Research On Low-power Vibrothermography For Detecting Kissing-bond Defects Of FRP Bonded Structures

Fiber Reinforced Polymer(FRP)is widely used in automobile industry,aerospace,civil engineering and other engineering fields due to its high strength,low density,corrosion resistance,fatigue resistance and other excellent material properties.It is widely used in engineering to bond FRP materials to form engineering structures.However,due to the method and technology of bonding or the quality of adhesive,various internal defects often occur at the bonding area.Kissing-bond defects are difficult to be detected by conventional nondestructive testing techniques such as ultrasound C-scan because of their surfaces close contact with each other and lack of significant air gaps.Therefore,it is of great significance to develop the nondestructive testing technology for the kissing-bonding defects of FRP bonded structures to ensure the safe service of the structures.As a new nondestructive testing technology,low-power virbrothermography provides a new idea for the detection of FRP bonded defects due to its advantages of high detection efficiency and intuitive detection results.In this paper,the kissingbond defects of FRP bonding structure are taken as the object,and the key problems related to the detection technology of low-power vibrothermography are studied.Firstly,based on theoretical analysis,the finite element model of the whole process of detecting kissing-bond defects by low-power vibrothermography was established.The evolution law of physical field was obtained by numerical simulation,and the detection mechanism was revealed.The results shown that the frictional and viscoelastic effect were the reasons for the heat generation of the kissing-bond defects in the FRP structure under ultrasonic excitation,and the frictional heat played a dominant role.Under excitation,the FRP bonding structure could generate sufficient heat at the position where the defect was closely bonded and diffuse to the surface of the specimen,which verified the feasibility of this technique for the detection of such defects.FRP bonded structures with kissing-bond defects made of carbon fiber reinforced plastic(CFRP)and glass fiber reinforced plastic(GFRP)were taken as the object,and influencing factors were analyzed based on the above numerical model.The results of finite element simulation shown that the maximum amount of heat could be generated at the defect when the excitation amplitude was larger(150V)and the two ends of the specimen were fixed.Finally,the thermal generation of CFRP-CFRP specimens with different sizes of kissing-bond defects was simulated by using the optimal parameters.It was found that when the size of defects was5mm?5mm,the thermal generation of defects was less,which indicated that this technology had a certain size limitation in the detection of such defects.The specimens with different types of FRP bonded structures with kissing-bond defects were made to carry out low-power vibrothermography experiments.Firstly,the effectiveness of the low-power vibrothermography was further proved based on the experimental results.Then,the influence mechanism of different factors on the detection process was analyzed.Finally,the low-power infrared thermal imaging detection technology proposed in this study was compared with other infrared thermal imaging detection technologies.The experimental results shown that this technology could detect such defects efficiently and intuitively,and the experimental results such as temperature distribution were in good agreement with the finite element simulation results.Meanwhile,the proposed low-power vibrothermography detection technology had obvious detection advantages over other technologies.A YOLOv3-KB defect identification method was established based on YOLOv3 algorithm,which was suitable for the detection of kissing-bond defects in FRP bonded structures.By comparing the identification results under different training epochs and the selection of optimizer,the identification accuracy and positioning accuracy of the kissing-bond defects with different features on the infrared thermograms were evaluated,and the optimal detection model was obtained.Further detection of the video files provided guidance for the realization of realtime detection of such defects.