Study On Detection Method Based On Phononic Crystals Theory For Delamination Defect In FRP Laminates

As a new type of material possessing characteristics of high specific strength,large specific modulus,good durability,excellent weather resistance and low density,fiber reinforced polymer（FRP）laminates have been widely used in manufacture of wind turbine blade spar.The manufacturing lengths of blades have exceeded 100m.Such ultra-long blades exhibit heavy weight,low stiffness and complex structure.They chronically operates under complex wind and wave load conditions,and are affected by severe environment such as freezing,rain erosion,etc.Delamination defect is an interlayer defect arising from the interface between two adjacent laminates,being the leading type of defect in FRP laminates,delamination defects are the primary problem to be solved in defect detection of FRP laminates.This kind of defects can cause the structural strength and stiffness of laminates to decrease greatly.Even with small alternating load energy,the defects can reduce the load carrying capacity of FRP laminates by more than 50%.Timely discovery of delamination defects in the spar can reduce catastrophic loss and damage of equipment caused by blade failure,and ensure that the quality and performance of the blade structure meet the requirement of 20-25 years of operation.Therefore,studying on detection method for delamination defects in FRP laminates used in wind turbine blade,and realizing fast and accurate detection are of great theoretical significance and huge social and economic value for avoidance of major accidents with safe and reliable operation of blades and wind turbines.This thesis is based on one National Natural Science Foundation of China（Project No.:U1809219）and two Major Scientific Research Projects of Zhejiang Province（Project No.:2022C01178,2023C01123）.Based on phononic crystals theory,the delamination detection method for FRP laminates was studied,focusing on the key problem of fast and accurate defect detection.Phononic crystals theory,periodic method of FRP laminates,spectral element analysis method,delamination defect characteristic analysis and delamination defect detection method were studied.The main research contents and innovative works include the following five aspects:（1）.The basic theroy of Bragg scattering phononic crystals,lattice theory and Bloch theorem were analyzed.The theory of delamination defect detection of Bragg scattering phononic crystals was studied,the wave equation of infinite periodic structures and the dynamics equation of finite periodic structures were established,and the dispersion characteristic of infinitely periodic structure and the frequency response characteristic of finitely periodic structure were analyzed.On this basis,the dynamic characteristics of delamination defective elements were analyzed,and the equivalent stiffness of delamination defective elements was constructed.Coupled with the dynamic characteristics of other structural elements,a phononic crystals beam multi-particle spring oscillator model with delamination defective structures was constructed.Also,the dynamic equation of Bragg scattering periodic structure with delamination defects and the mass matrix and stiffness matrix of periodic structure with delamination defects were established.The theory basis of defect detection based on Bragg scattering phononic crystals was realized.（2）.The construction method of Bragg scattering phononic crystals was studied,a method of concentrated mass-Bragg scattering phononic crystals construction was proposed,the influencing factors such as concentrated mass form,mass ratio,quantity of concentrated mass and beam thickness were analyzed,and the validity of the construction method was verified.Based on above works,the Bragg scattering periodic structure with delamination defects for FRP laminates was constructed and the defect modes characteristics were analyzed.The analysis results showed that the defect modes frequency could effectively reflect the delamination defect information,and defect modes frequency is connected with the length,location and depth of delamination defect.The detection of frequency response curve and defect modes frequency of delamination defect structures were realized.（3）.The theory of spectral element model was analyzed,and the modeling method of periodic structure of FRP laminates with delamination defects was studied.A method for spectral element modeling of delamination defect structure in FRP laminates was proposed,and the spectral element model and stiffness matrix of FRP laminates with delamination defects were established,which can be used to analyze defect modes characteristics of delamination defects at different lengths,locations and depths.Compared with traditional finite element analysis method,the calculation speed was greatly improved under the premise of consistent calculation accuracy,which was conducive for acquisition of a large number of calculation data of the defect modes frequency of FRP laminates periodic structure and data statistical analysis.The experimental system was built and experiments were carried out.The experimental results proved that the modeling simulation results were consistent with the experimental test data,and the defect frequency error was 4.33%,which verified the accuracy of the modeling method and supported the establishment of hierarchical defect characteristic indexes,quantitative evaluation models and defect detection methods.（4）.The characteristics of defect modes related with the length,location and depth of delamination defects were studied.The excitation method on two sides of the periodic structure of FRP laminates was proposed.It was found that there were significant differences in the frequencies of defect modes corresponding to the vibration propagations generated by excitations on the left and right side of the periodic structure of FRP laminates with delamination defects.The algebraic addition of the frequencies of the defect modes induced by excitations at the two ends of the beam,and the length of the delamination defects exhibited an approximately linear relationship.The index DL_length were proposed to characterize length of the delamination defects;The algebraic substraction of the frequencies of the defect modes induced by excitations at the two ends of the beam,and the length of the delamination defects exhibited an approximately linear relationship.The index DL_location were proposed to characterize location of the delamination defects;It was also found that the relationship between the depth of the delamination defect and the algebraic product of the frequencies of the defect modes generated by excitations at the two ends of the beam was close to a quadratic curve.The index DL_depth was also proposed to characterize the delamination defects depth.By establishing the three characteristic indexes,the quantitative characterization of delamination defects was realized.（5）.The defect characteristic indexes for different orders of band gap of the periodic structure of FRP laminates were studied and quantitative evaluation model was established.The addition of defect modes（ADM）method for length detection,the substraction of defect modes based on ADM（SDMBA）method for location detection,and the multiplication of defect modes based on SDMBA（MDMBS）method for depth detection for delamination defects in FRP laminates were proposed.The influence factors for the method,the detection errors,and the disturbing effect from local resonance were analyzed.The results showed that the detection methods are of good detection accuracy and stability.The experimental system was built to carry out the test,and the results showed that the error of delamination defect length was 1.8%,the error of delamination defect depth was -4.8%,and the delamination defect location was accurate.The modeling simulation and experimental results proved that the detection method can efficiently and accurately detect the delamination defect length,location and depth.In addition,ADM,SDMBA and MDMBS methods can provide useful reference for delamination defect in metallic composites.