PSI - Issue 36

O.L. Derkach et al. / Procedia Structural Integrity 36 (2022) 71–78 O.L. Derkach et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction and Problem Statement At present, rocket-space and aircraft engineering impose strict requirements to the design of equipment, which involve the weight reduction of structural elements in the assurance of their strength and fatigue resistance. This fact implies a wide use of anisotropic composite materials. At the same time, local damages, particularly fatigue cracks and dents, occur during operation under the action of external dynamic loads and working environment. These damages can cause the loss of functional performance of the objects. Therefore, there is a need for the development of scientifically justified approaches to the diagnostics of such damages. Cawley and Adams (1979), Manivasagam and Chandrasekaran (1992) show that determination of the presence of cracks in composite elements is a more complex task as compared with that one for structural elements made of homogeneous isotropic materials. As is known their presence in the structure affects the modal and dissipative properties of the elements’ componen ts. The natural frequencies and modes of vibrations on the objects under investigation in Zinkovskii et al. (2015, 2018) and Kruts et al. (2019) are used to obtain information on the damage presence. It should be noted, in these works the influence of the damage taking into account on both stiffness and inertia properties of the beam. In the same time, as have been shown in Krawczuk and Ostachowicz (1995), Song et al. (2003), Kisa (2004), and Kim et al. (2019) for composite structures, the level of influence of the damage parameters on the specified modal parameters depends considerably on the properties of composite material. The development of vibration diagnostic procedure for damage detecting from the results of determination of modal parameters of the composite elements in the structures remains to be topical. It is required to develop the models and calculation- experimental investigations on the determination of the damage parameters’ effect and structure of the composite material on the formation of the spectrum of natural frequencies and vibration modes. The goal of the paper is to determine the effect of local damage in the form of a rectangular notch (as a dent model) on the surface of structural elements and anisotropy of the elastic properties of the unidirectional composite materials on the principal frequency of flexural vibrations of the cantilever beam with the constant rectangular section. 2. Object of Investigation, Its Model and Its Validity Consider a cantilever beam in the constant rectangular section (Fig. 1). It is made of the composite material which consists of a soft isotropic matrix and stiff unidirectional fibers of the circular cross-section at the angle θ to its axis.

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x C

h C

θ

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h

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Fig. 1. Cantilever composite beam, which is reinforced with unidirectional fibers, with damage in the form of rectangular notch.