Control of gaps in technical structures during ground vibration testing

OBRABOTKAMETALLOV MATERIAL SCIENCE Том 20 № 3 2018 EQUIPMEN . INSTRUM TS Vol. 3 No. 2 2021 The papers [26, 28] demonstrate how the gaps in the moving connections of the force-displacement application systems might be identified on the basis of the GVT. It was suggested to measure overloads in the moving connections of its substructures and plot it as Lissajous figures. The paper [29] shows that the results acquired by means of the portraits of oscillations and the Lissajous figures appeared to be the same. Figure 2 shows an example of the displacement application system – a control wiring of deflectable surfaces. а b Fig. 2 . A control wiring of deflectable surfaces а – an exemplary control wiring scheme; b – cables and rockers with sensors; 1 – cables; 2 – rockers; 3 – acceleration sensors Fig. 3 . The natural frequency as a function of the oscillation amplitude The presence of the gaps in the moving connections of the control wiring results in the increased displacement of the deflectable surface. That is why the rotating frequency of the surface depends on the oscillation amplitude. Figure 3 illustrates the intrinsic natural frequency dependency on the oscillation amplitude of the deflectable surface which is neutrally imbalanced (there is a static force in the system). As shown in Fig. 3, A – the oscillation amplitude of the control point of the surface; ω – the natural frequency of the deflectable surface (the resonance frequency); A 0 – the oscil- lation amplitude when the static force in the control wiring is achieved; ω 0 – the natural frequency of the system without the gap; ω e – the minimal value of the natural frequency. If the displacement of the deflectable surface exceeds a permissible value due to the increased gap in one of the moving connections, the damaged structural part is located by the ξ parameter and the size of the gap is estimated according to the equation ([28]): 2 2 0 0 0 1 4, 2 1 1, 39 . e e A           ω ω       τδ = − − +           ω ω             (2) Where δ – the size of the gap; τ – the fraction of the displacements of the sensors located near the damaged and the control points. It is worth to be noticed that if the decrease in the rotating resonance frequency of the deflectable surface due to the gap is less than 12 % and the oscillation amplitude is not higher than 50 % of the А 0 value, the size of the gap (2) is calculated with the precision error no more than 10 %. The equation (2) should be used specifically. For instance, to determine the ω е frequency of the deflect- able surface at the moment when the gap is disclosing, one has to descend the frequency of the driving force. The oscillation amplitude А 0 is calculated when the distortions of the portraits of oscillations emerge,