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In recent years, the novel lightweight honeycomb sandwich structures have been applied in a wide range of industries. However, daily operational conditions, fatigue, as well as various defects developed during exploitation lead to the risk of structure failure. To meet safety and economical requirements, such structures must be tested. In this paper, an ultrasonic pitch-catch technique based on ultrasonic guided waves (UGW) for detection of defects in honeycomb sandwich structures is proposed. The technique is based on simultaneous scanning of a pair of contact type transmitting and receiving transducers positioned at fixed distance from each other. The proposed technique has been verified by the experiments and simulations on a honeycomb sandwich structure with an aluminium core and carbon fiber skin. The results have shown that different types of internal nonhomogeneities, such as changes in internal multilayered structure, skin delaminations or disbonds between the skin and the core, give the effects of amplitude and phase velocity deviations, which can be detected with accuracy reasonable for practical applications. [ABSTRACT FROM AUTHOR]

The object of the investigation is a honeycomb structure of composite sandwich made of glass-epoxy laminating layers and a honeycomb core of epoxy impregnated paper. Large composite tanks possessing cylindrical shape are produced using the winding process. Therefore, the final products have uneven thickness and fibre orientation lamina layers, and also an unevenly impregnated honeycomb layer. The aim of this research is to develop an economically attractive embedded ultrasonic measurement technique for on-field diagnostics of complex composite structures used for production of large liquid storage tanks. Development of the relatively cheap and easy to operate embedded diagnostic/monitoring technique is important aiming to assure safety of liquid storage tanks (monitoring structural integrity against overpressure, etc.) and detection of accidental defects that may appear during transportation, installation and exploitation of those structures. Typical defects that are aimed to be detected are relatively large delaminations/disbonds (area having diameter of 150-200 mm) of skin layers caused by low energy impacts that cannot be detected visually and show severe influence on the structural strength and safety of liquid tanks. This work presents results of numerical modeling and experimental research in low frequency (50 kHz) ultrasonic guided waves (UGW) propagation in large honeycomb composite structures. Finite element (FE) simulation of UGW propagation has been made aiming to reduce quantity of ultrasonic transducers and optimize their placement on composite structure. The possibilities to place ultrasonic transducers and receivers on both sides (internal or external) of composite structures and to use such a proposed technique for detection of delamination/debonding areas caused by low energy impacts or alternating semi static loads (e.g. filling and draining of liquid from storage tank) and monitoring of partial self-healing of relatively rigid composite structures were proved by experimental testing. [ABSTRACT FROM AUTHOR]