| Stitched Carbon fiber composite laminates adopt the stitching process,which improves the disadvantage of weak inter-laminar performance of traditional composites by inserting stitches in the thickness direction and improves the damage tolerance of the material.The damage tolerance of stitched composites is critical for flight safety as the strength and stiffness of stitched composites decrease due to low-speed impacts.In this work,the experimental and numerical simulation work on the out-of-plane central impact,edge impact,and compression after impact properties of stitched carbon fiber composite laminates are carried out with different conditions of impact on the stitched laminates as the research object,and the main contents of the whole work are as follows:(1)Based on the progressive damage theory,a low-velocity impact finite element model of composite laminate was established.The constitutive relationship and progressive damage theory of composite laminate were described,and the application of different criteria was discussed in detail.The 3D Hashin and Hou criterion were adopted as the initial criterion for intralaminar damage,the linear reduction was chosen as the stiffness degradation solution,and the characteristic length was imported into the degradation solution to decrease the dependence on the mesh size,and a low-velocity impact finite element model of the composite material was established.The correctness of the finite element model and the subroutine were verified by the experimental results in the literature,which laid the foundation for the low-velocity impact damage analysis of the stitched laminate.(2)An experimental investigation of the damage characteristics of out-of-plane low-velocity impact and compressive residual strength of stitched laminates was carried out.With the assistance of a simple stitching device,the specimens with different thicknesses and stitch densities were prepared by the VARTM process,the low-velocity central impact test was accomplished by drop weight testing machine,the compression test was performed on the damaged specimens,and the force-time and force-displacement curves of the specimens were obtained.The morphology of the delamination damage was obtained with ultrasonic C-scan equipment.The impact damage mechanism and compressive failure mode were investigated based on the test and inspection results.The results showed that the stitching contributed to the improvement of the impact resistance and the compressive residual strength of the laminate,and the denser the density was,the more obvious the enhancement effect was,when the impact energy was 20 J,the peak impact force of the specimen with the highest density of stitching was increased by 21.48%,and the residual strength was increased by 27.48%.In addition,the form of failure in compression after impact was different for stitched and unstitched specimens.(3)The finite element model for out-of-plane low-velocity impact and compression after the impact of stitched laminates was developed.Based on the progressive damage theory,the equivalent stitch resin cylinder model was established according to the improved lock stitch characteristics.The 3D Hashin and Hou failure criteria were selected,and combined simulation and analysis of impact and compression were proposed to establish the finite element model for the out-of-plane impact and compression after impact of the stitched laminate.The accuracy of the simulation was verified by comparing it with tests,and the effects of the thickness and density of the laminate on the impact damage and damage tolerance of the stitched laminate were analyzed.The results of the study showed that impact damage was mainly matrix tensile damage and delamination,and the stitches assisted in reducing matrix tensile damage and decreasing the area of delamination.The bending stiffness of the thin plate was small,and the damage was mainly manifested as matrix tensile damage;the bending stiffness of the thick plate was large,and the damage was mainly manifested as delamination damage,which dissipated the energy of the impact through the delamination.(4)The experimental research on low-velocity edge-on impact and compression after impact tests of stitched laminates was conducted.By referring to the standard of impact test,we designed and manufactured the fixture for edge-on impact,and carried out the edge-on impact and post-impact compression test of stitched laminates.The internal and external damage was characterized by microscope and Micro-CT equipment,and the effects of stitch density and impact energy on the impact damage mechanism and compression failure mode were investigated.The similarities and differences between edge-on impact and out-of-plane central impact were studied in terms of mechanical response and damage morphology.These results showed that the stitches could improve the edge-on impact damage tolerance,and it has a more obvious inhibition of the delamination damage away from the dent to inhibit its expansion.The peak impact force of the specimen with the highest stitch density was increased by 19.12% when the impact energy was 15 J.(5)The model of edge-on and compression after impact of stitched carbon fiber composite laminates was established.Based on the continuum damage mechanics theory,this work adopted the modified Camanho criterion to characterize the stiffness degradation,established a numerical model for the whole process of edge-on impact and compression after the impact of stitched laminates,and clarified the effects of stitch density and impact energy on the edge-on impact damage and residual strength.The findings showed that the edge-on impact damage was mainly delamination and compression damage,the stitching assisted in reducing the compression damage and inhibiting the delamination extension,and the delamination had no obvious directional characteristics.The compression failure damage after edge-on impact occurred in the centre of the laminate and extended downward from the impact damage area across the width of the specimen.Eventually,the specimen failed. |
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