Effect of Proximity and Dimension of Two Artificial Pitting Holeson the Fatigue Endurance of Aluminum Alloy AISI 6061-T6Under Rotating Bending Fatigue Tests

G.M. DOMINGUEZ ALMARAZ, V.H. MERCADO LEMUS, and J.J. VILLALON LOPEZ

This work deals with the study of the two artificial pitting holes effects, caused by theirdimensions and proximity, on the fatigue endurance of aluminum alloy AISI 6061-T6 underrotating bending fatigue tests. Stress concentration induced by artificial pitting holes is analyzedand correlated with the experimental fatigue life. It is found that the stress concentrationincreases exponentially when the two pitting holes approach, and this induces an importantreduction in the fatigue life. Concerning the diameter variation of one pitting in regard to thesecond, no important influence was observed on fatigue life for a given separation betweenthem; this implies that the separation between the two artificial pitting holes and the associatedstress concentration is the principal parameter on the fatigue life under these conditions. Finally,results are discussed and conclusions are presented involving the fatigue life, proximity, anddimension of pitting holes, stress concentration factor, and fracture surfaces where the failureorigin is identified.

DOI: 10.1007/s11661-011-0799-4� The Minerals, Metals & Materials Society and ASM International 2011

I. INTRODUCTION

ALUMINUM alloys are largely used in modernindustries, such as aeronautical, automotive, sportinggoods, building construction, office and domestic furni-ture, marine vessels, electrical transmission, packaging,etc. Aluminum alloy AISI 6061-T6 is known as the‘‘aluminum alloy for general purpose use’’ because ofwide industrial uses: truck bodies and frames, screwmachine parts, structural components, aircraft andaerospace components, rail coaches, ship building,helicopter rotor skins, camera lenses, electrical fittings,and connectors, valves, driveshafts, brake components,and couplings.

Alloy 6061-T6 has excellent corrosion resistance toatmospheric conditions and good corrosion resistance tosea water and other environments.[1–5] Nevertheless, forsome industrial applications, fracture on aluminumalloys is associated with corrosion pitting holes.[6–9] Inorder to study the fatigue behavior of this materialunder rotating bending fatigue tests and pitting holes,artificial pitting holes were machined on hourglass shapespecimens.[10]

II. EXPERIMENTAL DETAILS

Table I contains the chemical composition of thisaluminum alloy and Table II the corresponding

mechanical properties. Figure 1 presents the testingspecimen and dimensions in millimeters, and Figure 2the numerical stress distribution results for two closepitting holes with diameter 1 mm, separated 100 lm, inthe transversal direction regarding the principal axis ofspecimen.[11]

The high Von Mises stress in Figure 2 is 249 MPawhen the specimen is loaded in the vertical directionwith P = 39 N, as is shown in Figure 1. The specimenis fixed at 14 mm from the left side in order to obtain therotating bending condition. Numerical results for thespecimen without pitting holes under identical loadingcondition yields 81 MPa for the high Von Mises stress;this implies a stress concentration factor of aboutKt � 3.07.In both artificial pitting holes tests, the narrow section

diameter was D0 = 4.1 mm, the nominal Von Misesapplying load was rn = 79 to 81 MPa (P = 37 to39 N), and the testing frequency was 50 Hz of testingfrequency. The number of tested specimens was 64; 4specimens for each of the following classifications: 2pitting holes with identical diameter (1 mm) separated100, 200, 300, and 400 lm in longitudinal and transver-sal direction, and the same for 2 pitting holes withdifferent diameters (1 and 0.8 mm). All artificial pittingholes were hemispherical; then, the depth was the pitradius: 500 and 400 lm. A cooling system was imple-mented with cool air in order to maintain the highesttemperature at the narrow section below 263 K (70 �C);under this condition, no important crystallographictransformation on tested material was expected. Imagesof two artificial pitting holes located at the narrowsection and separated 100 lm in transversal and longi-tudinal directions are presented in Figure 3. The stressconcentration factor Kt was close to 3 in both cases.

G.M. DOMINGUEZ ALMARAZ, Professor, V.H. MERCADOLEMUS, M.Sc. Student, and J.J. VILLALON LOPEZ, UMSNHEngineer, are with the Faculty of Mechanical Engineering, Universityof Michoacan (UMSNH), 58000 Morelia, Mich., Mexico. Contacte-mail: dalmaraz@umich.mx

Manuscript submitted March 22, 2011.Article published online September 7, 2011

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Furthermore, for two pitting holes presenting differ-ent diameters (1000 and 800 lm) and separated 100 lmin the transversal direction, the stress concentrationfactor was again Kt � 3 (Figure 4); whereas for thesame conditions, but in the longitudinal direction, thiswas Kt � 2.6 (Figure 5).

