Low-Velocity Impact Test And Numerical Simulation Of 2.5D Woven Composites

With the demand for lightweight and high-performance materials.The application fields of composite materials are gradually expanding.The application types of composite materials have also expanded from laminated composites to three-dimensional woven,knitted,and braided composites.In the actual application of different service environment,in order to meet the reasonable application of composite materials and the safety requirements of components,it is very necessary to explore the mechanical properties of composite materials under specific working conditions.2.5D woven composites is a kind of composite material with good impact resistance,which can overcome the shortcomings of traditional laminated composite material.From the aspects of experiment and numerical simulation,this article explores the remaining mechanical properties of the material in different directions and the corresponding decrease and change trend in view of the excellent impact resistance and high anisotropy of mechanical properties of 2.5D woven composites.It has great engineering significance for accurately evaluating the service performance of the materials in the impact-prone environment.In the aspect of experiment,firstly,low-velocity impact test with two kinds of energy(21.6J,47.6J)were carried out on five specimens with different off-axis angles(0°,30°,45°,60°,90°).The mechanical response information of 2.5D woven composites under impact was obtained.Optical microscope and Micro-CT scanning technology were used to observe the surface and internal damage types and morphology of the composites after impact.The low-velocity impact test causes almost the same impact damage to the samples,which lays the foundation for the subsequent compression after impact test.Then the in-plane static compression test was going on the samples with impact damage.Getting the compressive mechanical response,residual compressive strength and decreasing trend of 2.5D woven composites in five directions.The whole compression process was recorded by DIC equipment,and gained the in-plane strain distribution of the composites in all directions.Combined with the damage morphology of the samples after compression,the occurrence and development of compression damage after impact were analyzed.In the aspect of numerical simulation,first of all,a full thickness unit-cell model was constructed according to the actual structure of 2.5D woven composites.And using the general periodic boundary and UMAT subroutine written in FORTRAN language to predict the macro-mechanical properties of composites.The stress distribution and damage area of the three-dimensional unit-cell model under various loads were also analyzed and summarized.On this basis,a macro-meso hybrid model is proposed to predict the low velocity impact behavior.Combined with the material parameters predicted by the unit-cell model,the VUMAT subroutine is written to predict the low-velocity impact mechanical response,and the damage evolution law and energy absorption characteristics of the composites are analyzed.The numerical simulation results are verified by experimental data.Through the above research,it is proved that in different directions there are many differences in the distribution of compression damage,mechanical response,residual compressive strength and decreasing trend of 2.5D woven composites after impact.It is found that the microstructure of 2.5D wowen composites(such as the direction and distribution of yarn)makes them have obvious anisotropic characteristics,and the damage caused by impact shows strong directionality,which leads to great differences in compression properties along different directions.