Compressive Properties Of Reinforced Composite Hierarchical Grid Structure

The composite grid structure is a new type of lightweight composite sandwich structure with high specific strength,high specific stiffness and strong designable,it has received extensive attention and in-depth research by scholars at home and abroad since its inception.Unmanned underwater vehicle usually provide buoyancy for their dive by means of lightweight materials,the buoyancy materials are mostly made of glass beads/epoxy composites,but their density is high,water pressure resistance is poor,the buoyancy is small.The glass microspheres with a density of less than 0.3g/cm~3 can only work normally in the ocean at a depth of 200meters,therefore it is difficult to meet the technical requirements of UUV.The purpose of this work is to discuss the feasibility of applying the composite grid structure with excellent mechanical properties to the buoyancy structure of UUV.It found that the composite grid structure has excellent mechanical properties through the investigation and summary of domestic and foreign research status,however,in the low-density area,the wall of the grid is prone to elastic buckling failure and the bearing capacity is not high.In order to solve this problem,this paper adopts the design idea of multistage structure,thepolymethacrylimide(referred to as PMI foam)is added between the unidirectional reinforcing carbon fiber composite materials to prepare the core of the reinforced composite multistage grid structure.The out-of-plane compression,in-plane compression and water pressure resistance properties of the structure are investigated and the failure modes under three working conditions are revealed.Based on the design idea of multistage structure,the reinforced composite multi-level grid structure is designed and fabricated by the interlocking assembly process.Firstly,the experimental study on the out-of-plane compression performance of the structure shows that the out-of-plane compression performance is unstable,mainly because the processing accuracy of the PMI foam is not enough,resulting in poor multi-stage structure enhancement effect.Then based on multi-level grid optimization design theory,widening the optimal foam thickness interval,improving the stability of the out-of-plane compression performance of the multi-stage grid structure,and solving the problem of unstable mechanical properties of the PMI foam due to insufficient processing precision.Then,through theoretical prediction,numerical simulation and experimental means,the out-of-plane compression performance of multi-stage grid structure of carbon fiber composites with different equivalent densities is studied,and the curve prediction of the relationship between the out-of-plane compression performance and the equivalent density of the grid structure is drawn.When the density is between 0.04g/cm~3 and 0.2g/cm~3,the out-of-plane compressive strength of the grid structure increases linearly from 12.34MPa to 53.30MPa,and the experimental results are consistent with it.Therefore,the conclusion that the out-of-plane compressive strength increases with the increase of the equivalent density is obtained,and its out-of-plane compressive strength reaches 200×10~3m~2/s~2,which is more than three times higher than the out-of-plane compressive strength compared with the composite quadrilateral honeycomb.The simulation and experimental results show that the collapse failure of the grid wall composite and the simultaneous failure of Wrinkling are the main failure modes,which are consistent with the theoretical prediction results.The theoretical prediction formula of the multistage grid structure under in-plane compression load is derived and the failure mechanism diagram of in-plane compression is drawn.The size of the specimen is designed according to the failure theory mechanism diagram and the in-plane compression performance is tested.The accuracy of the failure mechanism diagram is verified by the experimental results,which provide theoretical guiding for the engineering application.Finally,the investigation on the water pressure resistance of the multistage grid structure under water pressure is carried out,and the water pressure resistance reaches18.0±1.3MPa.The failure mechanism of the multistage grid structure under water pressure is revealed by finite element simulation.It found that the composite panel is dented in the compression process,which caused extrusion damage to the PMI foam from the side and led to early failure of the multistage grid structure.This is the main reason why the water pressure resistance is worse than the out-of-plane compression strength.The water pressure resistance performance of the multistage grid structure has been improved by 3 times compared with that of the glass microspheres buoyancy materials,which can only withstand about 6MPa of water pressure.It indicates that the application of composite multistage grid structure with excellent mechanical properties to UUV can greatly improve the mechanical properties of buoyancy structure and reduce the weight and volume of buoyancy structure.This will enable the deep-sea unmanned platform to carry more payloads and improve its comprehensive performance such as endurance.