Research On Modal Displacement Irregularity Coefficient Method For Laminate Damage Detectio

The composite laminates with the advantages of high specific strength,corrosion resistance and good fatigue resistance are widely applied in aerospace,automobile,wind power and other fields.The intra-laminar damage and delamination of the laminates are prone to occur because of the special working environment,resulting in a significant reduction in strength and stiffness.The Non-Destructive Testing（NDT）can effectively detect the damage,and the modal analysis is often used to identify these damages because of its simplicity,flexibility and economy.But it is difficult to accurately determine the location and degree of the damage using the initial modal parameters.To this end,build a new modal characteristics detection index with higher accuracy has great theoretical significance and engineering value.The main research work and results are as follows:（1）Based on the APDL language of ANSYS,the Shell181 element is used to establish the simply supported laminate model,and the fiber in the layer is invalidated by the Camanho damage criterion.The natural frequency of the laminate and the modal displacement of the mesh nodes are obtained by modal analysis,and the Gaussian curvature of the curved surface is obtained by the difference method.The variation of the Gaussian curvature difference with the location and degree of the damage were studied.The square of the Gaussian curvature difference was used to increase the gap between the peak and other regions,and was defined as the unevenness coefficient of modal displacement to detect the location and degree of intra-laminar damage.The results show that the modal displacement unevenness coefficient improves the accuracy of damage location detection,and it increases with the increase of the damage degree.（2）The 4-layer Poly Tetra Fluoro Ethylene film of size 0.02 m×0.11 m is embedded respectively between adjacent carbon fiber prepregs being 0.115 m and 0.06m away from the left edge to prepare the laminates with and without damage.The force-hammer excitation experiment is carried out on the prepared laminates with fixed supports at both ends,and the frequency response curve of each measuring point is processed by the Polylscf algorithm in Dong-Hua Data Analysis System.The finite element model of delaminated laminates is established through adding new layers with film properties,and the modal displacements of nodes are compared with experimental data.The number of measuring points in the experiment is limited,making the precise characterization of surface abrupt characteristics difficult.The numerical results are used to calculate the Gaussian curvature difference between the modal shapes of intact and damaged laminates.The variation of the modal displacement unevenness coefficient with the location and degree of the damage were studied.（3）The Shell181 element is used to establish the laminate models with the fiber angles of[0°]_16s,[90°]_16s,and[0°/90°/0°/90°/0°/90°/0°/90°]_2s,respectively.The Camanho damage criterion is used to construct the matrix compression failure on multiple locations,and the natural frequency and mesh node displacement are obtained by modal analysis.The intact laminates with three fiber angles were prepared using the prepreg,and the force hammer excitation experiment was carried out.The modal displacement of each measuring point was obtained by Dong-Hua Data Analysis System,and compared with the numerical results.Compared with the experiment,the simulation can provide enough measuring points to accurately characterize the abrupt characteristics of the surface.Therefore,the Gaussian curvature of the numerical modal shape is calculated by the difference method,and the variation of the square of the Gaussian curvature difference of the single-order and multi-order with the location and number of the damage is studied.