The Impact Responses And Multiscale Structure Damage Mechanisms Of Three-dimensional Angle-interlock Woven Composites

Three-dimensional woven composites(3-DWCs)with the reinforcing yarns in the thickness direction have good properties such as high specific strength,specific stiffness,interlaminar shear resistance and impact damage tolerance,etc.They can avoid the disadvantages of traditional laminate composites,such as easily being delaminated.They are widely applied in aerospace,wind turbine blades,ship manufacture,automotive industry and other fields.3-DWCs are inevitably subjected to impact loading during manufacture and service,and their dynamic damage mechanisms and damage modes are significantly different from those of laminate composites.The dynamic mechanical properties of composite materials are different from their static mechanical properties.It is of great significance to study the mechanical properties of 3-DWCs under impact loading for the design and application of impact-resistant composites.This thesis aims at the impact responses of 3-DWCs in the drop weight and the split Hopkinson pressure bar(SHPB)tests.With the assistance of meso-structure and continuum structure finite element models,the woven structure effects of 3-DWCs on strain distribution,crack distribution,interfacial damage distribution and stress propagation are investigated.The multi-scale structure impact deformation and damage mechanism of 3-DWCs are discovered.The main contents of this thesis are as follows,(1)Three impact energies 3 J,13 J and 23 J are selected for the drop weight impact tests,and three damage modes including barely visible impact damage,visible impact damage and clearly visible impact damage are obtained.The digital image correlation technology and highspeed camera are used to record the process of deformation and damage propagation on the composite surface.A damage tracking algorithm is proposed to investigate the woven structure effect on surface damage.The composite mechanical properties are analyzed in terms of impact responses and surface damage morphologies.The woven structure effect on internal damage is explored with micro-computed tomography(Micro-CT)images.An image segmentation algorithm is proposed to extract composite boundaries and cracks.(2)A meso-structure finite element model of 3-DWCs with one quarter of full size is established.The ductile damage criterion and the shear damage criterion are used to define the composite damage initiation.The specified damage evolution laws are grounded on the energy dissipated during the damage process.A surface-based zero-thickness cohesive zone model is employed to define the interface properties.The finite element results are compared to the experimental results to investigate the woven structure effect on the surface strain distribution,internal crack distribution,stress propagation on yarns and interfacial damage distribution.(3)Three impact pressures of 0.2 MPa,0.3 MPa and 0.4 MPa are selected for the SHPB impact tests.The damage variable curves are obtained from CT images to analyze the composite internal damage degree.A meso-structure finite element model under SHPB tests is established to explore the woven structure effect on composites in terms of internal crack distribution,stress propagation and interfacial damage distribution.(4)Based on the periodic boundary conditions and master-slave node techniques,the multiscale finite element method is applied to predict mechanical properties of the fiber tow,the mesoscale composite and the macro-scale composite.The impact responses of the continuum structure finite element model of 3DAWCs are investigated with the vectorized user-defined material(VUMAT)subroutines written in Fortran language.The predicted results of the Hashin criterion,the Hashin-Shokrieh criterion and the maximum stress criterion in macroscopic damage of composites under SHPB impact tests are compared with experimental results,and the woven structure effects on crack distribution and stress propagation are investigated.(5)From thousands of CT scans,a small number of images with distinctive features from each other are selected and manually marked to distinguish warp and weft yarns.A deep learningbased U-Net model is trained for segmenting warp and weft yarns in the remaining CT images.A new automatic crack classification algorithm including a series of morphological operations is proposed to classify the overall cracks in images from the U-Net model predictions.The crack classification results are statistically analyzed.The watershed algorithm is used to get individual yarns and the crack proportions in them are obtained.The global and local distribution patterns of yarns and cracks are obtained with the help of 3D reconstruction technique from the U-Net model segmentation results and crack classification results.The main conclusions of this thesis are as follows,(1)(1)In the case of 3 J,the surface damage appears at 0.80 ms and increases rapidly,and the height of the damage(along the weft direction)is greater than the width of the damage(along the warp direction).After 5.00 ms,the damage height,damage width and damage area gradually reach stable values.The straight arrangement of weft yarns facilitates damage propagation,and the curved warp yarns prevent the damage propagation.The damage height is always larger than the damage width throughout the damage evolution.