Fatigue crack growth in bolted metallic joints is one of the most serious and concerning types of damage in aircraft structures. The problem of how to quantitatively track the size of a crack in order to determine structural integrity has attracted much attention. In this paper, a novel eddy current array sensing film, with one exciting coil covering the entire thickness and several sensing coils distributed along the axial length of the hole, bonded onto the bolt, is proposed to quantitatively monitor a bolt-hole crack in the radial and the axial directions. Finite element simulation is utilized to investigate the eddy current disturbance caused by the crack growth and to optimize the coil configuration of both exciting and sensing coils in order to improve the crack monitoring capability of the sensing film. The simulation results show that the proposed sensing film has very good capability for tracking cracks when the traces of the exciting coil are designed to be similar to the sensing coils’ and have opposite current direction at the boundary of two adjacent sensing coils. An experiment is conducted to verify the feasibility and effectiveness of the proposed eddy current array sensing film to quantitatively track hole-edge crack growth in bolted joints.
Bolted joints are the key components for enduring primary load and play a vital role in ensuring structural safety for aircrafts. But it is very difficult to analyze the strength and failure modes of bolted joints due to their complexity and nonlinear coupling factors. Therefore, it is greatly essential to estimate potential failure mode at the early age and quantitatively track the damage parameters for calculating residual life and determining structural integrity. In this article, the concept of two-dimensional eddy current array–based sensing film is proposed to estimate the failure modes by identifying the circumferential location of the damage around the bolt hole and quantitatively track the damage parameters including the damage size in the radial direction and the damage depth in the axial direction of bolt hole. Finite element simulation is utilized to study the interaction between eddy current signals and the damage, and optimize the configuration of sensing film. Simulation result shows that a noticeable difference of eddy current signals of sensing film can be clearly seen that damage is located at different circumferential location around the hole. In addition, the exciting coil with opposite current directions at the boundary of axially adjacent coils has a better capacity of differentiating the damage sizes in the radial and axial direction than that with same current directions. The experiment is conducted to demonstrate the effectiveness of the proposed sensing film for estimating fatigue modes and quantitatively tracking damage growth.
There is an urgent need to monitor the structural state of composite bolted joints while still remaining in service; however, there are many difficulties in analyzing their strength and failure modes. In this paper, a built-in distributed eddy current (EC) sensor network based on EC array sensing film is developed to monitor the hole-edge damages of composite bolted joints. The EC array sensing film is bonded onto the bolt and consists of one exciting coil and four separate sensing coils. Experiments are conducted on unidirectional composite specimens to validate the ability of the EC array sensing film to quantitatively track the damage that occurs at the hole edge and to investigate the performances of the EC array sensing films with different configurations of the exciting coil. Experimental results show that the induced voltage of sensing coil changes only if the damage appears on the laminate structure where that particular sensing coil is located, whereas the induced voltages of the other sensing coils on other laminate plates remain unchanged. Numerical simulation based on the finite element method is also carried out to investigate and explain the phenomena observed in the experiments and to analyze the distribution of the EC around the bolt hole. Both experimental and numerical simulation results demonstrate that the developed EC array sensing film can effectively identify not only whether there is damage at the hole edge but also the damage location within the thickness and quantitative size.
In this study, the nonlinear equation of motion of the beam on the elastic foundation is obtained via the Newton's second law of motion, and its free vibration nature is investigated. Considering the inextensionai condition, the planar model of the beam accounting for the effects of the rotary inertia is derived. Then, the linear vibration and nonlinear vibration of the beam on the elastic foundation are examined. It is shown that the cut-ojf frequency can be observed in the frequency spectrum of the beam response. The effects of the rotary inertia on the natural frequencies are systematically investigated. Einally, the frequency differences, due to the different foundation models, and the possible modal interaction of the beam are discussed.
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