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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.

Carbon/epoxy fiber laminates are increasingly used in critical structures of aircrafts, ships, etc. Fatigue resistance is an important requirement for such structures. Material degradation due to the gradual accumulation of fatigue damage is accompanied by a decrease in stiffness. A decrease in stiffness during fatigue damage may result in a change in the natural frequencies of the structures. The aim of this study is to obtain experimental data on the variation of the elastic characteristics of a carbon/epoxy fiber laminate as a result of progressive fatigue damage. As the studied characteristics of the laminate, four elasticity parameters of the monolayer are considered (two Young's moduli, the shear modulus and Poisson's ratio). These characteristics are commonly used in laminate strength and durability models. The research technique is based on an experimental study of natural frequencies and eigenmodes of samples under fatigue testing. Fatigue tests of the samples are carried out under cyclic tension while maintaining constant amplitude and the average load value. The fatigue test of the sample was stopped several times to perform modal tests. The modal tests were carried out using scanning laser vibrometry. The determination of the four above-mentioned elastic parameters of the monolayer is considered as the inverse identification problem using the results of modal tests. The dependences of the mentioned elastic characteristics on the relative fatigue life are obtained. These dependencies can be used to develop methods for predicting the fatigue life of carbon fiber parts.

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.

In order to determine the elastic parameters of polymer composites, a mixed numerical-experimental method of identification was developed in the 1990s - 2000s, based on the use of experimental data on natural frequencies and eigenmodes of oscillation of samples. Its practical application involves the choice of a shape and a size of samples, a set used to identify the natural frequencies and eigenmodes of vibration, a methods of their experimental determination, a finite-element model for modal analysis, and an algorithm for solving the identification problem.The object of the research is laminated polymer composite materials reinforced with carbon or glass fibers. The aim of the work is to develop practical aspects of implementation and assessment of the accuracy of the mixed experimental-calculation method for identifying of the elastic parameters of such materials based on experimental data on natural frequencies and eigenmodes of vibration of standard samples. Parameter identification for the material is considered as an optimization problem with an optimization function, which is a weighted sum of squares of differences between the experimental and calculated values of natural frequencies. A procedure was developed to implementing the main steps of the suggested technique: experiments, calculations and control of results. An error analysis of main steps was carried out, and the accuracy of the determined parameters of the ply was estimated. To determine the natural frequencies and eigenmodes of oscillation of the samples, the method of three-component scanning laser vibrometry was used. The experimental technique was established and parameters of the technique were chosen to ensure the necessary accuracy in determining of the natural frequencies. The parameters of the identification procedure and the finite-element model of the sample were selected. To control the obtained values of the elastic parameters, the natural frequencies of the samples were calculated, including those not used in the identification procedure. The error assessment of the in determined elastic parameters was performed on three different series of samples of carbon-fiber-reinforced laminates with the same material of the plies and different ply numbers and stacking orders. The developed technique can be recommended to determine the parameters of material models required for strength and vibrations assessment of products manufactured with layered composites.

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.