Preparation Of Three-dimensional Curved Flat-knitted Spacer Fabric Composites And Its Low-velocity Impact Property

As a new kind of three-dimensional(3-D)knitted fabric composite material,the 3-D knitted fabric composite material has strong impact resistance and it has a promising future in the fields of bridge,automobile,aerospace and other industries.The spacer fabric reinforcement has the advantages of good structural integrity,high hardness and light weight,and the hollow integrated structure is stable and firm,the spacer layer has good heat resistance,sound insulation and impact resistance.Such spacer-reinforced composites are sensitive to impact loads,and damage caused by impact loads can seriously affect their mechanical properties.In the actual production and use process,various accidental impacts may occur,such as tool drop,flying stone impact,etc,and the composites are often used in the form of curvature in practical applications.Therefore,it is of great practical significance to study the mechanical properties of composites with different curvatures,especially low-speed impact performance and finite element simulation prediction.In this paper,the weaving process of flat-knitted spacer fabrics is studied,and the reinforcements with different curvatures are developed.The composites with different curvatures are made to study the compression properties,low-velocity impact and residual strength of composites.The finite element model of composites with different curvatures is established.Compare with the experimental results,test whether the established model and theoretical analysis method are correct,and then predict the mechanical properties of other composites with different curvature.The main research contents are as follows:First of all,a new double-layer rib-knitting spacer fabric with aramid yarns was knitted on a computer flat knitting machine,and the knitting steps and knitting process were discussed.Then the aramid reinforcement yarns are integrated into two surface layers of 3-D spacer fabrics in the weft direction to enhance the tensile properties.By changing the weaving process,flat-knitted 3-D speacer fabrics curved in different angles were developed.Tensile tests on the universal material testing machine are carried out in the weft and warp directions of two flat spacer fabrics(with reinforcement yarns and without),and their stress-strain curves are compared.The results revealed that reinforcement yarns reduced deformation,and improved tensile stress and dimensional stability of 3-D spacer fabrics when other experimental conditions being equal.The 3-D flat-knitted spacer fabric was woven into two kinds of plane and curved surface.Fourteen kinds of composite materials with different curvatures were prepared by vacuum assisted resin transfer molding technique(VARTM),and the radius of curvature was 300 mm,160mm,120 mm and +∞ respectively.The development of fabric reinforcements with better mechanical properties and composites laid the foundation for the study of mechanical properties of subsequent composites.Secondly,the compression properties of the 3-D flat-knitted spacer fabric composites were investigated.The composites with different curvatures were placed on an electronic universal material testing machine to test the compression performance,and the compression load-displacement curves were compared in groups.The results show that,when the other conditions are the same,for the curved composites:(1)whether it is a flat spacer fabric composites or a curved spacer fabric composites,the lateral compression peak of the non-woven fabric composites is significantly higher than the vertical;the lateral compression peak of the material increases as the curvature of the sample increases,and the longitudinal compression peak decreases as the curvature of the sample increases.(2)whether it is a flat-spaced fabric composites or a curved-spaced fabric composites,the transverse compression peaks of the weft-inserted fabric composites are significantly higher than the longitudinal direction;the same group of composites with the increase of the curvature of the sample,the compressive load peak is also gradually increased.The compressive properties of the composites were studied to provide data for the comparative study of the low-velocity impact and residual compression strength of the subsequent curved composites.And then,the low-velocity impact properties of the 3-D flat-knitted spacer fabric composites and the residual compressive strength were studied experimentally.The compression direction is divided into two types: horizontal and vertical.The residual compressive strength of different reinforcement composites is studied by using the optimal method.After the optimal structure is selected,the curvature and residual compressive strength are compared.The conclusions are as follows:(1)Using the method of measuring damage area and single variable to study the low-velocity impact performance of composites and its influencing factors,in the research process,the operation is convenient and the results are intuitive,which is a better method for similar research.(2)The influence of the flat fabric reinforcement and the curved fabric reinforcement on the low-velocity impact performance and residual compressive strength of the composites is different.The single-variable method is used to obtain the positive influence of the curved fabric reinforcement on the two performances.And the degree of influence is large.(3)The weft insertion of the fabric will also affect the low-velocity impact performance and residual compressive strength of the composites.The weft insertion factor shows a positive effect in most of the tests,but the factor has little effect.(4)The greatest influence on the low-velocity impact properties and residual compressive strength of composites is the final radius of curvature of the composites.The composites with smaller radius of curvature has a larger curvature,and the gain effect is more prominent in the low-velocity impact experiment and the lateral compression of the residual compressive strength.Through comprehensive systematic comparison of experimental data,the failure morphology of typical samples is analyzed and described,which provides experimental basis for subsequent finite element numerical simulation and prediction.Finally,finite element numerical simulation is carried out.Finite element model of composites with different curvature was established by using finite element software ABAQUS.The damage area,stress distribution,energy absorption,failure mechanism and other aspects were calculated and analyzed by finite element method through the principle of single variable,and compared with the experimental results to verify whether the established model and theoretical analysis method were correct.In addition,two compositesl models with curvature radius of +∞ and 100 mm were added to study the trend of the specimens’ impact resistance through data comparison,so as to predict the mechanical properties of other composite materials with different curvature.The results show that the greater the curvature of the composite,the stronger the bearing capacity;as the curvature increases,the impact resistance performance decreases.The combination of experiment and simulation shows that the established finite element model can predict the impact resistance of composites with different curvatures,which provides a theoretical basis for the application of the composite.