leads to significantly increasingDR values until specimen failure.The CAT performed with a ra = 220 MPa reaches

2 9 106 cycles without failure. This result correspondsvery well with the endurance limit determined in theload increase test shown in Figure 4.

C. Microscopic Investigations

As shown in Figures 7(b) and 8, the electrical resis-tance of cast irons changes in a characteristic mannerduring cyclic loading. To investigate the deformationprocesses and mechanisms causing the change in elec-trical resistance DR in more detail, specimens in definedfatigue states were observed in the SEM.In Figure 10, selected SEM micrographs of the cast

iron EN-GJL-250 are related to a characteristic DRcourse. Figure 10(a) shows the change in electricalresistance DR for a constant amplitude test withra = 120 MPa. Already at N = 50 cycles, whichcorresponds to 0.5 pct Nf, localized stresses at the tipsof the graphite lamellae result in graphite-matrixdebonding and in the initiation of first micro-cracks(compare with Figure 10(b)). Graphite-matrix debond-ing as well as the initiation of micro-cracks conse-quently leads to an increase of the DR values. Butnevertheless, during further cycling, decreasing DRvalues were measured as a result of superimposedeffects like closing of micro-pinholes (compare withFigure 6). With increasing number of cycles, addition-ally micro-crack closure, which can be observed atN = 200 (compare with Figure 10(c)), results in afurther decrease of DR. Both the closure of micro-pinholes and micro-cracks, respectively, leads to an

Fig. 7—Cyclic deformation curves (a) and cyclic resistance curves (b) for constant amplitude tests with a reference number of cycles ofNf = 1.2 9 104 for the cast irons EN-GJL-250, EN-GJV-400, and EN-GJS-600.

Fig. 8—Cyclic resistance curves for constant amplitude tests with220 £ ra £ 340 MPa for the cast iron EN-GJS-600.

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increase in the effective cross section in the specimen’sgauge length and consequently to decreasing DR values.The reopening of micro-cracks at N = 1 9 103 (com-pare with Figure 10(d)) as well as the initiation andpropagation of additional micro-cracks (compare withFigure 10(d)) lead again to an increase of the DRvalues. Finally, macroscopic crack-growth and pro-nounced graphite-matrix debonding results in a sharpincrease of DR until specimen failure (compare withFigure 10(e)). These results underline the capability ofelectrical resistance measurements to investigate onlinefatigue-dependent microstructural changes of cast irons.

V. CONCLUSIONS

The fatigue behavior of the cast irons EN-GJL-250,EN-GJV-400, and EN-GJS-600 was characterized inload increase and constant amplitude tests on the basisof mechanical stress-strain hysteresis and electricalresistance data. The plastic strain amplitude (ea,p) aswell as the change in electrical resistance (DR) can beused equivalently to describe cyclic deformation pro-cesses of the investigated cast irons.

In load increase tests, the endurance limit was reliablyestimated with one single specimen. Furthermore, thistest procedure allows the determination of appropriatestress amplitudes for constant amplitude tests. The R0

values of virgin specimens can be correlated with thenumbers of cycles to failure of a series of specimens inconstant amplitude tests. Specimens characterized bythe highest (lowest) R0 value tend to have the shortest(longest) number of cycles to failure and vice versa. Thisproves that the electrical resistance can be used toidentify fatigue-relevant differences in the microstruc-ture of cast irons.In stress-controlled constant amplitude tests, the

cyclic deformation behavior is dominated by cyclichardening until macroscopic crack initiation. The com-parative evaluation of the fatigue behavior of theinvestigated cast irons reveals a decrease of ea,p andDR in the sequence EN-GJL-250, EN-GJV-400, andEN-GJS-600. In SEM investigations, the changes inelectrical resistance DR could be correlated with micro-crack closure as well as with different states of graphite-matrix debonding and micro-crack growth. Theseresults underline the high potential of DR measurementsto assess the fatigue behavior of cast irons.

Fig. 9—SEM micrographs of graphite-matrix debonding at the fatigue states N = 50 pct Nf (a) and N = 100 pct Nf (b) for the cast ironEN-GJL-250 as well as at the fatigue states N = 25 pct Nf (c) and N = 100 pct Nf (d) for the cast iron EN-GJS-600.

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ACKNOWLEDGMENTS

The support of the German Research Foundation(Deutsche Forschungsgemeinschaft) is gratefullyacknowledged.

REFERENCES1. R. O’Rourke: Adv. Mater. Process, 2001, vol. 159, pp. 65–68.2. S. Dawson and T. Schroeder: Trans. Am. Foundry Soc, 2004,

vol. 112, pp. 813–21.3. S. Shao, S. Dawson, and M. Lampic: Materialwissenschaft und

Werkstofftechnik, 1998, vol. 29, pp. 397–411.4. D. Veldkamp: Wind Energ, 2008, vol. 11, pp. 655–72.5. R.W. Landgraf, J.D. Morrow, and T. Endo: J. Mater, 1969, vol. 4,

pp. 176–88.6. P. Lukas and M. Klesnil: Mater. Sci. Eng, 1973, vol. 11, pp. 345–

56.7. J. Polak: Int. J. Fatigue, 2005, vol. 27, pp. 1192–1201.8. J. Polak: Czech. J. Phys, 1969, vol. 18, pp. 315–22.9. U. Holzwarth and P. Schaaff: Phys. Rev. B, 2004, vol. 69, pp. 1–12.10. B. Sun, L. Yang, and Y. Guo: Fatigue Fract. Eng. Mater. Struct,

2007, vol. 30, pp. 1052–62.

11. E.-W. Huang, R.I. Barabash, B. Clausen, Y.-L. Liu, J.-J. Kai,G.E. Ice, K.P. Woods, and P.K. Liaw: Int. J. Plast, 2010, vol. 26,pp. 1124–37.

12. P. Starke, F. Walther, and D. Eifler: Int. J. Fatigue, 2006, vol. 28,pp. 1028–36.

13. P. Starke, F. Walther, and D. Eifler: Fatigue Fract. Eng. Mater.Struct, 2007, vol. 30, pp. 1044–51.

14. P. Starke and D. Eifler:Materialprufung, 2009, vol. 51, pp. 261–68.15. H. Germann, P. Starke, E. Kerscher, and D. Eifler: Procedia

Engineering, 2010, vol. 2, pp. 1087–94.16. M. Shirani and G. Harkegard: Eng. Fail. Anal, 2011, vol. 18,

pp. 12–24.17. M. Wohlfahrt, C. Oberwinkler, S. Tunzini, A. Rauscher, R. de la

Prida Caballero, and W. Eichsleder: Procedia Engineering, 2010,vol. 2, pp. 1337–41.

18. C. Guillemer-Neel, V. Bobet, and M. Clavel: Mater. Sci. Eng. A,1999, vol. 272, pp. 431–42.

19. P. Hubner, G. Pusch, and L. Krodel: Adv. Eng. Mater, 2004,vol. 6, pp. 541–44.

20. H.Q. Xue, E. Bayraktar, and C. Bathias: J. Mater. Process. Tech,2008, vol. 202, pp. 216–23.

21. K.-O. Lee and S.-B. Lee: Key Engineering Materials, 2007, vols.345–346, pp. 367–70.

22. D. Dengel and H. Harig: Fatigue Fract. Eng. Mater. Struct., 1980,vol. 3, pp. 113–28.

Fig. 10—Cyclic resistance curve (a) as well as SEM micrographs for the fatigue states N = 50 (b), N = 200 (c), N = 1 9 103 (d), and N = Nf

(e) of a constant amplitude test at ra = 120 MPa of the cast iron EN-GJL-250.

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