Impact Damage Detection In Patch-Repaired CFRP Laminates Based On Ultrasonic Nonlinear Response

Carbon fiber-reinforced polymer(CFRP)laminates are widely used in aerospace and other high-tech manufacturing industries due to their high specific strength,relative thermal stability,and low weight.CFRP laminates inevitably generate barely visible impact damage(BVID)during production and application,which lead to a great potential threat to the safety of aircraft structures.Therefore,it is necessary to detect and repair the damaged structures.External patches have been extensively applied in industrial production taking advantage of their convenience,economy and feasibility.However,conventional nondestructive detection methods cannot meet the demand of industrial production because of coarse grain size,serious frequency dispersion phenomenon and complicated micro-damage in anisotropic CFRP laminates.Recent studies have shown that nonlinear ultrasonic technology has the advantages of small attenuation,long propagation distance and high detection efficiency in damage propagation of composite materials.The structural health monitoring(SHM)method based on nonlinear Lamb waves is currently considered to be a promising method for use in the aviation field.In this work,the effects of low-speed impact(LVI)resistance in CFRP laminates repaired with patches of various design parameters is discussed via numerical and experimental methods based on nonlinear ultrasonic responses technology.And the interaction between LVI damage and nonlinear ultrasonic propagation is investigated.According to the different methods of frequency excitation,two kinds of integrated finite element models that combines LVI with nonlinear Lamb wave detection were developed to predict the nonlinear Lamb wave behavior in LVI-damaged patch-repaired CFRP laminates and patch repair effect.The main works of this paper are as follows.1.Based on the correlation theory of nonlinear ultrasonic Lamb wave,and the nonlinear ultrasonic dynamic equation in anisotropic media is derived.Then,the dispersion equation is obtained based on the propagation characteristics of Lamb waves in multi-layer composite plates.And the phase velocity and group velocity dispersion curves are calculated to further determine the excitation frequencies of nonlinear ultrasonic Lamb wave higher harmonics and mixing frequencies modes in the experimental and finite element simulations.The analytical solution of the wave equation is calculated by the perturbation method.The relative acoustic nonlinearity parameters(RANPs)for single frequency and mixed frequency excitation were introduced to evaluate the impact damage of adhesively bonded repair CFRP laminates,which provides a theoretical foundation for the development of nonlinear ultrasonic Lamb wave detection technology in composite plates.2.The three main time-frequency analysis methods of short-time Fourier transform,Hilbert-Huang transform and wavelet transform are comparatively investigated.The results show that the wavelet transform is more superior in the extraction and analysis the nonlinear ultrasonic second harmonic,third harmonic and mixing-frequency wave signals of LVI-damaged patch-repaired CFRP laminates.Wavelet decomposition technology can effectively filter the noise in non-stationary signals and completely reflect the response signal in the detection process.3.In the past,when studying the propagation of nonlinear Lamb wave in impact damage of composite materials,only the impact damage behavior of composite or ultrasonic Lamb wave response in materials with artificially preset damage was simulated.And this finite element modeling approach did not accurately reflect the interaction between nonlinear Lamb wave propagation characteristics and impact damage,resulting in deviations of the results.Therefore,aiming at a systematic integrated evaluation procedure that combines LVI with nonlinear ultrasonic high-order harmonic response is proposed,the LVI process of CFRP laminates repaired with different patch design parameters and the propagation characteristics based on nonlinear Lamb wave detection technology are comprehensively studied.The system includes,firstly,the finite element model of nonlinear Lamb wave propagation in LVI-damaged patch-repaired CFRP laminates is established.Then,the BVID of CFRP laminate repaired with patches of various sizes is evaluated based on second RANP and third RANP.Finally,the reliability of the finite element model is verified by drop-weight impact tests and RAM-5000 SNAP nonlinear ultrasonic high-order harmonic detection system.Based on the finite element model,the nonlinear Lamb wave high-order harmonic responses of patch shapes,patch locations,patch thicknesses and stacking sequences in the LVI-damaged single-sided patch-repaired structure are discussed.4.In order to effectively reduce the interference of nonlinearity from the test system,a nonlinear Lamb wave collinear mixing detection technique is proposed to investigate the BVID caused by LVI in single-side patch-repaired CFRP laminates using experimental and numerical methods.Firstly,a mixing detection FE model of LVI-damaged patch-repaired CFRP laminates is established to predict the propagation response of nonlinear Lamb wave in LVI-damaged CFRP laminates that were repaired with various patch design parameters.The numerical procedure is verified by drop-weight impact tests and RAM-5000 SNAP ultrasonic mixing detection system.The BVID of patch-repaired specimens in nonlinear Lamb wave-mixing detection is evaluated based on mixing relative acoustic nonlinearity parameters(MRANPs).Then,the patch-repair effects of CFRP laminates containing holes that were repaired with different sizes,numbers of layers and stacking sequences are analyzed based on numerical simulation methods.5.Due to the different RANPs for impact damage of adhesively bonded structures repaired with various patch design parameters,their existence of multiple variables,and the correlation of these design parameters on the impact performance is highly complicated.Thus,it requires a systematic optimization strategy to identify the optimum patch design parameters.In this study,a mathematical surrogate model was constructed by the Latin hypercube sampling(LHS)and response surface method(RSM)with second RANP,third RANP,sum-frequency RANP and difference-frequency RANP as the response value,while the different patch radius,patch thickness and patch rotation angle as independent variables.And the reliability of the model is verified by analysis of variance(ANOVA)and significance test,relative error value statistics analysis,normal probability distribution of residuals,three-dimensional response surface plot and its contour plot and so on.The multi-island genetic algorithm(MIGA)in Isight software was used to multi-objective optimization the surrogate model,and a group of optimal patch processing parameters are obtained,which makes the patch repaired CFRP laminate have strong impact resistance and good repair effect,and provides a theoretical reference for the further study of nonlinear ultrasonic Lamb wave propagation in impact damaged structures that were repaired with different patch design parameters.The research work of this paper explores the correlation between multiple typical patch parameters and detection response signals to evaluate the impact resistance and patch repair effect in LVI damage of patch-repaired CFRP laminates by nonlinear Lamb wave detection technology.A collinear mixing technique is proposed to eliminate system interference,and the method of solving the optimal patch parameter combination by multi-objective optimization is established,which provides a necessary theoretical basis for accurately monitoring the structural health of composite materials in the future.