4 S. Yoshida and T. Sasaki 2.19% 2.24% 2.28% 2.32% 2.37% 2.41% 25 mm 10 mm Fig. 1.3 Successive fringe images taken post yield points. Yield strain is 1.00%. Specimen: aluminum alloy AA7075 Table 1.2 Parameters used for numerical analysis Pulling rate (mm/s) σ (1/s) G (N/m2) ρ (kg/m3) λ(N/m2) α 0.45 1×10−8 2.2 ×10−4 1 8.8 ×10−4 0 4 0 5 -5 1.5 -1.5 7 -3 5 -5 1.5 -0.5 (a) (b) (c) Fig. 1.4 Experimental fringe patterns (upper) and numerical analysis (lower) the numerical analysis. Note that this condition was assimilated to the experiment through the wave velocity. The set of parameters shown in Table 1.2 provides the transverse wave velocity close to experimental values [11]. Figure 1.4 compares the experimental fringe images and the numerical patterns of vx andvy for three representative stages. Stage (a) is when the normal strain is approximately 2% (the same stage as Fig. 1.3). In this stage, the vx and vy patterns move at a constant speed. The numerical patterns clearly indicate the feature that vx exhibits longitudinal wave characteristics where the contours are approximately perpendicular to the tensile axis andvy exhibits semicircular pattern as shown. Careful examination reveals that the numerical vx pattern is denser near the right end of the specimen. This seems to correspond to the concentrated fringes near the middle of the specimen observed in the experimental vx pattern. This indicates that the numerical result shows strain concentration although its location is not the same as experiment. It is likely that the strain concentration occurs at a crest of the longitudinal wave in vx. The experimental image at stage (b) was taken when the normal strain is approximately 3%. At this stage, the vx fringes are more concentrated and curved as compared with stage (a). The vy fringes are not semi-circular. Instead, they are rather
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