92 F. Magi et al. 0.025 8 7 6 5 4 3 2 1 0 0.02 0.015 0.01 0.005 0 Amplitude 0-Peak [m] 116 118 120 122 124 126 Frequency [Hz] 128 130 132 134 116 118 x 10-3 120 122 124 126 Frequency [Hz] 128 130 132 134 Transmissibility V/A [m/s]/[m/s2] 0.5V before a b 0.4V before 0.3V before 0.2V before 0.1V before 0.1V after 0.2V after 0.3V after 0.4V after 0.5V after Fig. 8.9 Response (a) and transmissibility (b) of Test 3 before and after fatigue 134 132 130 Resonant frequency [Hz] 128 126 124 0 2 4 6 Number of Cycles Frequency 2nd harmonic x 106 8 10 1 1.06 1.12 Normalized 2nd harmonic amplitude 1.18 1.24 1.3 Fig. 8.10 Increase of the second harmonic of the vibration velocity The modal analysis carried out before and after the endurance proved to be useful for understanding the structural modification that the specimen has undergone. Nevertheless, it does not provide any information about the temporal evolution of the damage and the appearance of nonlinearities. Measuring the ODSs and controlling the presence of higher harmonics in the response vibration while fatigue testing helps to identify in time the damage evolution. By using the LDV while the PLL and the amplitude control are active, it is possible to gather rapid information about the level of nonlinearity. For the reference test, the presence of higher harmonics was not captured during the 10 million cycles. However, referring to Fig. 8.10 as soon as the first damage occurs, the vibration becomes highly nonlinear, with a second harmonic that increases by 4 % after the first drop in frequency at 3 million cycles. After that, as a consequence of delamination propagation, the second harmonic reaches a value 20 % higher than its initial value. In addition to the nonlinear behaviour, delaminations cause changes in the ODSs. The SLDV scanning performed at regular intervals captured changes of the deflection shape due to developing damages. Referring to Fig. 8.11, it is possible to see how the onset of the delamination in the right side of the specimen causes a torsional movement, changing the target strain field. Thanks to this observation, it was possible to understand that referring the fatigue test to a constant strain after the delamination initiation is misleading.
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