it would be important to perform some linearity checks on the sample to predict where the resonant frequency might move to during the HCF test. Figure 9 presents an FRF of a similar sample to the one shown earlier, whose frequency of interest was around 560Hz. This example aims to explain more clearly the reason for tuning a test rig to a frequency matching the one run during the HCF. In fact, the rig was tuned to a frequency of 560Hz which, with the increase of the excitation force, did not match the sample resonance anymore thereby reducing the amplification effect of the mechanical test rig. Figure 10 Non linear responses of a specimen 7 Conclusions This work has presented a simple but practical study of the effect of impedance mismatch between an electromagnetic shaker and a test rig used for performing HCF measurements. The endurance measurement is becoming increasingly important for building data sets of fatigue properties of new composite materials. The Figure 9 Test rig (a), connecting mechanism and shaker armature (b) MTS MHB MSA KRod KTS KSA CTS CRod CSA (a) Test rig (b) Shaker Armature MTM Connecting mechanism 511
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