Chapter 2 Fatigue Analysis of 7075 Aluminum Alloy by Optoacoustic Method Tomohiro Sasaki, Hiroshi Ono, Sanichiro Yoshida, and Shuich Sakamoto Abstract The influence of fatigue damage on the elastic response of AA7075 aluminum alloy was investigated through a combination of optical and acoustical experiments. Specimens were previously subjected to fatigue cyclic loads at various fatigue levels within the fatigue life. Macroscopic deformation process under a certain load below the yield point (elastic region) for the pre-fatigued specimen was visualized by electronic speckle pattern interferometry (ESPI). At the same time, the acoustic velocities of vertical and shear waves propagating in the fatigued specimen were measured using an ultrasonic probe. The acoustic analysis showed the following change in residual stress by the fatigue cyclic load; an increase in compressive residual stress with the number of pre-fatigue cycles (NP) below 10 3, and relaxation of the residual stress NP over 10 3. The visualization using ESPI demonstrated that the strain heterogeneity in the macroscopic elastic regime was enhanced with increase of the pre-fatigue cycle. The correlation between the optical and the acoustical measurement results is discussed based on the change in the residual stress, localized plastic deformation, and the crack initiation. Keywords Fatigue • Optical method • Acoustic method • Speckle patter interferometry • Aluminum alloy 2.1 Introduction Fatigue of metals is generally interpreted as a process of crack initiation by localized deformation, and crack propagation, leading to final fracture under cyclic loading. A number of fatigue inspection methods such as those use X-ray, ultrasonic wave, and acoustic emission have been established [1–4]. These methods mainly aim at detection of the presence of fatigue crack, and fatigue life is predicted by monitoring the crack length based on fracture mechanical parameters. On the other hand, the stage of localized plastic deformation prior to the crack initiation is a complex phenomenon and not fully understood. Thus, it is generally difficult to detect the fatigue damage at the earlier fatigue stage particularly in ductile metals, because most of the fatigue life is spent by the stage before the crack initiation. The localized plastic deformation occurs not only in “Low cycle fatigue”, but also in “High cycle fatigue” that is characterized by lower stress condition below the macroscopic yield stress. This study focuses on the influence of the localized plastic deformation on macroscopic deformation behavior of metals. Our previous works using a full field optical method [5, 6] demonstrated that the strain concentration, which is observed in the macroscopic elastic regime, is enhanced depending on the degree of fatigue cycles. On the other hand, a similar effect of fatigue damage on the elastic behavior of metals was also confirmed through acoustical methods by several researchers [7–10]. These methods are based on stress dependence of elastic wave velocity that propagates through the material, termed as “acousto-elasticity”. The fatigue damage was detected as a change in the elastic wave velocity or attenuation attributed to residual stress induced by the localized plastic deformation. Both the results obtained from the optical and the acoustical methods are associated with the elastic response of metals, and indicate the possibility of analyzing the elastic response as a mean of fatigue diagnosis. T. Sasaki ( ) • H. Ono • S. Sakamoto Graduate school of Niigata University, 8050 Ikarashi-ninocho, Nishi-ku, Niigata-shi, Niigata, Japan e-mail: tomodx@eng.niigata-u.ac.jp S. Yoshida Department of Chemistry and Physics, Southeastern Louisiana University, SLU 10878, Hammond, LA, 70402, USA e-mail: syoshida@selu.edu © The Society for Experimental Mechanics, Inc. 2018 L. Lamberti et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-63028-1_2 7
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