Modal characteristics (natural frequencies and vibration modes) estimation is a common way to avoid resonance vibrations in constructions. It requires reliable data about mechanical properties of the material. In the case, when the polymer composites' determination of such characteristics is difficult (in comparison of isotropic materials), because of a greater amount of elasticity characteristics and their dependency on wide range of structural and technological factors. Also, most of the literature and open sources contain controversial data about composite material properties, and in case of crucial constructions calculations, it would be better to perform an additional experimental identification of these properties. The aim of this paper is to create elastic vibration model parameters identification method for polymer composites using experimental modal analysis. The object of research is laminated carbon fiber-reinforced plastic based on a full strength carbon material widely used in aviation. An experimental determination of natural frequencies and corresponding vibration modes was performed using 3D scanning laser vibrometry. Finite-element analysis was used for numerical determination of modal characteristics. The material model used in calculations is a laminated composite structure with orthotropic, linear, and elastic layers. Identification of parameters was performed as a minimization problem of discrepancy between natural frequencies for corresponding vibration modes obtained numerically and experimentally. The problem solving was performed using a quasi-random search method. The proposed method can be recommended for material properties determination required for a modal analysis of polymer composite structures.

It is a widely known fact that the stiffness of polymer composite materials decreases with the accumulation of fatigue damage under cyclic loading. The purpose of this article is to develop a method and obtain experimental data on decrease of the elastic characteristics of a fiber-reinforced laminate, as a result of progressive fatigue damage. The developed technique consists of two stages. At the first one, the natural frequencies and eigenmodes of the samples during their fatigue testing are experimentally obtained. The dependences of the natural frequencies of the samples on the number of loading cycles are found. At the second stage, the four elasticity parameters of the laminate monolayer (two Young modules, the shear module and Poisson's ratio) are identified via the natural frequencies. The inverse numerical/experimental technique for material properties identification is applied. The dependences of the natural frequencies and mentioned elastic characteristics on the relative fatigue life are obtained as experimental results of both modal and fatigue tests. The results can be useful to study the fatigue behavior of the investigated materials and to create methods for calculating fatigue life.

This article describes a method for choosing the shape of structurally-similar elements made of polymer composite ma-terials and the scheme of their loading during fatigue tests. These tests are an important step of the complex of efforts to prevent fatigue failure of aviation critical structures made of polymer composite materials. The shape of structurally- similar elements and the loading scheme must be substantiated by a series of preliminary calculations. When testing such samples, the deformation field in the critical zone of the part must be re-produced. The ability to achieve fatigue failure of specimens on the selected equipment shall be verified. The application of the proposed approach to the justification of the choice of structure and loading of structurally similar elements is demonstrated on the example of a typical aircraft structure.

In the present work, a comprehensive experimental study of the patterns of fatigue failure of thick rods made of laminated carbon fiber is carried out under two loading schemes: cyclic tension and cyclic three-point bending with static tension. In the process of fatigue loading, along with the loading parameters of the samples, the resonant vibration frequency of the sample, acoustic emission parameters, strain fields, and temperatures on the sample surface were continuously recorded. The main mechanism of destruction of laminated carbon fiber in both studied loading schemes is the appearance and development of delaminations. Experimental data on the change in the resonant frequency of sample oscillations and the parameters of acoustic emission (the number of events per unit time and the energy of events) are obtained during fatigue testing. There is a stepwise change in the number of acoustic emission events due to the appearance or sharp growth of cracks, which coincides in time with the phenomena of an abrupt change in the resonant frequency and the appearance of high-energy acoustic emission events. In the zones of fatigue failure, local areas of increased self-heating of the samples appear. The fields of three components of the strain tensor on the surface of the studied samples during the fatigue loading were obtained by the method of digital image correlation. In the zones of delaminations, the component of deformations transverse with composite layers sharply increases. The recording of acoustic emission parameters, deformation fields and temperatures, resonant oscillation frequency during fatigue loading makes it possible to reveal in a complex the facts and moments of the appearance and development of fatigue damage in samples.

Carbon fiber reinforced polymers (CFRP) are increasingly being used in heavily loaded parts of aircraft engines. CFRP fan blades, vanes must successfully resist fracture due to high-cycle fatigue. One of the consequences of fatigue damage is a decrease in the rigidity of the material and a drop in the natural vibration frequencies of parts. These effects are of interest when developing fatigue fracture models and predicting durability. The purpose of this article is to develop a method and obtain experimental data on the decrease of elastic characteristics of CFRP as a result of progressive fatigue damage. The developed technique consists of two stages. During the first one, the natural frequencies and eigenmodes of the samples during their fatigue testing are experimentally obtained. During the second stage, the four elasticity parameters of the CFRP laminate monolayer are identified via the natural frequencies. The inverse numerical/experimental technique for material properties identification is applied. The dependences of elastic characteristics on the relative fatigue life are obtained as experimental results of both modal and fatigue tests. The results can be useful in studying the fatigue behavior of the examined materials and in creating methods for calculating fatigue life.