8 T. Sasaki et al. In the present study, in order to establish the correlation of the elasticity of metals and the fatigue damage, the deformation behavior of aluminum alloys previously subjected to cyclic loading are visualized with electronic speckle pattern interferometry (ESPI). At the same time, the change of elasticity due to the fatigue damage is evaluated by the acoustic measurement. We demonstrate the influence of microscopic factors including the localized plastic deformation, the presence of micro-cracks, on strain heterogeneity, and discuss the change in elastic response of the material. 2.2 Experimental Procedure 2.2.1 Fatigue Test An AA7075-T1 aluminum alloy sheet with thickness of 3.0 mm was used for experiments. Necked shape specimens as shown in Fig. 2.1 were cut from the sheet for fatigue tests by electronic discharge machining. The yield strength of specimen measured by a static tensile test was approximately 8.0 kN. The fatigue test was conducted for the specimen under load control condition with a sinusoidal waveform of 5 Hz. The maximum tensile load ranged from 5.0 kN to 7.0 kN, and the minimum load was a constant at 0.1kN. Figure 2.2 shows the maximum load – number of cycle curve obtained from the fatigue test. From this curve, the maximum load in the pre-fatigue test was determined to 5.0 kN which was approximately the fatigue limit. The numbers of cycles used in the pre-fatigue test, NP, were 10, 10 2, 103, 104, and 105, respectively. 2.2.2 Acoustic Measurement Acoustic wave velocity in the specimen was measured using an ultrasonic transducer driven by a square wave pulserreciever with 35 MHz bandwidth (Olympus 5077PR). As shown in Fig. 2.3, the measurement was conducted for the necked center of specimen. Velocities of vertical wave propagating in the thickness direction (Vzz), and shear vertical waves in the displacement directions of x and y (Vzx, Vzy) were respectively measured. Coupling media used was distilled water for the vertical wave measurement, and glycerin paste for the shear wave measurement. 2.2.3 Dynamic Observation of Deformation Behavior of Pre-fatigued specimen Deformation behavior during tensile tests was visualized with a two-dimensional ESPI in the same manner described in a previous work [6]. Figure 2.4 shows the optical setup in this study. Two optical interferometers were arranged to horizontal Fig. 2.1 Specimen used in this study
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