Nondestructive Testing Of Interface Damage Of FRP Strengthened Structure Based On Infrared Thermal Imaging With Laser Thermal Excitation

FRP composite material has the advantages of light weight,strong corrosion resistance,convenient construction,good heat resistance and durability.In recent years,seismic and reinforcement of structural columns,walls,beams,plates and panels in civil engineering Applications are becoming more widespread.However,due to the construction technology,the quality of the adhesive,the deterioration of the environmental conditions,etc.,the peeling damage often occurs in the interface of the FRP-reinforced concrete members,which is usually called the peeling damage layer.There are various damage detection methods for the peeling damage layer of FRP-reinforced concrete members during use.Among them,infrared thermal imaging detection technology is widely used because of its advantages of simple operation and wide detection range compared with other non-destructive testing techniques.However,infrared thermal imaging excited by traditional heat sources has high resource consumption,measurement accuracy decreases rapidly with the increase of heating distance,and it is impossible to detect the peeling damage of FRP reinforcement structure at a long distance(generally less than 2 m).In order to solve this problem,this paper proposes the use of linear laser thermal excitation to achieve long-distance,high-accuracy and low-power infrared thermal imaging to locate the peeling damage.Through the corresponding numerical analysis and experimental research,the feasibility and accuracy of the method for the location of the interface peeling damage based on the line laser thermal excitation plate are demonstrated.This paper introduces the law of infrared radiation and the principle of active infrared thermal imaging technology,and proposes the evaluation parameters of the results of infrared thermal imaging.Using the moving scanning linear laser heat source to locate the peeling damage of the FRP plate reinforcement member,the distribution formula of the surface temperature field of the strengthened specimen under laser thermal excitation is derived,and then the temperature difference equation between the non-destructive zone and the damaged zone of the specimen is obtained.The atmospheric absorption characteristics of laser waves in atmospheric propagation were analyzed,and the effects of different laser powers,FRP reinforcement layers on the surface damage of the strengthened specimens and the surface temperature difference of the non-destructive zone were studied.Based on the infrared thermal imaging method,the maximum detection distance of the specimen peeling damage can be determined by considering the principle of the surface thermal temperature of the infrared camera and the basic parameters of the minimum resolution.Numerical simulations were performed using COMSOL finite element analysis software.Firstly,the three-dimensional numerical analysis of the internal hole damage of the concrete slab of single material is carried out.(1)Verify the correctness of the finite element model of the single material with damage,and(2)Verify that the method and the traditional heat source excitation method are located in the superficial damage.The measurability provides a theoretical basis for the detection of FRP-reinforced concrete interface damage.Then the factors such as the size of the damage,the depth of damage,the distance between the heat source and the surface of the test piece were discussed.The results show that the method can identify the damage inside the concrete well.With the increase of the buried depth of the damage,the surface thermal spectrum of the concrete slab tends to be blurred.When the buried depth of the damage is less than 3 mm,the method can be clear.The position of the internal hole is distinguished;in the buried depth range of 4 mm-5 mm,the method is not suitable for detecting such depth.Similarly,compared with the traditional heat source excitation method,the method proposed in this paper can greatly improve the heating distance.For the identification of the same size and depth of damage,the line laser heat source has obvious advantages over the traditional heat source.The FRP-reinforced concrete slab model with peeling layer was established.The damage conditions of different sizes under different FRP layers were analyzed.The linear laser thermal excitation method and the traditional heat source excitation method were used to numerically simulate the heat source energy.After comparison,it is found that the linear laser heat source(15 W)consumes less energy than the conventional heat source(3000 W)at the same heating distance and time,and the accuracy of damage detection and positioning is higher than that of the conventional heat source;The increase of the surface heat spectrum of the specimen under the traditional heat source tends to be blurred,and the line laser thermal excitation can still clearly identify the damage location;The method can determine the position of the damage,and can recognize the long-distance damage and improve the detection accuracy compared with the conventional method.The experimental platform was built to produce FRP-reinforced concrete specimens.The method of processing experimental data was introduced.The surface temperature distribution of the specimens with different size and different reinforcement layers under different heat source excitation methods was investigated.The final surface thermogram of the experimental specimen was used to analyze the pros and cons of the two methods.The experimental results show that the traditional heat source of two high-energy halogen headlights(each 2500 W)basically loses the detectability of different size peeling damage under 1-3 layers of FRP-reinforced concrete slabs when the heating distance is more than 3 m,and 15 W The line laser heat source still maintains the detection capability of different size damage positions under the 1-3 layer reinforcement layer when the heating distance is 5 m.The experimental results obtained are basically consistent with the finite element results.The theoretical and experimental feasibility of the proposed line laser thermal excitation method is demonstrated.It shows that the method overcomes the shortage of the traditional method for the detection accuracy of the heating distance and the peeling damage while greatly reducing the energy consumption,and is more conducive to the inconvenience of engineering or even the close detection of the peeling damage of the FRP reinforcement structure.