Research On The Nondestructive Testing Technology Of Voids In Carbon Fiber Reinforced Composite Laminates

Carbon fiber reinforced polymer composites have been widely used in the field of aerospace because of their excellent properties such as low density,light weight,high corrosion resistance,good stiffness and fatigue performance.In a series of processes such as laying,manufacturing,assembling,and servicing,the damage such as voids formed in the composite due to load-bearing and environmental changes will greatly affect the mechanical properties of the material,and even indirectly lead to the failure of the material structure,resulting in incalculable and disastrous consequences.In the field of aerospace,the quality inspection requirements of composite laminates used for actual load-bearing components are strict.The overall porosity of the material is required to be no more than 2%,the porosity of key structures is required to be less than 1%,and defects with a size of more than 6mm are not allowed.However,due to the anisotropy and structural diversity of composites,as well as the complexity of the formation and distribution of voids in CFRP materials,the efficient and accurate characterization and detection of voids in CFRP materials have always been the difficulties of the nondestructive testing of voids.Focusing on the two key issues of"Accurate porosity detection and rapid testing of large-scale defects in carbon fiber reinforced composite laminates",this paper deeply studies the formation and morphology of internal voids in CFRP materials,analyzes the influence of voids on ultrasonic signal propagation and attenuation,and proposes a new detection method for accurately predicting the overall porosity in composites,the rapid detection and location of large-size defects in large-scale complex structural composites are studied.The specific research contents and conclusions of this paper are as follows:Aiming at the formation and distribution of voids in CFRP composites,the changes of curing parameters affecting voids with the progress of the curing process are deduced based on the composite curing model.The nucleation critical size and growth critical size of voids in the curing process are deduced.Based on the theoretical research,the orthogonal experiments of preparing composite laminates were designed under different curing parameters,and 16 groups of quasi isotropic composite laminates were obtained;Through the statistical analysis of 31093 CT tomography results of the prepared laminates,the distribution law of the number of voids with different sizes is obtained,and the accuracy and consistency between the theoretical derivation and the actual detection results of void sizes and distribution are verified;Through the analysis of the orthogonal test results,it is found that the curing pressure has the greatest influence on the sample preparation,and the curing time has the greatest influence on the void content formed in the curing process.By controlling the curing process,the optimal porosity in the laminate is 0.92%.It lays a foundation for controlling the formation and growth of voids in the manufacturing process and studying the influence of voids on the detection signals in materials.Based on the studies of the distribution and morphology of voids in composites,combined with the theoretical study of ultrasonic propagation and attenuation in composites with voids,the effects of void size,material thickness,and porosity on ultrasonic attenuation in composite laminates are calculated and analyzed;The propagation and attenuation of ultrasonic in composites with different void contents are simulated by finite element analysis method;Through the ultrasonic attenuation experiment,the influence of voids on ultrasonic attenuation is verified,and the variation of ultrasonic transmission signal waveforms under different void contents and material thicknesses are analyzed to demonstrate the consistency between the calculation,analysis and the actual signal propagation law,which provides a scientific basis and lays a theoretical foundation for extracting the features of the ultrasonic signal and the correlation between voids and signal waveforms in the next part.Aiming at the accurate detection of the overall porosity of microvoids in composites,a new porosity detection method based on the artificial neural network,ultrasonic transmission method and X-ray tomography technology is designed and proposed.Firstly,some composite laminate samples with low porosity are prepared.3600 groups of ultrasonic propagation signals of different samples and detection positions are collected by the ultrasonic transmission method.Nine features of signal waveforms are designed and extracted as the input value of the neural network.The accurate porosity results of detection positions are obtained by X-ray tomography technology as the output layer of the neural network,The neural network is trained and verified using the experimental data.Through repeated training and correction,the structure and optimal values of various parameters of the three-layer BP neural network are determined,the most stable and accurate neural network model for predicting the porosity of composites is obtained.The accuracy of this method is97.22%,with an average accuracy of 86.76%.This method is not limited by the influence of voids,composite components and structures on the complex propagation process of ultrasonic signals.For materials prepared by the same process,only the transmission waveform result is required to accurately predict the material porosity.This method can be extended to the porosity detection of curved structures which are more difficult to explain the signal propagation mechanism,so as to realize the replacement of manual operation by machine learning method.This method provides a new choice for accurate and rapid porosity detection of carbon fiber composite laminates with complex shapes.Aiming at the large-scale composite structural parts in load-bearing and service state in practical application,the resolution and limitations of various nondestructive testing methods for composite defects are analyzed,and the accuracy and application range of ultrasonic impact resonance method for detecting large-size voids in composite materials are studied.The knock strength can be flexibly adjusted by handheld equipment,and the defects above 1mm in the laminates can be accurately measured on the spot without a coupling agent and additional environmental requirements;A large-scale composite defect detection and positioning platform is built,and a detection software which can locate and visualize the defect position and size in real-time is designed and developed.By controlling the mobile robot to drive the detection equipment,the defects more than 3mm inside the composite plate can be detected automatically,and the detection error does not exceed 10%;Through calculation and verification,the minimum curvature radii of materials that can be detected when the test direction is D_y and D_x are obtained;Through the manual detection of large bending structural parts of a certain model,the defects above 3mm in the laminates are accurately checked,which verifies the feasibility and accuracy of this method for complex shape structural parts,which can solve the field real-time defect detection problem that the traditional detection methods can not effectively and accurately complete due to the excessive size of materials and the limitation of field environments.