In the case of the transversal direction, the high stressmoves from the bottom of the pitting hole for a singlehemispherical pitting hole[12,13] to the common wallwhen the two pitting holes approach, as shown inFigures 2 and 4. In the case of the longitudinaldirection, the high stress path follows a diametricaldirection, perpendicular to the principal axis of thespecimen (Figure 5).

III. RESULTS

Figure 6 presents the fatigue endurance evolutionunder rotating bending fatigue tests when the specimenpresents two pitting holes with identical diameter(1000 lm) in the transversal and longitudinal directionsand 100, 200, 300, and 400 lm of separation.

Results for experimental tests with two pitting holesof different diameters are plotted in Figure 7. Concern-ing the rotating bending fatigue life confrontationbetween the specimen without and with artificial pitting,Figure 8 presents some of these results.

IV. FRACTURE SURFACES

The fracture surfaces for two pitting holes withsimilar diameter (1 mm) and separated 100 lm in the

transversal and longitudinal directions are shown inFigures 9(a) and (b), respectively.Concerning the fracture surface for two pitting holes

with different diameters (1 and 0.8 mm) and separated100 lm, the pictures are shown in Figures 10(a) and(b) for the transversal and longitudinal directions,respectively.

V. DISCUSSION

Results plotted in Figures 6 and 7 show a cleartendency: in reducing the proximity of two close pittingholes, the fatigue life is reduced dramatically, for boththe transversal and longitudinal directions. A factor of 2on fatigue life is observed when it is compared with theseparation between 100 and 400 lm for both directionsand identical or different diameters of the two artificialpitting holes; nevertheless, in the case of pitting holeswith different diameter, the fatigue life is reduced somethousands of cycles compared with the similar diameterpitting holes case, particularly for the 100 and 400 lm ofseparation. In Figure 6, the longitudinal pitting holesfor the four separations present a fatigue life slightlylower than the transversal pitting holes. The sear stressin the plane XY for the longitudinal pitting holes ishigher in all the cases compared to that in the transver-sal pitting holes; this should be at the origin of this

Table I. Chemical Composition for Aluminum Alloy AISI

6061-T6

Chemical Composition (Wt Pct)

Al 95.8 to 98.6Cr 0.04 to 0.35Cu 0.15 to 0.4Fe max. 0.7Mg 0.8 to 1.2Mn max. 0.15Si 0.4 to 0.8Ti max. 0.15Zn max. 0.25Other each max. 0.05Other total max. 0.15

Table II. Mechanical Properties for Aluminum Alloy AISI6061-T6

Mechanical Properties

Density (kg/m3) 2700Hardness, Brinell 95ry (MPa) 270ru (MPa) 310E (GPa) 68.9Poisson ratio 0.33Elongation at break (pct) 17

Fig. 1—Specimen picture and dimensions (mm).

Fig. 2—Numerical simulation for the specimen with two close pit-ting holes with the same diameter (100 lm of separation) at the nar-row section and in transversal position.

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Fig. 3—Two pitting holes separated 100 lm, in the (a) transversal and (b) longitudinal directions.

Fig. 4—Von Mises stresses for the two pitting holes 1000 and800 lm, in the transversal direction.

Fig. 5—Von Mises stresses for the two pitting holes 1000 and800 lm, in the longitudinal direction.

Fig. 6—Fatigue life with the proximity of two pitting holes with identical diameter (1 mm), placed on transversal and longitudinal directions;fatigue life on one pitting specimen.

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Fig. 7—Fatigue life with the proximity of two pitting holes of different diameters (1 and 0.8 mm), placed on the transversal and longitudinaldirections; fatigue life on 0 pitting specimens.

Fig. 8—Fatigue endurance of aluminum alloy for three types of specimens: without pitting and 1 and 2 artificial pitting holes.

Fig. 9—Fracture surface for two similar pitting holes with 100-lm separation: (a) transversal and (b) longitudinal.

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difference in fatigue life. No clear tendency was observedfor the 200 and 300 lm of separation in Figure 7;nevertheless, fatigue life for the longitudinal direction ishigher than in the transversal direction. The points forthe 100 and 400 lm of separation in the last figure areoverlapped.

Concerning the transversal pitting holes, an exponen-tial increase of the stress concentration factor isobserved when the two pitting holes approach: the highstress moves from the bottom of the single hemisphericalpitting hole under uniaxial loading[12,13] to the commonwall of the two pitting holes when approaching, asshown in Figure 11 obtained by numerical simulation.

Concerning the fracture surfaces, the stress concen-tration factors were localized close to the pitting holes,causing an important plastic deformation at this zone,as shown in Figures 9 and 10. The granular zonesrelated to fast crack growth and low plastic deformationare localized at the opposite of pitting holes; the fractureorigin is, then, associated with pitting holes and thehighest stresses close to this zone.

