shaker. In fact, the excitation can be produced: (i) by flutter, as happens in the constant flow air jet, or (ii) by periodically impacting the structure with pulsed air jets. The first method can usually be performed on first-order modes- for example, 1F, 2F and 1T, all of which can be “easily” excited by flutter whereas it is more difficult to excite high order modes with this method. A pulsed air jet offers a better control of the excitation because it is produced by a perforated rotating disc whose rotational speed governs the excitation frequency of the tested component. This machine can produce excitation in a frequency bandwidth which depends on the max rotational speed and the number of holes of the sampling disc. The excitation force is applied to the component through a nozzle and its level depends on the pressure of the air impacting the component. The larger the impacting area, the larger is the force applied for a given constant pressure. However, a larger impacting area could produce a smaller response when higher order modes are excited because of nodal lines close to the excitation position. An EM shaker is a versatile exciter which can control the excitation frequency more precisely and for a wider frequency bandwidth as compared with previous exciters. Force transducers can be installed for measuring the force input into the structure allowing the measurements of a Frequency Response Functions (FRF). This cannot be done using the other two exciters because in there the force level can be only estimated. Generally, when an HCF test is performed using an EM shaker, the test component is installed in a test rig. The simplest rig set up would be to connect a holding block to a shaker using a push rod to perform a base excitation. A component can be so constrained to the holding block which transfers the excitation force. Such a test rig configuration can be very sensitive to energy dissipation because of the many components connected. However a solution for enhancing its capability is to design a test rig to be tuneable to one resonance of a test piece. 3 Model of test rig configuration A simple lumped parameter spring-mass model, representing a test rig and specimen, was used to simulate forced response so as to calculate some design parameters of the rig. Figure 1 presents a simple model of the test rig in which the dashed boxes represent the test structure and the shaker armature, (a) and (b) respectively. This system provides three variables which can be used for designing and tuning the test rig, and these are: (i) the mass of the holding block (MHB), (ii) the stiffness of the rod (KRod) and (iii) the mass of the armature (MSA). A first run of simulations was produced for designing the rod dimensions, diameter and length, and other simulations were performed for sensitivity analysis. The latter was needed to identify the parameters which could be easily modifiable for fine tuning of the rig. Results of the simulations showed that the mass of the holding block was quite insensitive for tuning whereas the rod dimensions and the mass of the shaker armature demonstrated to be very sensitive. A modal test of the test structure, constrained in the holding block, was produced so to identify a mode shape and its natural frequency. Using the measured natural frequency the analytical model calculated a set of dimensions for the push rod. Table 1 reports the modes, natural frequencies and loss factors, and it is interesting to note that the composite material used here shows higher values of damping than a metallic equivalent one. We can say that, for a given excitation force, the levels of vibration achievable with metal components can be higher that the one achievable with composite ones. Hence, a shaker needs to work harder to excite a composite specimen to equivalent vibration level. The component was constrained with good accuracy by gluing two 505
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