| With the gradual improvement of national public safety industry standards and road traffic safety regulations and the gradual improvement of people’s safety awareness,the demand for helmets is increasing day by day,and its safety performance requirements are getting higher and higher.Although the existing textile composite helmets can meet the basic requirements of safety protection performance,its preparation process mostly adopts the way of cutting and laying prefabricated parts,which is prone to structural defects at the cutting place,and there are large waste of raw materials and low production efficiency in the production process,high manual participation,poor molding quality and dimensional stability of products.It is difficult to meet the current green development concept of energy saving,emission reduction and clean production,which limits the development of textile composite helmet.Therefore,the design and development of integrated helmet prefabricated parts and their helmets,and the study of their safety protection performance and damage mechanism under impact are of great significance to the design and production of helmets with low energy consumption and high quality.In this paper,two kinds of three-dimensional(3D)flat-knitted inlay fabric were designed,and the basic mechanical properties and impact resistance of the fabric and its composites were explored;Secondly,based on the designed fabric structure with outstanding mechanical properties and the full-form flat-knitting technology,the integrally formed helmet shell preform was developed and the helmet was prepared;Finally,the safety protection performance of fully formed horizontal braided helmets was evaluated with reference to the helmet impact test standard,and its impact damage characteristics were analyzed.The main research work are as follows:(1)The structure of 3D flat-knitted inlay fabric was developed,and the basic mechanical properties of the fabric were studied.In order to solve the problem of large extensibility and poor energy transmission in impact process of knitted fabrics,two kinds of 3D flat-knitted inlay fabric with non-buckling reinforcement yarns were developed and designed using Ultra-high Molecular Weight Polyethylene(UHMWPE)yarns,and the basic mechanical properties of the two kinds of 3D flat-knitted inlay fabric with different structures were compared and analyzed by quasi-static tension,bursting and bending respectively.The results show that the tensile and bending properties of the 3D flat-knitted inlay fabric based on interlock structure are obviously better than those of the 3D flat-knitted inlay fabric based on plain stitch because of the better binding effect of the interlock structure on the inlay yarn and the integrity of the structures is more prominent;The 3D flat-knitted inlay fabric based on plain stitch has relatively extensibility,large deformation and relatively large energy absorption in the process of bursting;In general,3D flat-knitted inlay fabric based on interlock structure has more development potential in rigid composites because of its advantages in tensile properties,bending properties and tensile stability.(2)The tensile and impact properties of 3D flat-knitted inlay fabric reinforced composites were studied.Firstly,the tensile properties of 3D flat-knitted inlay fabric reinforced composites were tested based on acoustic emission technology,and the tensile damage patterns and acoustic emission characteristics under different damage forms were analyzed.The results showed that the 3D flat-knitted inlay fabric based on interlock structure had higher tensile strength and stable tensile fracture process.In addition,the acoustic emission results show that the damage forms of fiber matrix debonding and matrix fracture during tensile damage have the characteristics of large cumulative acoustic emission energy,many events and long duration,while fiber fracture has the characteristics of large amplitude,high frequency,low cumulative acoustic emission energy,few events and short duration.Secondly,the impact resistance and damage modes of3 D flat-knitted inlay fabric reinforced composite laminates based on interlock structure and plain weave fabric reinforced composite laminates are compared and analyzed.The results show that 3D flat-knitted inlay fabric reinforced composite has good impact resistance,and the main damage modes include fiber breakage,fiber-matrix debonding,matrix cracking,tow splitting and plastic deformation.Finally,the elastic constants and strength parameters of 3D flat-knitted inlay fabric reinforced composites are obtained by multi-scale modeling finite element method,and the impact process and damage forms of 3D flat-knitted inlay fabric reinforced composites are simulated and analyzed by macroscopic homogeneous modeling.(3)The preform technology of full-form flat-knitted helmet based on 3D flat-knitted inlay fabric structure was designed,and the preform knitting and helmet preparation were carried out.The 3D solid model of helmet shell was obtained by reverse development technology,and the model was unfolded based on transverse knitting and longitudinal knitting in full-form flatknitting technology,and the two-dimensional unfolding diagram which can be used for process design was obtained.Combined with the structure of 3D flat-knitted inlay fabric and local knitting technology,the full-form flat-knitted preform of helmet shell was designed,and the sinker and pulling device of the computered flat knitting machine were designed and modified,the full-form flat-knitted preform of helmet shell with UHMWPE as raw material was successfully knitted.After that,based on the preform of full-form flat-knitted helmet,the helmet was fabricated by air bag pressurization method.(4)The safety protection performance of the full-form flat-knitted helmet during impact were evaluated,the energy absorption damage mechanism of the helmet was analyzed,and the impact process of helmet was simulated.Referring to GB811-2010 test standard of motorcycle occupant helmet,the safety protection performance of different parts of full-form flat-knitted helmet under the action of plane anvil and spherical anvil was tested on helmet impact tester.The data showed that full-form flat-knitted helmet had outstanding safety protection performance,and the safety protection performance of different parts was relatively balanced.By analyzing the damage morphology of helmet after impact,it can be found that the damage of helmet shell and foam layer under plane anvil impact was small,while the damage of helmet shell and foam layer under spherical anvil impact was serious,and its damage forms mainly include fiber-matrix debonding,matrix cracking,foam crushing and fracture,and plastic deformation of helmet shell and foam layer.In addition,the stress distribution during helmet impact was simulated by establishing a finite element model of helmet impact.Compared with the test results,it was found that the model has certain guiding significance in customized design of helmet and prediction of safety protection performance.Through the development,design and preparation of full-form flat-knitted helmet,as well as the research on its impact resistance and damage mechanism,the further development of textile composite helmet is promoted,and a good theoretical foundation for the development of prefabricated parts with complex shape structures by full-form flat-knitting technology. |
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