load P=18KN. As one example, Figure 5 is the relationship between resistance variations measured by sensor spliced on one 7074Al specimen and test load cycles. As the set alarm value was ΔR = 0.05Ω in the test, the crack was detected at approximate 5.8x104 cycles. The crack length was about 0.3mm corresponding to ΔR=0.05Ω which was measured on the fracture surface by stereoscope analysis. ICM under different loading ways, such as tension-tension, tension-push and three point bending, has the similar results for crack detection, as depicted in Figure 6. Particularly, ICM sensors are able to monitor the riveted lap and butt joints where fatigue cracks likely commence at holes edges. From above coupon results, it is thought that ICM is suitable to detect small fatigue cracks and monitor their growth. Figure 5 The curve of resistance and load cycle from 7074Al specimen test results 2.3.2. Component tests Full-scale fatigue tests were carried out using real horizontal tail shafts as specimens. Taking this advantage, ICM system with 5 special sensors spliced on the fatigue critical areas of the shaft determined by finite element analysis has been used during the test, as shown in figure 6. The histories of cracks from initiation, growth till fracture in the shafts were monitored by ICM system during tests. Figure 7 is a resistance-hour curve recorded by ICM system for one of shafts which can verify the effectiveness of crack detection for ICM system in real component. Figure 6 Some works done for extending the fatigue life of the shaft in a type of fighter aircraft
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