The results of an experimental study of the effect of operation-induced damage in equitype elements on vibration of regular system are illustrated by the example of testing a fork type specimen. The in-service frequency detuning of equitype elements is found to have an influence on the formation of vibrations of regular systems similar to their technological detuning.Keywords: damage, equitype element, regular system, fork type specimen, frequency detuning.
Problem Formulation.It is a matter of general experience that the systems that are functionally designed to be regular ones have some unavoidable deviations from identity and regularity of arrangement of their equitype elements (subsystems) due to various factor related to design, manufacture, and operation. For instance, in the case of turbomachine impellers (a typical example of such systems) the aforesaid shows up in the fact that the turbine blades differ in elastic, dissipative, and aerodynamic characteristics. Based on numerous theoretical and experimental investigations, the blade frequency detuning within the applicable manufacturing tolerances (i.e., the technological detuning) is responsible for a considerable scatter in vibrational stresses in the blades. However, when in operation the blades may experience nick-type damages due to the ingress of foreign bodies, fatigue cracks, etc., which in fact constitutes the in-service disturbance of the impeller regularity (i.e., the in-service detuning).Analysis of the available research findings has demonstrated that the investigations of the influence of fatigue crack type damage on the formation of a vibration pattern in mechanical systems addressed mostly the individual (separate) structural elements such as beams, bars, shafts, blades, plates [1,2]. This type of damage was found to affect their vibration frequencies [2] and dissipative characteristics [3]. There are almost no data on the effect of damage in equitype elements on the vibration of regular systems. We are aware of only one work [4] that provided results of a numerical study of the influence of a blade crack on the vibration of a bladed disk.The objective of the present work has been to study experimentally the effect of a fatigue crack type damage on vibration characteristics (frequency and amplitude) of a regular system.Experimental.The tests were carried out on a fork type specimen with prismatic bars. It was made of as-delivered D16 aluminum alloy. The specimen design is schematically shown in Fig. 1. It constitutes a physical model of a regular system consisting of two equitype elements. From the design standpoint, the specimen is similar to that used earlier [5] for the study of the influence of technological frequency detuning of subsystems on the vibration of the regular system. This enables us to compare the experimental data obtained.Damage in one of the bars was simulated by a 1-mm-wide notch of depth a, which was made crosswise on the test portion at a distance l from the root section. From this point on, the intact and damaged ba...
We present the results of investigations of the aerodynamic stability of flexural-torsional vibrations of compressor blades under conditions of attached and separated flow with regard for the cross and mutual aerodynamic links of the blades in a broad range of variations of the phase shift, the ratio of the amplitudes of translational and angular components of their displacements, and the angle of attack. A procedure and equipment used for the experimental determination of the nonstationary aerodynamic characteristics of the blades are described on the basis of the model of flat-plate airfoil cascades.Statement of the Problem. In trying to get the highest possible specific parameters of the aircraft gas-turbine engines (AGTE), we inevitably encounter the problems of vibrations of blades in the rotors of these engines. Thus, one of the methods used to make turbomachines more compact and decrease their weight is based on the application of blades with high aspect ratios. This leads, on the one hand, to a shift of the spectrum of natural frequencies of vibrations of the blades to the region of lower frequencies and, on the other hand, to the excitation of of high-frequency flexural-torsional modes of vibrations of the blades characterized by the presence of a phase shift between the translational and angular components of displacements of the sections and different values of the ratio of their amplitudes.It is known [1] that the role of decisive factor in the loss of the stability of a wing in the case of classical flutter is played by the phase shift between the translational and the angular components of displacements of the wing responsible for the formation of aerodynamic crosslinks and the supply of energy from the flow. The principal features of the classical flutter of airplane wings are also typical of the loss of aerodynamic stability of the analyzed blades [2]. However, this process is complicated by the mutual influence of neighboring blades in the blade ring. A similar phenomenon is also typical of the lightened blade rings of fan and compressor stages containing either compliant disks or damping (antivibration) shroud platforms. The self-excited vibrations of these rings are realized in the form of running strain waves with various numbers of the nodal diameters and circles. Note that the phase shift between the translational and rotational components of displacements in the sections of blades located near the shroud 200 0039-2316/07/3902-0200
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