Duque-Rojas, Nairobi B.

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    AFM studies on fractured surfaces of AF1410 steel and A7075-T6 aluminum alloy
    (2006) Duque-Rojas, Nairobi B.; Suárez, O. Marcelo; College of Engineering; Sundaram, Paul A.; Pérez, Néstor; Department of Mechanical Engineering; Perales-Pérez, Oscar J.
    In the present research, atomic force microscopy (AFM) was used to characterize fracture surfaces of the AF1410, ultra-high strength steel, and a high strength AA7075-T6 aluminum alloy. Both materials are used by the U.S. Navy in aircraft parts. The implementation of a reliable qualitative and quantitatively technique that could reveal three-dimensional characteristics of the material fracture topography and the study of roughness parameters was necessary to explain the fracture surfaces (nano-fractography). Compact tension specimens made of AF1410 steel and AA7075-T6 alloy were exposed to different conditions at fatigue and impact-fatigue testing respectively. The bottom half of the fractured fatigued samples in the AF1410 steel and both of the impact fatigued sample for the aluminum alloy were used for AFM observations. The surface roughness was a parameter that aided to quantify the surface texture of the steel sample. Using AFM software, six roughness parameters were calculated over a determined area: the roughness average (Sa), the mean value (Sm), the root mean square (Sq), the valley value (Sv), the peak value (Sp), and the range between the peak and the valley value (Sy=Sp-Sv). The crack growth in fatigue testing of the AF1410 steel was analyzed in air and under 1, and 3.5% saline solutions. In the analyzed areas, the results indicated that decreasing NaCl concentration significantly enhanced the roughness average (Sa) in the crack growth direction due to an aggressive process of metal dissolution. When the material was tested at low frequencies and under low sodium chloride concentration, a much higher roughness average was obtained compared to the 3.5 % NaCl condition. With the AA7075-T6 alloy, grains orientation effect was analyzed using two specimens. In this work different crack propagation rates were found on the fracture surfaces, both for the longitudinal and for the transversal specimen.