Conference Papers
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Item Influence of retrogression and re-ageing heat treatment on the fatigue crack growth behavior of 7010 aluminum alloy(Elsevier B.V., 2019) Nandana, M.S.; Udaya Bhat, K.; Manjunatha, C.M.Aluminum alloys are widely used in aircraft structural components where light weight, high strength and good corrosion resistance are the primary requirements. These alloys are generally used in peak-aged (T6) condition in which they are susceptible for stress corrosion cracking. In the recent years, retrogression and re-ageing (RRA) treatment on aluminum alloy is carried out to enhance their corrosion resistance maintaining the ultimate tensile strength. The aim of this work was to study the influence of RRA treatment on the fatigue crack growth rate (FCGR) behavior. The 7010 aluminum alloy was heat treated to two different conditions i.e., T6 and RRA. The microstructures of these alloys were characterized by using TEM. Standard compact tension (CT) specimens were prepared and FCGR tests were carried out by using a 100 kN servo-hydraulic test machine as per ASTM E647-15e1. The constant amplitude FCGR tests were carried out at a stress ratio, R = 0.5 using sine wave loading pattern at 10 Hz. Crack length was monitored by following compliance technique. Microstructural studies show that RRA treated alloy contain fine and densely populated precipitates in the matrix along with coarse and discontinuous precipitates in the grain boundary. The fatigue crack growth rate was observed to reduce along with an increase in the threshold stress intensity factor range (ΔKth) for RRA treated alloy compared to the T6 alloy. The mechanisms for reduction in fatigue crack growth rate of RRA treated alloy is attributed to the microstructural modifications. The increased resistance is expected to enhance the damage tolerance capability of the alloy. © 2019 The Authors. Published by Elsevier B.V.Item Effect of Microstructure on the Fatigue Crack Growth Behavior in Al–Zn–Mg–Cu Alloy(Springer Science and Business Media Deutschland GmbH, 2020) Nandana, M.S.; Udaya, B.K.; Manjunatha, C.M.High-strength Al–Zn–Mg–Cu alloys are used in airframe structures, such as bulk heads, wing spars, and lug joints. In this investigation, the effect of RRA microstructure on the fatigue crack growth rate (FCGR) behavior is studied. The 7010 aluminum alloy was heat treated to two different conditions, i.e., T6 and RRA. The microstructure of the heat-treated alloy is characterized by using transmission electron microscope (TEM). The FCGR tests were performed as per ASTM E647 standard by using a 100 kN servo-hydraulic test machine. The tests were performed using standard compact tension (CT) specimens with a stress ratio, R = 0.7 using a sine wave form at 10 Hz in a standard laboratory air environment. The matrix microstructure of the RRA-treated alloy consists of fine scale η´ (MgZn2) precipitates with increased interparticle spacing when compared to closely packed η´ precipitates in the standard T6-treated alloy. The grain boundary precipitates are coarsened and discrete in the RRA-treated alloy, while it is continuous in T6 condition. An improvement in the threshold stress intensity factor range (ΔKth) by about 0.65 MPa√m is observed in RRA-treated alloy compared to the T6-treated alloy. The FCGR was observed to be lower by 2 times in RRA-treated alloy compared to T6-treated alloy over the major portion of FCGR curve. The increased free slipping distance between the matrix precipitates in RRA-treated alloy is correlated to the improved fatigue crack growth resistance of the RRA-treated aluminum alloy. © 2020, Springer Nature Singapore Pte Ltd.Item Influence of heat treatment on near-threshold fatigue crack growth behavior of high strength aluminum alloy 7010(Springer Science and Business Media Deutschland GmbH, 2020) Nandana, M.S.; Udaya, B.K.; Manjunatha, C.M.In this study, aluminum alloy 7010 was subjected to three different ageing treatments i.e., peak ageing (T6), over ageing (T7451) and retrogression and re-ageing (RRA) to study the influence of precipitate microstructure on the fatigue crack growth rate (FCGR) behavior. The microstructural modifications were studied by using TEM to examine the change in size and morphology of the precipitates. The size of the precipitates in the matrix range from 16-20nm in T7451, 5-6nm in RRA and 2-3nm in T6 alloys, respectively. The FCGR tests were performed on standard compact tension (CT) specimens as per ASTM E647 standard in a computer controlled servo-hydraulic test machine with applied stress ratio, R = 0.1 and loading frequency of 10 Hz. The crack growth was measured by adopting compliance technique using a CMOD gauge attached to the CT specimen. The fatigue crack growth rate was higher in T7451 and lowest in RRA treated alloy. The RRA treated alloy showed higher (formula presented) compared to T7451 and T6 treated alloys. The measured (formula presented) was 11.1, 10.3 and (formula presented) in RRA, T6 and T7451 alloys, respectively. In the near-threshold regime, the RRA treated alloy exhibited nearly 2-3 times reduction in the crack growth rate compared to the T6 alloy. The growth rate in the RRA alloy was one order lower than that of the T7451 condition. The surface roughness of RRA treated alloy was more pronounced. The reduction in FCGR observed in RRA alloy was correlated to partial crack closure due to tortuous crack path and partially due to increased spacing between the matrix precipitates. The reduction in near-threshold FCGR and increase in (formula presented) is expected to benefit the damage tolerant capability of the aircraft structural components under service loads. © Springer Nature Switzerland AG 2020.