VI. CONCLUSIONS

Stress concentration factors induced by pitting holesreduce dramatically the fatigue life of tested speci-mens.[14,15] The stress concentration factor for twopitting holes increases exponentially in reducing itsseparation;[10] furthermore, fatigue life is reduced by afactor of 2 when the proximity between the two pittingholes moves from 400 to 100 lm, all others parametersresting unchanged.These results were obtained within a low-cycle fatigue

regime for all tested specimens: from 104 to 106 cycles.Further investigations will be undertaken in order toinvestigate the effect of artificial pitting holes on therotating bending fatigue endurance, in the high-cyclefatigue regime.The fatigue endurance for the longitudinal direction

seems to be slightly lower than in the transversaldirection, particularly for the pitting holes of the samediameter. This should be related to the highest searstress in the XY plane presenting in the case oflongitudinal pitting holes. Fracture in the case of twolongitudinal pitting holes with different diameters wassystematically observed passing by the high pitting hole(Figure 10(b)); this should be associated with a dimen-sion effect on pitting holes and the stress concentrationfactor.[14,16–18]

The fracture surfaces show an important plasticdeformation close to the pitting holes, where highstresses are induced.

ACKNOWLEDGMENTS

The authors express their gratitude to the Universityof Michoacan (UMSNH) for the facilities received inthe development of this work. A particular mention ofgratitude is extended to CONACYT (National Coun-sel for Science and Technology, Mexico City) for thefinancial support destined to this research project.

Fig. 10—Fracture surfaces for two pitting holes with different diameters and separated 100 lm: (a) transversal and (b) longitudinal.

Fig. 11—Localization of high stress for two close pitting holes onthe transversal direction and uniaxial loading.

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REFERENCES1. C.K. Toh and S. Kanno: J. Mater. Sci., 2004, vol. 39 (10),

pp. 3497–3500.2. S.J. Pawel and E.T. Manneschmidt: J. Nucl. Mater., 2003,

vol. 318, pp. 355–64.3. J.R. Davis, ed.: Aluminum and Aluminum Alloys, ASM Specialty

Handbook, ASM International, 1994.4. P. Traverso, R. Spiniello, and L. Monaco: Surf. Interface Anal.,

2002, vol. 34 (1), pp. 185–88.5. R. Braun, U. Alfaro Mercado, and G. Biallas: Mater. Sci. Forum,

2006, vol. Aluminium Alloys 2006-ICAA10, pp. 1113–18.6. W.L. Xu, T.M. Yue, H.C. Man, and C.P. Chan: Surf. Coat.

Technol., 2006, vol. 200, pp. 5077–86.7. S. Ishihara, Z.Y. Nan, A.J. McEvily, T. Goshima, and S. Sunada:

Int. J. Fatigue, 2008, vol. 30, pp. 1659–68.8. D.L. DuQuesnay, P.R. Underhill, and H.J. Britt: Int. J. Fatigue,

2003, vol. 25, pp. 371–77.9. H. Allachi, F. Chaouket, and K. Draoui: J. Alloys Compd., 2010,

vol. 491, pp. 223–29.

10. G.M. Domınguez Almaraz, V.H. Mercado Lemus, and J.J. Vil-lalon Lopez: Procedia Eng, 2010, vol. 2, pp. 805–13.

11. M. Cerit, K. Genel, and S. Eksi: Eng. Failure Analysis, 2009,vol. 16, pp. 2467–72.

12. W.D. Pilkey and D.F. Pilkey: Peterson’s Stress ConcentrationsFactors, 3rd ed., John Wiley & Sons, New York, NY, 2007.

13. P.C Paris, T. Palin-Luc, H. Tada, and N. Santier: CrackPaths 2009, International Conference on CRACK PATHS(CP 2009), Proceedings, Vicenza, Italy, Sept. 23–25, 2009,pp. 495–502.

14. S.I. Rokhlin, J.-Y. Kim, H. Nagy, and B. Zoofan: Eng. Fract.Mech., 1999, vol. 62, pp. 425–44.

15. R. Perez Mora, G. Domınguez Almaraz, T. Palin-Luc, C. Bathias,and J.L. Arana: Key Eng Mater, 2010, vol. 449, pp. 104–13.

16. M.R. Bayoumi, A.A. Ismail, and A.K. Abd El Latif: Eng. Fract.Mech., 1996, vol. 53, pp. 141–51.

17. K. Sun: SPE Product. Facil., 2005, vol. 20, pp. 334–39.18. G.S. Chen, K.-C. Wan, M. Gao, R.P. Wei, and T.H. Flournoy:

Mater. Sci. Eng. A, 1996, vol. 219, pp. 126–32.

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