Study On Response Characteristics Of Air-Coupled Impact-Echo In Curved Panel

With the continuous development of China's transportation industry,tunnel engineering is widely used.Due to design defects,construction quality,load effect and aging of the structure itself,tunnel structure inevitably produces different degrees of defects and damages in its life cycle.it is very important to detect rapidly and accurately the damage of tunnel to reduce the maintenance cost of the tunnel,improve the durability of the lining structure and reliably ensure the normal operation function of the tunnel.In this paper,from the point of view of air-solid coupling,using air as the coupling medium of stress wave propagation between the lining structure and the detection system,the rapid detection and evaluation of tunnel lining internal damage based on air-coupled impact-echo is studied.Through numerical simulation and experimental analysis,curvature,hammer diameter,hammer point location and defect depth are comprehensively analyzed.The main research results of this paper are as follows:(1)The applicability of air-coupled impact-echo method for curved plate structure is studied by numerical simulation.According to different curvature radius of curved plate structure,a series of finite element models are established to analyze the response characteristics of impact-echo.It is found that when the ratio of curvature diameter to thickness of the curved plate is more than 20,the local impact-echo modes at the impact point are basically the same as those of the flat plate structure,while the curvature is not a negligible factor when the curvature radius is small,which has a great influence on the impact-echo frequency.(2)By means of numerical simulation and experiment,the diameter of the exciting ball,the position of the knocking point,the distance between the sensor and the knocking point,and various defects at different depths in the damage detection of curved surface plate were compared and analyzed.The bigger the diameter of steel ball,the lower the frequency of incident wave and the slower the attenuation.However,the faster the attenuation of stress wave excited by small diameter steel ball,the shallower the defect can only be detected.The relative position of impact point and defect has less influence on the peak frequency of response signal.The leaky wave attenuates rapidly in the air,so the closer the microphone is to the tapping point,the stronger the signal energy received,which makes the better detection effect.For the defects of different depths,the main frequency of impact-echo of deep defect is in good agreement with the theoretical value,while that of shallow defect echo is mainly bending mode response with low frequency.(3)The main frequency of air-coupled impact-echo often shows a certain randomness because of the randomness of the impact intensity and position.The traditional damage imaging method based on the main frequency of impact-echo often produces a large error due to the fluctuation of the peak frequency of the signal.This paper proposes a solution to this problem.An improved method of image judgment index is proposed.Considering the frequency fluctuation range of the defect-free part,a band-pass filter is introduced and a new judgment index is finally formed according to the ratio of peak frequency energy.Then,the damage imaging of a planar plate and a standard metro tunnel surface lining is validated.It is found that the method is more accurate to identify the scope of defect.