Identification Of Double Delaminations Of Fiber Reinforced Polymer Beams Based On Vibration Frequency And Curvature

Fibre Reinforced Polymer(FRP)is widely used in aerospace,automotive,sports and civil engineering due to its advantages of light weight,high strength,corrosion resistance and ease of molding.For FRP laminated structures,the bonding force between layers is mainly determined by the strength of resin matrix,so the strength along the thickness direction is very low,which makes delamination damage become the main failure mode of FRP materials in practical engineering applications.The occurrence of delamination damage will greatly weaken the mechanical properties and service life of the structure,and may even cause the failure of the whole structure,leading to serious consequences.Therefore,it is of great significance to find a reliable,accurate and efficient method for damage detection and identification of FRP materials.For the problem of FRP layered damage identification,the existing literature mainly focuses on single-layer damage identification,while the research on double/multi-layer damage identification is less.Considering that the natural frequencies are easy to measure and have good reliability and reproducibility,and the curvature of the mode is very sensitive to the local changes of the structure,etc.In this paper,FRP laminated beams with double delamination damage are studied.Frequency-based and curvature-based methods are used to identify double delamination damage.By constructing the theoretical model of FRP laminated beams with double delamination damage,the corresponding damage parameters can be predicted by using genetic algorithm(GA)with the frequency variation of each order corresponding to the damage parameters as the identification index.The finite element model of FRP laminated beams with double delamination damage is constructed to obtain all order pure bending mode data.The mode curvature is transformed into mode curvature by central difference method for damage identification.The validity of the two methods is verified by numerical and experimental verification.Compared with single-layer identification algorithm and double-layer identification algorithm,the results show that the double-layer identification method is more universal and can accurately identify single/double-layer damage,while the single-layer identification algorithm is not effective in predicting double-layer damage.The double-layer identification algorithm is validated by numerical and experimental cases.The average error of each damage parameter in the prediction theory case is within ±12.00%,while that in the actual case is about ±5.00%.Therefore,the double-layer identification algorithm can effectively identify single/double-layer damage.Based on curvature mode,three damage identification indexes(absolute difference of curvature mode,instant damage factor and difference of adjacent points of curvature mode change)are proposed to identify double-layer damage of FRP laminated beams.The numerical results show that the three damage identification indicators can effectively identify double-layer damage,especially the difference between adjacent points of curvature mode change value is the best.There are still many clutters in the measured curvature modal data after filtering.It is difficult to distinguish whether the clutter is caused by damage or disturbed by the measurement process,so it is difficult to identify the damage by the measured data.To sum up,the natural frequency should be used as the identification index when the layered damage is identified based on the variation of vibration parameters.This index can identify both single and double layered damage in FRP materials.