Principles And Methods Of Damage Monitoring Of CFRP Laminates Based On FBG Sensing Technology

In recent years,there is a growing demand for new materials in various industries.Carbon Fiber Reinforced Plastic(CFRP),featuring “light weight and high strength”,is becoming an important supporting force for the revolution of new materials in China.As the most cost-effective carbon fiber composite structural component,CFRP laminates can not only be used as load bearing part of a structure,but also form a bonded joint with adhesive in composite materials,which are widely used in many fields of our daily life.However,due to defects in workmanship,improper transportation and other causes,CFRP laminates often have some hidden primary damages that are commonly known as barely visible damages,and these damages will get worse during use,which may possibly increase the risk or consequences of major accidents.Therefore,it is of great significance to detect the location of primary damages in the CFRP laminates,and to identify its damage and propagation process.Compared with other sensing methods,the fiber Bragg grating(FBG)sensor provides many unique advantages of compact size and easy formation of sensing networks,which is most suitable for location and identification of CFRP laminates damages.In this dissertation,the CFRP laminates subjected to tensile load were taken as the research object,and the relationship between strain distribution and damage location and propagation was analyzed in detail,so as to obtain the damage location and identification methods of CFRP laminates based on strain information.In addition,the damage monitoring system of CFRP laminated plates was constructed with the distributed FBG sensor as a key component,and the damage location and identification tests of CFRP laminates were conducted according to FBG sensing technology.The main research contents of this dissertation are as follows:Firstly,based on the mechanics theory of composite materials and the theory of damage mechanics,the material failure criterion and performance degradation scheme corresponding to the inner unit and inter-layer unit of CFRP laminates were determined,and the finite element simulation analysis of CFRP laminates under tensile load was carried out by using the ABAQUS software.Secondly,the CFRP laminates with different prefabricated damage locations were simulated and analyzed in ABAQUS software,so that the strain distribution information corresponding to different damage locations was obtained.The damage location and strain information were input into the BP neural network to obtain the damage location neural network model of CFRP laminates.Combined with the created model,the principle of damage location of CFRP laminates based on strain information was explicitly explained in the dissertation;the strain information of inter-laminar between the first and second layers of CFRP laminates under different tensile displacements was extracted from the ABAQUS results file.The mapping relationship between the change of inter-laminar strain distribution and the damage propagation evolution of CFRP laminates was obtained through detailed analysis.With the combination of the mapping relationship,it systematically expounded the principle of damage identification of CFRP laminates based on strain information.Finally,the damage monitoring system of CFRP laminated plates based on FBG sensing technology was built,and the principle of damage location and identification was given a detailed explanation;the test specimen of CFRP laminated plate was prepared,and a distributed FBG sensor was attached to its surface in order to perform a damage location test of CFRP laminated plate based on FBG sensor.The maximum error of the actual output value and the expected output value was not more than 3.43 mm,verifying the accuracy of damage location method of CFRP laminates.Therefore,the location of primary damage of CFRP laminates can be achieved;the test specimen of CFRP laminate with embedded FBG sensor in the inter-laminar was prepared,which was designed to conduct a damage identification test based on FBG sensor.The results of experiment and simulation tests were basically consistent,with the maximum error less than 16%,which verified the accuracy of damage identification method of CFRP laminated plates.Therefore,the identification of damage and propagation process of CFRP laminates can be successfully realized.