2 Fatigue Analysis of 7075 Aluminum Alloy by Optoacoustic Method 13 "yy D @v @y ; xy D @u @y C @v @x (2.2) Figure 2.8 shows the result plotted against the tensile load up to 400 N. The tensile strain, "yy monotonically increased with the increases of applied load as shown in Fig. 2.9a. The pre-fatigued specimens exhibited higher slopes than that in the non-fatigued specimen. In addition, a larger difference was seen in the shear strain xy as shown in Fig. 2.9b. The increase in xy implies strain heterogeneity that is probably due to the localized plastic deformation during the fatigue test. The slopes of "yy and xy, (compliance d"yy/dF, and d xy/dF) obtained by a least square method are shown in Fig. 2.10. Variation of normal compliance, d"yy/dF by the pre-fatigue cycle was consistent with the measurement result of shear velocity along the tensile direction (Fig. 2.5b); d"yy/dF showed an increase at NP < 10 3 and a decrease at NP > 10 3. In addition, the shear compliance, d xy/dF showed a slight increase at NP >10 3 and a rapid increase at NP <10 3. These results demonstrate that the (a) Tensile strain, eyy, (b) Shear strain, gxy Fig. 2.10 Mean value of strain plotted against the applied load. (a) Tensile strain, "yy, (b) Shear strain, xy Elastic compliance Fig. 2.11 Elastic compliance
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