signals to the main interrogator by cables. One sub-interrogator can connect with as many as 8 sensors. A main interrogator comprises of following units: 1) communication unit for sending and receiving the message from each sub-interrogator; 2) storage unit for saving all working parameters and data; 3) date-time unit for recording the date and the time as the resistance value changes of each sensor; 4) man-machine interaction for monitoring working conditions of all sensors through the lights in the main-interrogator and downloading the message to a ground–portable computer through the USB interface for further processing; 5) central process unit for managing all the above units and 6) DC power converting module with wide inputting voltage. Figure 3 The construction of ICM system hardware The weight of sensor can be ignored and the weights for a typical sub-interrogator and main interrogator are less than 180g and 80g, respectively. The dimensions of a normal sub-interrogator are 67mmx50mmx16.5mm and the dimensions of a normal main interrogator are 110mmx50mmx 20mm. The software component is a special developed program for managing the ICM system which can be installed on a portable computer. The program can download and process all the data recorded and saved by the ICM system. Since the system has a huge internal memory, it can store up all recorded data by existing sensors during their lifetime, the intervals for off-board downloading and processing the data can be conducted by users according to their own needs. The possessed data can help users determine whether the ICM integrity has been compromised or real damage like cracks has been appeared on the monitored locations. In addition, the program can also realize real-time monitoring. 2.3. Verification tests 2.3.1. Coupon tests Figure 4a Five types of specimens Figure 4b The picture of coupon testing Fatigue tests were carried out on five types of specimens including riveted lap joint and screwed connection made of 4340M steel, 7074 aluminum alloy and TC4 titanium alloy, as shown in Figures. 4 and 5. All specimens were fatigue loaded with 15 Hz, load ratio R=0.2 and the maximum tension Sensor Main- interrogator Sub- interrogator Main- interrogator
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