| Personal impact protection equipment,such as helmets and sports gear,is essential in both military and civilian fields.There are two basic requirements for the design of mechanical personal protective equipment.The first is that it is strong enough to prevent personal protective equipment from being damaged,and the second is that it can absorb impact energy,reducing the load transmitted to the wearer.The strength of personal protective equipment has essentially reached the requirement of resisting mechanical impact with the advancement of science and technology.However,dynamic impact deformation of protective equipment can still result in fatal non-penetrating injuries to the wearer.As a result,it is critical to reduce the local back deformation of personal protective equipment under impact load in order to prevent serious injury to the wearer.Cushioning energy-absorbing materials with high deformation capacity,such as polymer foams,rubbers,and textile fabrics,can be used as backing materials in this case.These soft backing materials can deform locally,preventing energy from being transmitted to the human body and thus improving mechanical impact protection.Despite the fact that these flexible protective materials have been extensively researched in the field of personal protection,research on the mechanical properties and energy absorption mechanism of protective materials with soft and flexible characteristics under impact load is still in its early and exploratory stages.The study of its energy absorption mechanism under impact load,as well as the application and development of flexible protective materials with high specific strength,low density,high protective ability,and high performance,are of great importance in the application and development of flexible protective materials,which is in line with the development trend of personal protective equipment.When developing new soft personal protective equipment,it is not only necessary to consider the new protective materials being developed,but it is also necessary to fully understand and correctly apply structural part design principles.As a result,three types of flexible materials with high energy absorption capacity,namely polyurethane foam(PUF),shear thickening gel(STG),and UHMWPE fabric,were chosen in this paper from two aspects of substrate design and structure optimization.A series of studies on the mechanical properties of PUF modified by STG and the low-velocity impact properties of UHMWPE fabrics/polyurethane foam sandwich materials were conducted using the sandwich structure.The effect of STG on the structure and properties of PUF was analyzed.The relationship between the parameters of the surface layer and the core layer and the low-velocity impact performance of the sandwich material was discussed.The numerical simulation method of flexible sandwich material was established to thoroughly examine the deformation and failure of UHMWPE fabric/polyurethane foam during low-velocity impact and reveal the mechanism of energy absorption.The main contents are as follows:Shear thickening gel/polyurethane foam(STG/PUF)composites were prepared by mechanical stirring method.The effects of opening agent contents and isocyanate contents on the density,hardness and microstructure of the foamed composites were discussed,and the optimum content was determined to improve the cell structure of the foamed composites.The chemical structure,dynamic thermomechanical and thermal stability of STG/PUF were characterized and analyzed.With the help of SEM images,the cell diameter distribution and cell density of foamed composites with different STG content were counted,and the influence of STG contents on the cell structure of foamed composites was systematically studied.The results showed that the content of STG had a great influence on the cell structure of PUF,and there was a suitable addition amount to make the cell structure of foamed composites reach the best.The mechanical properties of foamed composites with different STG contents under quasistatic tension,quasi-static compression and low-velocity impact load were studied.The deformation process,strain rate sensitivity and energy absorption capacity of foamed composites under three loading modes were explored.The apparent morphology of foamed composites before and after cyclic compression and low-velocity impact was characterized,and different failure modes of foamed composites under different loads were obtained.The results showed that the mechanical properties of STG/PUF under different loading modes were strongly correlated with the loading rate,and were influenced by the content of STG.The addition of STG changed the cell structure of PUF and the solid skeleton matrix of foamed material,thus affecting the mechanical properties of foamed composites.In the experimental range of this paper,when the weight of STG was 5～15 parts,it can enhance the strength of polyurethane foam.When the weight part of STG was 20 parts,it can enhance the toughness of polyurethane foam.Based on the drop hammer impact test system,the low-velocity impact properties of UHMWPE fabric/polyurethane foam sandwich materials with different materials and structural parameters were studied.The effects of STG content,foam density,fabric structure,punch diameter and impact energy on the low-velocity impact properties of sandwich materials were analyzed.Four indexes for evaluating the energy absorption characteristics of sandwich materials were defined,and the energy absorption capacities of experimental samples with different parameters were evaluated.The results showed that with the increase of STG content,the energy absorption mode of sandwich materials tended to the structure dissipating energy through more plastic deformation.With the increase of foam density,sandwich materials tended to produce more elastic deformation energy to resist impact penetration.Under the same impact energy,high-density foam can reduce the pit depth after impact,which can reduce the nonpenetrating damage.For the surface fabric structure,under the same fiber volume content,the woven structure was more recommended as the impact-resistant material.Transverse isotropy was used to describe the physical properties of fabric materials,and LOW DENSITY FOAM model was used to describe the physical properties of foamed materials.The tensile properties of UHMWPE yarn were tested,and the elastic modulus parameters of the yarn were obtained.Based on the stress-strain curve obtained by quasi-static compression experiment and the classical Sherwood model,the constitutive relation of polyurethane foam was fitted by considering the factors of density and strain rate.The fitting results were compared with the experimental results,and the input curve of LOW DENSITY FOAM model was obtained.A numerical simulation method of UHMWPE woven fabric/polyurethane foam damaged by low-velocity impact was established,which included nonlinear large deformation and foam strain rate correlation effect,and the simulation results were compared with the experimental results.The results showed that the maximum errors of peak force,maximum displacement and buffer time predicted by the finite element model established in this paper were 4.7%,5.7% and 7.6% respectively,and the simulation results were highly reliable.Based on the finite element model of low-velocity impact performance of UHMWPE woven fabric/polyurethane foam,the failure mode and evolution of sandwich structure were analyzed and expounded,and the energy absorption mechanism of sandwich material under low-velocity impact was revealed.In addition,three influencing factors such as friction coefficient,elastic modulus of yarn and interfacial fracture energy were studied parametrically.The results showed that the energy absorption mechanism of the sandwich material under the impact energy of 50 J was mainly that the main yarn of the upper fabric deforms greatly,the foam layer bears more compression load,the foam under the main yarn fails under the combined action of tensile load and shear load,and the interfacial layer between the fabric and the foam fails to debond to dissipate part of the energy.The kinetic energy of punch was mainly converted into the strain energy of fabric and foam,the energy absorbed by interface contact,the kinetic energy of structural parts and the energy dissipated by friction in the system.In addition,when the friction coefficient was between 0.75 and 0.15,and the elastic modulus of the yarn was greater than 30 GPa,the sandwich structure had better impact resistance under the impact energy of 50 J.In this paper,STG/PUF with high strain rate sensitivity was prepared based on the material design concept,and the influence of shear thickening gel on the cell structure and mechanical properties of polyurethane foam was revealed.Based on the concept of structural optimization,the effects of fabric structure,STG content,foam density,punch diameter and impact energy on the low-velocity impact performance of flexible sandwich materials were studied.Combined with the finite element model,the energy absorption mechanism of UHMWPE fabric/polyurethane foam was expounded,which provided a basis for the design and application of flexible protective materials. |
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