(2)For the composites under 13 J and 23 J,the cracks propagate mainly along the warp yarn on the sample front side and along the weft yarn on the back side.From the cross-sections of warp yarns,the crack distribution shows the ’ △ ’shape,and from the cross-sections of weft yarns,the crack distribution is in the shape of ’ ▽ ’,and the overall cracks show the ’ + ’ shape.(2)(1)For the composites under 3 J,the transverse tensile strain in the composite center continuously increases,and the contraction strain on both sides of the composites keeps increasing.The strain distribution in the impact center along the warp direction is such that,the strain near the center remains positive,while it is negative away from the center.The strain along the weft direction remains positive.(2)For the composites under 23 J,from the cross-section of the weft yarns,the weft yarns are broken at the impact center but still maintain a relatively intact shape.There is severe resin shedding at the center of the bottom of composites from the crosssection of warp yarns,and the interfacial damage range is far beyond that from the weft crosssectional view.(3)For the composites under 23 J,the value and distribution range of stress in the weft yarn direction is larger than that in the warp direction,which is due to the high strength and straight alignment of weft yarns.(4)In the impact model under 23 J,during 0～0.30 ms,the interfacial cracks appear in the finite element model,and the cohesive damage is mainly concentrated at the bending positions of warp yarns.During 1.20～3.95 ms,with the breakage of warp yarns,the interfacial damage on the weft yarns keeps increasing and the interfacial damage increase in different magnitudes.(3)At the impact pressure of 0.2 MPa,(1)as the distance between the slice images and the impact center decreases,the damage variables obtained from the cross-section of the warp yarn increase rapidly,while changes in the damage variables obtained from the weft cross-section are relatively gentle.(2)From the warp yarn cross-section,the degree of yarn crushing,the degree of interfacial damage and its distribution range at the impact center are obviously more serious than those in the weft yarn cross-sectional view.It indicates that the cracks tend to propagate along the straight-arranged weft yarns.(3)The stress propagation on weft yarns is more uniform and extensive than that on warp yarns.(4)The volume fraction curve of cohesion damage shows a ’ stepped ’ shape,and the interfacial damage distribution on the warp yarn is concentrated at the impact center,while a wider distribution of interfacial damage appears on the weft yarn.(4)(1)There is little difference in the mechanical parameters of fiber tows obtained by the mixed law method and the periodic boundary condition method.The accuracy of the finite element method has been verified and affirmed.The mechanics properties of meso-structure composites are predicted through the axial tension and in-plane shear stress distributions of inner representative unit cells(RUCs)and surface RUCs.And the macro-structure mechanics parameters of three-dimensional angle interlock woven composites are further predicted.(2)For the Hashin Shokrieh criterion,there are the smallest load deviation and the more similar damage morphologies to the experimental results,and the main crack extends along the weft direction.The values and distribution of stresses along the weft direction are larger than those in the warp direction for both the front and back sides of the composites under the Hashin-Shokrieh criterion.There is a high feasibility and validity of the Hashin-Shokrieh criterion for woven composite damage analysis.(5)(1)The trained U-Net model can recognize the warp and weft in CT images of composites under different impact pressures,with an accuracy of more than 90%,and the model has a good generality.(2)The accuracy of the automatic crack classification algorithm exceeds88%.The classification efficiency is thousands of times higher than the manual extraction of cracks.The proportion of interfacial cracks in composites in this study reaches 8%,which is much higher than that of other types of cracks.Enhancing interfacial adhesion is of great significance to improve the impact resistance of composites.(3)The statistical results of the watershed algorithm show that directly below the incident bar,the maximum crack volume fraction of an individual weft yarn is up to 22.8%,and the maximum crack volume fraction of an individual warp yarn is up to 4.9%.(4)The method proposed in this thesis can be used for crack quantification analysis of composites with similar woven structures.In this thesis,the drop weight impact and the SHPB impact responses of 3-DWCs are investigated.The woven structure effects on stress distribution,strain distribution,crack distribution and interfacial damage distribution of woven composites are analyzed by theoretical methods and numerical simulation methods.With the help of watershed algorithm and automatic crack classification algorithm based on deep learning,the classification and statistical analysis of cracks are achieved.We hope this work can provide theoretical basis for 3-D woven structure composites in dynamic loading,structure design and engineering applications.