Damage Detection In CFRP Composite Plate Structure By Nonlinear Frequency Mixing Technique Of Guided Wave

Carbon fiber reinforced polymer composite(CFRP)materials are susceptible to damage from low velocity impacts in engineering applications.Generally,the energy carried by low velocity impacts is small,which can lead to micro delamination damage that may occur within the structure.Because it cannot be detected directly at the surface,the presence of delamination damage can severely degrade the mechanical properties of materials and pose a significant potential threat to structural safety.Thanks to the sensitivity of nonlinear Lamb waves to microscale damage,this provides new ideas for the detection and assessment of micro delamination damage in CFRP composites.This thesis investigates the nonlinear frequency mixing effect between Lamb waves and delamination damage in CFRP composites under A0-S0 combined mode.Firstly,this thesis derives the basic principle of Lamb wave propagation in thin plates of isotropic materials.The dispersion characteristics,displacement vectors,wave structure,stress distribution,and contact acoustic nonlinearity of Lamb waves in homogeneous materials are described.The resonance conditions of frequency mixing in different combined modes are derived based on the material nonlinearity.The contact behaviors in the normal and tangential directions between the delamination interfaces are explored,and the sources of nonlinear frequency mixing effects in the delamination detection are revealed.Next,an experimental system for Lamb wave nonlinear frequency mixing detection was built.Artificial delamination damage in CFRP composite plates was fabricated and the A0 mode actuator was optimized.The material parameters of the CFRP composite plate were used to calculate the dispersion curve in Matlab software,and the frequency selection was performed according to the resonance conditions and the swept image of the piezoelectric wafer.Experimental measurements were conducted based on the selected frequency pairs,and the mode correctness of the two primary Lamb waves was determined by comparison with the dispersion curves.The nonlinear frequency mixing effect between the primary Lamb wave and the artificial delamination damage is analyzed,and the nonlinear frequency mixing effect can be highlighted by the signal subtraction method.The results show that nonlinear frequency mixing between counter propagating Lamb waves can greatly isolate the interference of material nonlinearity.Then,a model for the detection of delamination damage in CFRP composite plates by nonlinear frequency mixing of Lamb wave was developed in the software ABAQUS.The delamination damage was fabricated by the node offset method,and the grid size,time increment steps,excitation methods of A0 and S0 modes,and settings of contact behavior in the model were determined.The intensity of the nonlinear frequency mixing effect in three cases,S0-S0 combined mode,A0-A0 combined mode and A0-S0 combined mode,was compared by the experimentally selected frequency pairs as the primary frequencies of nonlinear frequency mixing.It is also confirmed that the intensity of the nonlinear frequency mixing effect in the A0-S0 combined mode is better than the other two combined modes.On this basis,the nonlinear frequency mixing effect in the combined A0-S0 mode is explored,and the relationship between the resonant waves generated under the nonlinear interaction and the propagation distance in the combined A0-S0 mode is analyzed.The results demonstrate that the nonlinear parameters decrease monotonically with the increase of the propagation distance of the resonant waves,which provides theoretical support for the determination of the signal receiving point location in experiments and simulations.In addition,The effects of different delamination shapes on the nonlinear parameters are discussed,and the results show that the nonlinear parameter grows with expanding delamination size and diminishes with increasing delamination depth and interface gap.Finally,both simulations and experiments demonstrate that the technique based on the nonlinear frequency mixing of counter propagating A0-S0 mode Lamb waves provides a potential and effective method to detect defects in the composite plate with heterogeneous structures.