13 DIC Measurement of Anisotropy for Plastically Deformed Thermoplastic 97 Fig. 13.3 Stress-strain curve evaluated from small specimens cut off from the necking part Fig. 13.4 Initial part of the stress-strain curve in Fig. 13.3 brittle materials. On the other hand, the stress-strain curves of the cutting angles of 45, 60, 75, and 90 degrees represent that the stress reaches each maximum value and then stress decreases to each constant stress value and strain continue to increase at the constant stress value. The necking is also observed in these specimens. The specimens with the cutting angles of 45 and 60 degrees fractured finally, but the specimens of 75 and 90 degrees did not fracture at the last strain measurement as arrows is indicated. As shown in Fig. 13.3, the maximum stress decreases with increasing the cutting angle from the necking part in the large specimens. Figure 13.4 shows the enlarged figure of the initial part of the stress-strain curves in Fig. 13.3. Figure 13.4 indicates that the initial stiffness depends on the cutting angle from the necking part. The elastic modulus is the largest in the direction of 0 degree. And the elastic modulus decreases with increasing the cutting angle from the necking part in the large specimens. In the present study, it is confirmed that the material as received is isotropic mechanical characteristics by the same uniaxial tensile tests. Therefore, the anisotropic mechanical characteristics appear in the necking part by plastic deformation. Figure 13.5 shows the images of the small specimens, the maximum strain, and the minimum strain distributions at the nominal strain of about 0.1. The maximum and the minimum strain in the small specimen of 0 degree direction is approximately uniform distribution. As shown in Fig. 13.5, the strain distribution in the small specimens with the cutting angle of 15, 30, 45, 60, and 75 degrees are not uniform, and the large strain band appears obliquely from bottom right to the upper left. The anisotropic elastic analysis represents that the large strain band is appeared obliquely in the specimen if the anisotropic elastic materials is subjected to uniaxial tensile force in the different direction to the principal axes of anisotropic elastic materials. Therefore, this experimental results correspond with the result of anisotropic elastic analysis. Figure 13.6 shows the relationship between the elastic modulus and the cutting angle from the necking part in the large specimens. The elastic modulus is calculated from the slope of the stress-strain curve in the range from 0.001 to 0.01. The results evaluated from the material as received are also shown in Fig. 13.6. It is found that the material as received is isotropic. On the other hand, the elastic modulus in the necking part decreases with increasing the cutting angle. Therefore, the
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