Nonlinear ultrasonic characterization of early degradation of fatigued Al6061-T6 with harmonic generation technique S.B. Gebrekidan a , To Kang b , Hak-Joon Kim a , Sung-Jin Song a,⇑ a School of Mechanical Engineering, Sungkyunkwan University, Suwon 440-746, 16419, South Korea b Korea Atomic Energy Research Institute, (34057) 111, Daedeok-Daero 989Beon-gil, Yuseong-gu, Daejeon, South Korea article info Article history: Received 21 September 2017 Received in revised form 29 December 2017 Accepted 29 December 2017 Available online xxxx Keywords: Third harmonic Voids Dislocation-precipitate Fatigue Higher order nonlinearity abstract In this paper, a third harmonic was used to investigate microstructural changes in Al6061-T6 due to dif- ferent fatigue cycles and a relationship between fatigue cycle and third order nonlinearity has been observed. Piezoelectric measurement harmonic generation technique was applied for the specimens with 0%, 55%, 75% and 85% fatigue cycles, respectively. The results shows that the third order harmonics grad- ually increased up to 55% and rapidly decreased after wards, it was attributed to the behavior of disloca- tion, dislocation-precipitation interaction and voids with increasing fatigue cycle. Further, it was verified with scanning electron microscope (SEM). We also observed that third order nonlinearity is more sensi- tive to small change in area of fraction of voids than second order nonlinearity after 55% fatigue life and could be a good candidate to investigate Al6061-T6 specimen with voids. Ó 2017 Published by Elsevier B.V. 1. Introduction Al6061-T6 is one of the widely used structural materials because of its low cost, high corrosion resistance and light weight. Its main application areas includes air craft wing construction, building water crafts, automotive, heat exchangers and so on. Dur- ing service, it is exposed to fatigue, creep, intermetallic corrosion, and other kinds of material degradation conditions. Most of the structural materials are vulnerable to fatigue irrespective of its working environment, therefore the study of microstructural change of Al6061-T6 associated with fatigue cycle has been an ongoing phenomena. Up to the date, different kinds of methods were employed to characterize microstructural change of materials due to an acting external conditions [1]. In particular, nonlinear ultrasonic techniques have been considered as a potential Nonde- structive Testing (NDT) method for the evaluation of microstruc- tural changes that are hard to assess using conventional ultrasonic technique. Nonlinear ultrasonic techniques principally depends on the interaction of an incident wave with the nonlinear- ity present in a material. When a monochromatic wave passes through a nonlinear medium, the interaction between the wave and nonlinearity generates a higher order harmonics which will be used to identify the degree of material nonlinearity. Second har- monic generation has been used extensively as a damage indictor since it is more sensitive to nonlinearity present in the material [2–4]. Various reports identified the possible way of analyzing sec- ond harmonics to track the microstructural changes of fatigued Al6061-T6 [5–7]. Depending on the interaction of ultrasonic wave and nonlinear- ities present in the material, various approaches have been used to map the higher harmonics with fatigue damage. A model of ultra- sonic wave–dislocation dipole interactions that quantifies the wave distortion by means of a material nonlinearity parameter beta was developed [9,10]. New relationship between the relative ratio of simplified second-order parameter and the relative ratio of simplified third-order parameter was derived from the relation- ship between the absolute second- and third-order parameters. The derived relationship was successfully verified on the experi- mental results obtained from Al6061-T6 processed for different heat treatment times [11]. Higher order harmonics generation pre- dominantly attributed to the formation of dislocations as the fati- gue load progresses [5,12,13]. Cantrell et al. model quantifies the nonlinearity in terms of lattice anharmonicity, dislocation plastic- ity and crack growth. In the contrary, Kim et al. links the nonlinear- ity to the change in third-order elastic constants of material [3]. Though precipitates and matrix transformations can contribute to the changes in nonlinearity parameters, the assessment of exact contribution of individual effects is yet difficult due to lack of avail- able elastic constant data pertaining to these effects. The effect of https://doi.org/10.1016/j.ultras.2017.12.011 0041-624X/Ó 2017 Published by Elsevier B.V. ⇑ Corresponding author. E-mail address: sjsong@skku.edu (S.-J. Song). Ultrasonics xxx (2018) xxx–xxx Contents lists available at ScienceDirect Ultrasonics journal homepage: www.elsevier.com/locate/ultras Please cite this article in press as: S.B. Gebrekidan et al., Nonlinear ultrasonic characterization of early degradation of fatigued Al6061-T6 with harmonic generation technique, Ultrasonics (2018), https://doi.org/10.1016/j.ultras.2017.12.011