88 F. Magi et al. 8.2 Material and Methods The test setup used here consists of a component which is excited at its first bending mode, clamped within the two rods of a steel fixture, and attached to the head of a shaker (see Fig. 8.1). As shown in Fig. 8.2, the component is a rectangular specimen, 100 mm wide and 260 mm long, with an added weight at 50 mm away from one edge, clamped at 110 mm and with ply drops in the centreline, at 130 mm. By adding the ply-drop, the component is forced to break in a desired location, along a line, away from the clamp. MONTEVERDI software, coded in LabVIEW, was specifically designed to automate the fatigue tests and used for this research work. The flowchart in Fig. 8.3 shows the entire test procedure, from the modal test to the endurance test. The software is made to perform a series of operations including Frequency Response Function (FRF), Stepped Sine Test (SST), calibration of strain-amplitude relationship, thermal imagining and endurance test. One accelerometer, attached at the fixture, was chosen for referencing the FRF and for the Phase Lock Loop (PLL) control during the fatigue test. The steps reported in Fig. 8.3 are required to study the dynamic behaviour of the system before, during and after the fatigue test. First of all the response of the structure is measured so as to identify the response target mode. After the first bending mode is identified, the stepped sine test is carried out at different vibration levels. In addition the strain–displacement calibration is made. This practice is adopted because the extreme dynamic environment, which either (1) forces the strain gauges to peel off from the specimen and/or (2) fatigues the cable after few thousands cycles. To overcome such an issue, the strain and the LDV output signal are synchronously measured, as shown in Fig. 8.4a. The measured data are then correlated Fig. 8.1 Fixture set-up Fig. 8.2 Specimen dimensions
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