90 F. Magi et al. Fig. 8.4 (a) Displacement (blue) and strain (green) time signals at one vibration amplitude used for the strain–displacement calibration. (b) The linear strain–displacement relationship at different vibration levels Fig. 8.5 Time sequence of the software operations • Resonant frequency decay • Changes in ODS during the endurance • Presence of extra harmonics in the response vibration • Changes in vibration amplitude response before and after the endurance. 8.3 Results The damage evolution in fatigue tests can be monitored by measuring the residual stiffness of the component. In a dynamic environment that means to monitor the resonant frequency decay (Fig. 8.6). Further, by capturing the surface temperature of the component, one can correlate the drops in frequency with the delamination initiation and propagation. From Fig. 8.7 one can observe how the temperature could help to identify the damage onset temporally (maximum temperature evolution over number of cycles) and spatially (thermal images). Test 2 was carried out at low vibration amplitude, its natural frequency never changed. It ran for 10 million cycles without developing damage. This test is considered as benchmark reference for comparisons with failed specimens. The outcomes
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