13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- normalized resistance R/R0 at the beginning of crack growth has been improved to about 1.5 times as the conventional. As a result, it is supposed to be an effective way to solve the unstable phenomenon at the beginning of crack growth shown in Figure. 4. Therefore, the measurement accuracy of the ion-sputtered metal film on “sputtered smart patch” was improved by changing the shape of the ion-sputtered metal film through FEM. Fatigue experiments will be conducted to verify the effectivity of this shape. 4.2. Wireless communication 4.2.1. Using RFID tag The conventional wireless measuring system, which composed of the ion-sputtered metal film, AD converter and RFID tag, was successfully applied to measure the crack length of sputtered smart patch during fatigue test. However, the communication distance was only 30 mm, and is not sufficient for wireless network system. 4.2.2. Using ZigBee By using the new wireless device, the communication distance was improved to more than 20 m. The wireless communication was proved to be possible among 1 receiver and 4 wireless devices. Moreover, within 20 m, the loss rate was almost 0, and more than 30 m, the loss rate was near to 5 percent. It could be expected to build a wireless fatigue monitoring system, which will be applied to the real structure such as bridge and ships, by using these new wireless devices. 4.3. Fatigue experiment Figure 11. Relationships between the normalized crack length and the crack growth rate In the high-stress low-cycle fatigue environment, the fatigue crack path of the sensor exhibited a straight line and is possible to measure the crack length easily. Figure 11 shows the relationships between the normalized crack length and the crack growth rate calculated by the incremental polynomial technique under each test condition. Tests performed two times under the same condition ( εmax=4x10 -3) showed good repeatability. The crack growth rate was almost constant with fatigue cycles in the range of α=0.54 to 0.86, and it decreased slightly before increasing in the range of α>0.86. The crack growth rate did not change a lot even through the maximum strain changed. This tendency is quite different from that of the low-stress fatigue environment. Further research is
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