12 T. Sasaki et al. u – 0 u – 200 u – 400 u – 600 u – 800 v –0 v –200 v –400 v –600 v –800 x y Fig. 2.8 Load-displacement curve and fringe pattern observed of a pre-fatigued specimen of NP D10 5 26.0mm 485 pixel 5.7mm (110 pixel) Analyzed area x y Np = 10^5 a b Fig. 2.9 Displacement vector field showing the deformation of necked part obtained from fringe analysis. The analyzed area is indicated in the image on the right part of specimen with increase of tensile load, showing the strain concentration. In the horizontal direction, x (u-fringe), the initial carrier fringes gradually became a bent shape. Difference between the non-fatigue and pre-fatigued specimen was observed in this strain concentration behavior. Figure 2.8 shows an example of the fringe patterns in a tensile test for the pre-fatigued specimen (NP D10 5). In all the pre-fatigued condition, the pre-fatigued specimens basically exhibited a similar concentration behavior of the v-fringe, whereas the curvature of u-fringe tended to be higher than that in the non-fatigued specimen. Figure 2.9 shows displacement vector field of the necked part obtained from phase analysis of the u, and v fringe contours. The analysis was conducted for an area of 110 485 pixel (5.7 mm 26.0 mm) around the necked part indicated by dashed line. The quiver plot on the left indicates that the strain concentrates at one side of the neck, implying strain heterogeneity. To discuss this strain concentration behavior in detail, the mean value of strain in the measured area was computed. Normal strain along the tensile axis "yy, and shear strain xy were obtained from the displacement components v and u as follows.
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