Analysis Of Prepreg Deformation At The Nodes And The Buckling Resistance Of Composite Grid

Composite grid structure is more and more popular due to its weight loss advantage and high damage tolerance.However,with the development of the manufacturing technology and the increase of the volume fraction of the carbon fibers,the fiber stacking at the grid nodes is a special problem to be dealt with.In this paper,placing-flattening technology and laying-rolling technology are studied to deal with the fiber stacking and protrusion at the nodes,as well as the influence of the fiber waviness caused by the two manufacturing technologies on the node stiffness and critical buckling load of the grid structure.?1?The deformation model of the prepreg at the nodes during the flattening process is established by using the rheology,and the numerical analysis is carried out.For the single-node grid structure,the parametric analysis of the total force,the length of prepreg between the nodes and the viscosity of the resin used in the prepreg is carried out.With the increase of total force,the time required for flattening the node decreases.With the increase of the length of prepreg between the nodes and the viscosity of the resin,the time required for flattening the node increases.The studying of different parameters provides a basis for consideration of cost and production cycle.For the multi-node grid structure,the time required for flattening process of a certain gird structure under the optimized parameters is 14.3 minutes.And from the middle node A to the edge node C,their respective force gradually decreases,and basically remains the same during the flattening process.The calculation results are verified by the actual laying and flattening test in the autoclave.?2?The deformation model of the prepreg at the node during the laying-rolling process is established by using the rheology,and the integral equation is derived.The laying speed,the radius of the roller and the length of the prepreg between the nodes are studied in parametric analysis.With the increase of the laying speed,the force needed to decrease the thickness of the prepreg by half increases.With the increase of the roller radius and length of prepreg between the nodes,the force needed to decrease the thickness of the prepreg by half increases.The strength check of carbon fiber in the rolling process is carried out to determine the boundary of each process parameter.When the viscosity of the resin is 10 Pa·s,the laying speed can be set to a larger value on the order of tens micrometers per second to raise the laying efficiency.If the speed of 1mm/s is an acceptable minimum laying speed,the maximum parameters l₀ of the grid structure can reach 4472mm.When the viscosity is greater than 1×10³ Pa·s,the maximum laying speed for the smallest value of l₀,50mm,is less than 0.5mm/s.?3?The sensitivity of the buckling critical load factor of a grid structure with rectangular cell to the nodal stiffness is studied by ABAQUS.Different engineering elastic constants are assigned to their nodes respectively.It is found that with the decrease of nodal stiffness,the critical load factor also decreases.However,when the nodal stiffness of the node is reduced to 10%of original stiffness,the critical buckling load factor of the grid member declines only by 15%.The average stiffness of the nodes with fiber waviness is calculated,and the average elastic modulus at the nodes is reduced to18.2%.At the same time,the critical buckling load factor of the grid structure is reduced by 10.7%.Compared with the grid structure with cutting-continuing nodes,although the shear continued joints have a layer of cut fiber at every other level,the anti-buckling ability of the grid structure made by the placing-flattening technology or the laying-rolling technology is not as good as that made by cutting-continuing technology in which the prepreg at the nodes is cut every other layer.