13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- However, 0.2% proof stress of each specimens shows different tendency from the former result. Fig. 7 plots 0.2% proof stress of each angle of as-received and 450°C annealed specimens. In both annealing conditions, 0.2% proof stress has anisotropy. Though ultimate tensile stress and maximum strain decreases their anisotropy of all angles by annealing, 0.2% proof stress changes of only 60° to 90° by annealing. In these angle, 0.2% proof stress become high by annealing, and it can be said that anisotropy become even strong in this part. In 0° to 45°, the value of 0.2% proof stress is almost the same in each annealing condition. It is the highest in 0° of all angles and gradually decreases to 45°. Figure 7. 0.2% proof stress of each angle of as-received and 450°C annealed specimens 3.3. AE measurement AE result is show in Fig. 8 and Fig. 9. AE measurement was conducted during the tensile test, from start of the test to just before fracture. In each graph, AE result and time stress curve are plotted together in order to connect result of AE measurement and mechanical properties. x axis is time, y axis is nominal stress and peak frequency of AE signals, and z axis is magnitude of AE signals. The value of peak frequency and magnitude is used after wavelet transformed. In all specimens, AE magnitude is high at yield point, so main AE source is related to yielding mechanism. After that, AE magnitude becomes lower gradually as strain increase. In both Fig. 8 and Fig. 9, there are many resemble point. AE magnitude is far higher in 90° than 0° and 45°. Peak frequency is wide range in yield point, but after yielding, AE magnitude is high in high frequency, from about 500 kHz to 600 kHz. The second highest AE magnitude is in frequency of about 300 kHz. In 0°, maximum strain is lowest of all angles, so AE signals is detected in the early time. After annealing, these tendencies are the same. However, AE magnitude of 0° becomes much lower than 45° and 90°. In order to analyze this AE result, the relationship between AE energy and nominal stress is plotted in Fig. 10. AE energy is calculated as square of AE magnitude and this AE signal is released when nominal stress increases 1 MPa. In Fig 10(a) to (c), AE energy peak (Epeak) is extremely high in 90°. AE energy is released from low nominal stress, about 50 MPa, to high nominal stress, about 250 MPa. It is well explained by the fact that AE signals are released at yield point of each specimen.
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