2 Fatigue Analysis of 7075 Aluminum Alloy by Optoacoustic Method 11 Fig. 2.6 The ratio of vertical and averaged shear wave velocity u – 0 u – 200 u – 400 u – 600 u – 800 v –0 v –200 v –400 v –600 v –800 x y Fig. 2.7 Load-elongation curve and fringe pattern observed at various tensile loads of a non-fatigued specimen. The tensile load is indicated in the load – elongation curve by arrows NP < 10 3 implies an increase of compressive stress in the x-y plane. Cyclic tensile load during the pre-fatigue causes the localized plastic deformation in the necked area. This fact might result in the increase of compressive residual stress, because the deformed area is restrained by the surrounding area. Therefore, the earlier pre-fatigue stage of NP < 10 3 can be interpreted as an increasing process of the residual stress in the necked part. Furthermore, the latter increase in the R-value at NP >10 3 indicates a relaxation of the residual stress. This relaxation behavior agrees with the result of surface acoustic wave velocity measurement by a scanning acoustic microscope in our previous work [6]. In the previous work, crack initiation was confirmed at the latter fatigue stage when the acoustic wave velocity decreased. It can be understood that the residual stress induced by the localized plastic deformation was relaxed with the crack initiation. 2.3.2 Deformation Behavior in Tensile Test Figure 2.7 shows a load-elongation curve and optical fringe patterns observed in a tensile test of the non-fatigued specimen (NP D0). These fringe patterns represent displacement contours along x, and y resulting from subtracting captured images at various tensile load levels by a base image before the tensile test (tensile load D0 N). Fringe pattern at 0 N (v-0 N or u-0 N) means the carrier fringes initially introduced. The fringe in the tensile direction y (v-fringe) concentrated to the neck
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