Control of gaps in technical structures during ground vibration testing

OBRABOTKAMETALLOV Vol. 20 No. 3 2018 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 3 2 21 а b Fig. 1 . The portraits of oscillations of a structure а – without a gap; b – with a gap The ground vibration tests were carried out via Test.Lab software. The subprogram was integrated into the software interface in order to analyze the portraits of oscillations. It allowed to calculate the distortions of the portraits of oscillations for all the measurement channels simultaneously, plot the distortion distributions of the structure and save it for further use. It enabled monitoring the gaps during vibration strength tests, as well as while the structures being used, by means of comparing the distortion distributions of the parameter ξ related to different states of the structure.Additionally, the plotting of the distortion distributions of the portraits of oscillations for each structural component was added to the subprogram so as to control the defects sequentially. While calculating the parameter ξ, the two types of normalization, which were conditionally named the global and local ones, were being used. In case of the global normalization, the value of Ψ was related to the amplitude of the first harmonic at the control point of the structure. The point where the amplitude of the first harmonic is maximal among the signals should be used as the control one. In case of the local normalization: ( ) 1 max , i i i A Ψ ξ = (1) where i – the measurement channel index; max|Ψ i | – the absolute maximum of the distortions of the por- traits of oscillations; ( A 1 ) i – the amplitude of the first harmonic. The global normalization is required to analyze the distortion distribution of the portraits of oscillations of the entire technical structure. A frequency-driven force usually acts near a natural frequency of the structure; that is why the false local maxima of the distortions have to be excluded. It occurs because of some acceleration sensors are situated close to mode shape nodes. The local normalization of the distortions of the portraits of oscillations is utilized to establish the locations of the gaps in the mechanical parts and structural connections. Such normalization facilitates comparison between the different gaps and control of its dynamics while being tested or utilized. Let us consider how one may make use of the proposed method to detect the gaps in force-displacement application systems. It is possible not only to identify gaps, but also to estimate its values based on experimental results. The increased gaps in the moving connections of the force-displacement application systems become to exist due to operating for a long period of time and fabrication errors. The gaps in transmissions of helicopters and automobiles lead to self-oscillations and make difficult to control it. Existing methods usually require the partial dismantlement of the structures.