# Experimental Verification of the Dynamic Model of Turbine Blades Coupled by a Sealing Strip
## 3 The Experimental Results
### Blades with Strips  
The strip is introduced in the slot on the cover of the air box and the compressed air system is turned on at the maximum pressure (0.75 MPa pressure at the inlet). The IP and OOP FRFs are then detected on the blades. In Figure 4a, the FRFs obtained with IP excitation are depicted. This time the two FRFs (FRF\({}_{\text{IP}}^{1}\) and FRF\({}_{\text{IP}}^{2}\) plotted in solid lines) are perfectly overlapped. In fact, the blades are no longer two separate entities: with the strip in the middle, they constitute a single dynamic system. The peak corresponds to a mode where the blades vibrate coupled by the strip together in \"in phase\" (IP mode). The FRFs obtained in case of OOP excitation are shown in Figure 4b. As for the IP case, the FRFs of the two blades with strip are perfectly overlapped. In this case the influence of the strip is more evident: it can be observed that the frequency of the OOP peak with a strip is higher than the corresponding OOP peaks without the strip. The close observation of Figure 4a,b yields the following highlights. Figure 3: (**a**) The four frequency response functions (FRFs) of the blades without strip and with the compressed air system switched off; (**b**) IP (in phase) FRFs of the blades without a strip in the case of compressed air turned on or off. * The FRFs of the system with strip are lower in amplitude than the FRFs without strip since globally the strip introduces stiffness and damping in the dynamic system. * IP motion: since the blades are no longer two separate entities, the two separate peaks (one for each blade) typical of the free condition have been substituted with one peak in the middle of the two. The IP peak (with strip) (Figure 4a) is in the same frequency range of the resonance peak of the free blades (i.e., without strip). The physical explanation of this can be found by observing the relative displacement of the blade platforms during IP blade vibration. As shown in Figure 5, this displacement is mainly vertical and consequently the strip constitutes a quite weak constraint (it may even detach from the platforms during part of the period of vibration). This is further analyzed in Section 5. The result is that the strip influence on the IP mode is quite weak both in terms of damping and stiffening, i.e., the amplitude and position of the peak does not change significantly. * OOP motion: the OOP peak (Figure 4b) is at higher frequency than the corresponding peak in free (i.e., no strip) condition. Furthermore, the peak appears considerably reduced in amplitude. The explanation can be found again by observing the relative displacement of the blade platforms. As further commented in Section 5.2, the OOP mode sees a purely horizontal relative displacement between platforms. This platform kinematics maximizes the strip damping and stiffening capability. Repeatability tests have been performed by repeating each test after having switched off the compressed air so that the strip has returned to its initial position on the air box cover. Each time  
Figure 4: (a) IP FRF of the two blades, blades with strip \(F_{E}=0.1\) N and without. (b) OOP (out of phase) FRF of the two blades, blades with strip \(F_{E}=0.1\) N and without. Figure 5: IP and OOP blades’ mode shape of vibration and corresponding strip behaviour. the air is switched on, the strip is repositioned against the blade platforms. The position of the strip with respect to the platforms may potentially differ from one test to the other. The FRFs detected for three repeatability tests performed at \(F_{E}\) = 0.1 N are shown in Figure 6 for blade 1, respectively, for IP excitation (Figure 6a) and for OOP excitation (Figure 6b). It can be observed that the IP curves are quite repeatable since, as commented before, the strip is less effective. The OOP curves look more noisy since they have lower amplitude and, for this reason, the differences are more evident.