OBRABOTKA METALLOV

Introduction. The main defect of the structures that occurs during its operation or strength tests is fatigue crack. Vibrational methods are promising to diagnose the cracks. The practical implementation of these methods on real structures is a relevant objective. Objective: to study the possibility of using distortions of the constrained vibration portraits as an identification feature of cracks in metal structures. Research Methods. Acceleration sensor are installed on the diagnose structure, and vibration oscillations are created in it by means of independent sources of harmonic vibrations. The dynamic consequence of the fatigue crack is the collisions of the “faces” and dry friction at the crest points under the action of the vibrational load. It is shown that the identification attributes of shock pulses and dry friction can be presented by nonlinear distortions of the vibration portraits, the vertical dissection of which is proportional to the signal of the acceleration sensor, and the horizontal one is proportional to the first harmonic of this signal. Such an vibrations portrait for a linear dynamical system is a circle. In order to estimate numerically distortions from the Fourier series, the first harmonic is subtracted for the vibration portrait, the absolute maximum is determined for the period of vibrations in the remainder of the series; the magnitude of the maximum is related to the amplitude of the first harmonic. Results and Discussion. The efficiency of the fatigue cracks detecting due to distortions in the vibration portrait is illustrated by the example of diagnosing the metal plane body panel with the size of 2285 × 975 mm. The panel had longitudinal and transverse power sets, as well as local reinforcements. The vibrations excitation of the panel was carried out by two electrodynamic exciters. 40 acceleration sensors were used to measure vibrations. Experimental studies of changes in natural frequencies and portraits of panel vibrations for the following defects have been carried out: a stress concentrator in the form of a drilled rivet and a through-cut in the center of the panel; fatigue cracks in the side stringer; crack in the side stringer after repair; crack in the central stringer at different stages of propagation. It is established that the largest changes in natural frequencies can be several per cent, but it is not possible to determine the location of the defects. The maximum distortion of the vibration portrait is hundreds of percent and reliably tracks the locations of defects and its magnitudes. It is noted that the method of normalizing the distortions of the vibration portrait and the amplitude of the panel vibrations affect the efficiency of the diagnosis of cracks.

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