| Bolted joints are widely used in aerospace domain for its advantages such as high reliability,high load-bearing ability,easy assembly and disassembly,easy maintenance and so on.However,connecting structures are the most vulnerable parts in aircrafts for the reason of stress concentration,complex load form,and tough flight conditions.Thus,it has become a research hotspot in Structural Health Monitoring domain to develop effective monitoring techniques for hole-edge damages,among which the intelligent bolt based on eddy current testing has shown its great development potential and widely application future in recent years.The newly proposed eddy current sensing film-based intelligent bolt composed of interleaving coil array though has the capacity to identify the hole-edge crack propagation condition in the radial and axial directions as well as its angle in the circumferential direction,it is of great practical significance to be further optimized for its large coil number,thickness,lift-off compensation demand,low angle resolution and linearity.In this paper,an intelligent bolt flexible sensing film with a new winding configuration of the sensing coil is designed,and its ability to detect the fatigue crack around the bolt hole edge is demonstrated by finite element simulations and experiments.Besides,the concept of composite intelligent bolt is put forward for composite connection structures.The main research contents are as follows:(1)An flexible eddy current sensing film composed of triangular coil array is designed.Rectangular exciting coil and triangular exciting coil are compared in the finite element simulation when a triangular array is utilized as inducing coils,and it is found that the latter is more sensitive to the hole-edge crack.Meanwhile,it is found that the same exciting currents at the adjacent positions of the circumferential adjacent coils are more conducive to detect crack growth than that when the opposite exciting currents are used.It is verified that the triangular sensing array along with triangular exciting winding is able to identify crack angle with better linearity than that of the interleaving eddy current array-based sensing film.It is also shown that the proposed sensing film can be applied to estimate the crack length and the crack depth.(2)A new eddy current sensing film composed of a parallelogram sensing coil array and a parallelogram exciting coil is proposed on the basis of the triangle sensing film,aiming to further reduce the number of the sensing coils but keep the same ability that the triangle film has.Two configurations of the parallelogram sensing film are compared on the effect of the crack angle detection.It is found that the parallelogram sensing coil with a strict step winding is better suited to provide crack angle information.Also,the ability of the proposed parallelogram sensing film to estimate the crack length and the crack depth are verified.(3)The investigation on the influence of temperature on the induced output voltage of the parallelogram sensing coil is carried out through experiments and simulations.The temperature ranges from 24℃ to 70℃ in the experiment and from-20℃ to 70℃in the simulation.The experimental and simulation results show that the induced voltage of the parallelogram sensing coil decreases linearly when the environmental temperature goes up.A temperature compensation method on the basis of least square regression is conducted and it is demonstrated the effectiveness to provide more accurate information about the crack length when the environment temperatures changes.(4)Considering the application background of composite materials serving in extreme environments,bolts made by composites have been developed to solve the thermal mismatch problem between metal bolts and composite joints.The concept of composite intelligent bolt based on embedded eddy current coil is proposed in this paper.The eddy current coil is embedded into the composite screw in the manufacturing process of composite bolt by technological means,aiming to monitor the hole-edge damage condition in composite materials.This paper mainly focuses on its feasibility to detect hole-edge damages in CFRP joints,and the performed experiments indicate that the proposed composite intelligent bolt has the ability to monitor the growth of the hole-edge crack as well as the compression damage. |
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