Effect of Proximity and Dimension of Two Artificial Pitting Holes on the Fatigue Endurance of Aluminum Alloy AISI 6061-T
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ALUMINUM alloys are largely used in modern industries, such as aeronautical, automotive, sporting goods, building construction, office and domestic furniture, marine vessels, electrical transmission, packaging, etc. Aluminum alloy AISI 6061-T6 is known as the ‘‘aluminum alloy for general purpose use’’ because of wide industrial uses: truck bodies and frames, screw machine parts, structural components, aircraft and aerospace 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 to atmospheric conditions and good corrosion resistance to sea water and other environments.[1–5] Nevertheless, for some industrial applications, fracture on aluminum alloys is associated with corrosion pitting holes.[6–9] In order to study the fatigue behavior of this material under rotating bending fatigue tests and pitting holes, artificial pitting holes were machined on hourglass shape specimens.[10] II.
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Table I contains the chemical composition of this aluminum alloy and Table II the corresponding G.M. DOMINGUEZ ALMARAZ, Professor, V.H. MERCADO LEMUS, M.Sc. Student, and J.J. VILLALON LOPEZ, UMSNH Engineer, are with the Faculty of Mechanical Engineering, University of Michoacan (UMSNH), 58000 Morelia, Mich., Mexico. Contact e-mail: [email protected] Manuscript submitted March 22, 2011. Article published online September 7, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
mechanical properties. Figure 1 presents the testing specimen and dimensions in millimeters, and Figure 2 the numerical stress distribution results for two close pitting holes with diameter 1 mm, separated 100 lm, in the transversal direction regarding the principal axis of specimen.[11] The high Von Mises stress in Figure 2 is 249 MPa when the specimen is loaded in the vertical direction with P = 39 N, as is shown in Figure 1. The specimen is fixed at 14 mm from the left side in order to obtain the rotating bending condition. Numerical results for the specimen without pitting holes under identical loading condition yields 81 MPa for the high Von Mises stress; this implies a stress concentration factor of about Kt 3.07. In both artificial pitting holes tests, the narrow section diameter was D0 = 4.1 mm, the nominal Von Mises applying load was rn = 79 to 81 MPa (P = 37 to 39 N), and the testing frequency was 50 Hz of testing frequency. The number of tested specimens was 64; 4 specimens for each of the following classifications: 2 pitting holes with identical diameter (1 mm) separated 100, 200, 300, and 400 lm in longitudinal and transversal direction, and the same for 2 pitting holes with different diameters (1 and 0.8 mm). All artificial pitting holes were hemispherical; then, the depth was the pit radius: 500 and 400 lm. A cooling system was implemented with cool air in order to maintain the highest temperature at the narrow section below 263 K (70 °C); under this condition, no important
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