| Nacre has excellent strength and toughness due to its unique “brick-mortar”microstructure.Imitating the microstructure of nacre and designing bionic composite structure can provide a new way for the development of new high performance impact protective structure.In this paper,using ceramics and polyurea as component materials and referring to the “brick-mortar” microstructure of nacre,the nacre-like ceramic/polyurea composite plate was designed and fabricated,and its damage and energy absorption characteristics under low velocity impact were studied.First,the low velocity impact damage characteristics of nacre-like ceramic/polyurea composite plate and its basic structure were studied by low velocity impact test,and the influence of structural parameters on the low velocity impact damage characteristics of nacre-like ceramic/polyurea composite plate was discussed;Then,in order to establish accurate numerical calculation models,the mechanical property tests of ceramics and polyurea materials were carried out,and effective methods were proposed to obtain the accurate mechanical property parameters of ceramics and polyurea materials;Finally,the stress distribution characteristics of ceramic/polyurea composite plates under low velocity impact were analyzed based on numerical simulation to deepen the understanding of the damage characteristics and mechanism of ceramic/polyurea composite plates.The specific work is detailed below:First,the nacre-like composite plate with “brick-mortar” structure can be regarded as the staggered stacking of segmented composite plates.In order to study its low velocity impact damage characteristics and mechanism,ceramic/polyurea laminated composite plate,segmented composite plate and nacre-like composite plate were designed and fabricated.The falling ball impact tests of three kinds of composite plates were carried out,and their damage characteristics were analyzed.Through the falling ball impact test with different drop heights,it is found that the laminated composite plate with 2 mm thick ceramic plate as the matrix and polyurea coated on the surface can greatly improve the low velocity impact resistance compared with the bulk ceramic plate.Due to the buffer effect of the surface polyurea,the critical failure energy can reach about twice that of the bulk ceramic plate when the area density is only increased by about 9%.Through the repeated falling ball impact test of the segmented composite plate,it is found that the segmented composite plate can effectively limit the crack propagation along the plane due to the buffer effect of polyurea between ceramic tablets,so that the damage is limited to the ceramic sheet at the impact position and its adjacent ceramic tablets,the damage area is small,and the structural integrity can be maintained under multiple impacts.Under the impact of falling ball with given energy,the damage on the front and back of nacre-like composite plate is slight,and the structure is complete,and the damage is still slight after 6 impacts,while the bulk ceramic plate with similar size is completely broken under the impact of the same energy.Nacre-like composite plate has better low velocity impact resistance and multiple impact resistance than bulk ceramic plate,which is mainly due to the buffer effect of polyurea between layers and polyurea between ceramic tablets in a single layer,which hinders the propagation of cracks along the in-plane and thickness direction of nacre-like composite plate.Second,the nacre-like ceramic/polyurea composite plates with similar overall size and different thickness of ceramic tablets were fabricated.The drop weight impact test was carried out.The contact force,absorbed energy and deformation of the composite plate were obtained.The influence of the thickness of ceramic tablet on the low velocity impact response of the composite plate was analyzed,and the composite plate was compared with the bulk ceramic plate with similar size.The test results show that,under the impact of 5 J energy,the bulk ceramic plate broke integrally and absorbed the impact energy of 1.07 J.The three kinds of nacre-like composite plates only have local damage,and the impact energy is basically absorbed.The nacre-like composite plate has better impact resistance than the bulk ceramic plate from the damage degree and energy absorption efficiency.The reason is that the cracks of the nacre-like composite plate deflects continuously along the direction of plane and thickness,which effectively dissipates the impact energy,reduces the damage degree of the composite plate and maintains good structural integrity;The damage degree of nacre-like composite plates(N-1,N-2,N-3)fabricated from 1 mm,2 mm and 3 mm thick ceramic tablets shows the trend of N-2<N-1<N-3,indicating that N-2 composite plates have the best impact resistance.It also shows that there is a reasonable thickness of ceramic tablet to optimize the impact resistance of nacre-like composite plate;The contact force between the composite plate and the punch shows a trend of N-1<N-2<N-3.The reason is that the overall stiffness of the composite plate increases gradually with the increase of the thickness of the ceramic tablet,resulting in the increase of the contact force;The punch displacement reflects the flexural deformation of the plate,and the deformation of the composite plate shows the trend of N-3<N-2<N-1,indicating that the N-1 composite plate has the best toughness.Third,the mechanical properties of ceramics and polyurea materials were obtained by different methods.For ceramic materials,the young’s modulus,shear modulus and poisson’s ratio of ceramic materials were obtained by pulse excitation method.For polyurea materials,first,the model parameters that can accurately characterize the tensile performance of polyurea were obtained based on the iterative inverse method and uniaxial tensile tests of polyurea.Then,the model parameters that can accurately characterize the compression performance of polyurea were obtained based on the response surface reverse method.Last,these two sets of parameters were used to establish complete uniaxial tensile and compressive stress-strain curves,and the hyperelastic intrinsic model parameters of different orders were obtained by least-squares fitting,and the obtained material parameters can be used to characterize the mechanical properties of polyurea in tension and compression.Fourth,combined with the low velocity falling ball impact test of ceramic/polyurea composite plate and the obtained material mechanical property parameters,the numerical simulation of low velocity impact response of ceramic/polyurea composite plates were carried out,and its stress propagation law was analyzed.The results show that,for bulk ceramic plate,the maximum stress at the same position of the specimen basically increases with the increase of drop height.At the same drop height,the back stress of specimen is greater than the front stress of specimen.When the drop height is 46 cm,the stress at the front and back of the ceramic layer of the laminated composite plate is much smaller than that of the bulk ceramic plate,which has a good cushioning effect and improves the impact resistance of the structure.The special structure and gap polyurea deformation and buffering effect of the segment composite plate effectively hinders the outward propagation of stress,limiting the stress to a single ceramic tablet,effectively protecting the integrity of the structure.Nacre-like composite plate combines the structural advantages of laminated composite plate and segment composite plate,effectively hindering the propagation of stress along the plane and thickness direction,when impacted,the frontal stress is concentrated in the impacted ceramic tablet,the back exists less stress. |
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