8 Scanning LDV Measurement Technology for Vibration Fatigue Testing 89 Fig. 8.3 Process flowchart and a linear calibration curve is obtained as in Fig. 8.4b. As long as the mode shape does not change and the resonance frequency does not change, it is fair to consider the strain amplitude relationship constant throughout the test. After the stepped sine test, the software saves the strain–displacement relationship to use during the fatigue test. The measured point of the laser is stored and the software requires a grid of points for a discrete scanning and/or the measurement parameters for executing CLSDV measurement technique; this latter requires both the scan rates and amplitudes of the sinusoidal voltages for the X-Y scanning mirrors. The measured area is controlled to be within the measurable range of theLDV. Referring to Fig. 8.5, when the fatigue test starts, the vibration phase and amplitude are constantly fed back to the software control. The PLL control locks the phase between the input and the output by changing the excitation frequency. As soon as the output phase exceeds an upper and lower threshold spaced apart the target phase, the controller is activated. The amplitude control is based on the same principle. In addition, while the test is running, nonlinear vibrations are monitored. Controls stop working at regular interval in order to allow the ODS measurement, which would capture any changes in ODSs by delamination propagation. When failure criterion is met or 10 million cycles are reached, the fatigue test stops and modal analysis is carried out. Several specimens were tested with this procedure but only few test data are reported in here. Thanks to the LDV system it was possible to study the evolution of four recordings:
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