160 M. Piraccini et al. In order to increase the reflection from the surface of the blade, reflective tape was used at the point of measurement. The forced response was acquired by using a LabVIEW programme and the frequency of excitation was set by using a control panel for regulating the speed of the electric motor of the exciter. By regulating the airflow by means of opening or closing the flow control valve of the chopped air machine, different excitation intensities were adopted in order to characterise the behaviour of the 1st flexural, 1st torsional, and 2nd flexural mode shapes of the three different layups at increasing displacements. After localising the resonant frequencies, tests were carried out on each of the first three mode shapes by exciting each blade in a frequency range around its natural frequencies and repeating the tests at different levels of excitation (i.e. different airflow intensities). 15.4.1 Experimental Results The following graphs summarise the global measured forced responses and give the natural frequency change with regard to the measured amplitude of the displacement. 15.4.1.1 Unidirectional: 1st Flexural Mode The 1st flexural vibration of the unidirectional 0ı-angled blade has been studied by means of exciting the component around its linear natural frequency (74 Hz) with six increasing airflow intensities. From forced responses (Fig. 15.6a) it can be seen that even with the lowest excitation intensities the amplitude of the vibration of the unidirectional blade at the first mode shape was extremely high. The behaviour of the blade vibrating with the 1st flexural mode shape is evidently linear as there is no significant frequency variation with varying amplitude even if the displacement becomes very large as can be seen from backbone curve in Fig. 15.6b. 15.4.1.2 Unidirectional: 1st Torsional Mode The vibration of the 1st torsional mode of the 0ı blade is extremely singular and markedly nonlinear. Nine increasing airflow intensities were investigated so as to be able to fully characterise such a varying behaviour. From Fig. 15.7a it can be seen that jump phenomena start to appear when the peak of the vibration approaches the value of the thickness of the blade (i.e. 3 mm) and the vibration abruptly changes its intensity. This is coherent with the known consideration that significant nonlinearities Fig. 15.6 First bending mode. (a) Forced responses of the unidirectional blade during vibration at its first flexural mode with six increasing intensities of airflow (labelled “int”). (b) Backbone curve of the unidirectional blade undergoing linear vibration at its first flexural mode with six increasing loading intensities
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