Study On Near Field Detection Of Defects In Foam-filled Honeycomb Composites

Honeycomb composite structures are widely used in transportation,construction,aerospace,and other fields due to their excellent characteristics such as light weight,high bearing capacity,and strong designability.The excellent mechanical properties of the honeycomb structure in the out-of-plane direction and the relatively weak in-plane direction affect its lateral application.Polyurethane foam-filled honeycomb is an effective method to improve its in-plane mechanical properties and expand its lateral application fields.However,there is no standardized method and process for polyurethane foamfilled honeycomb,and there are various structural defects in the filled honeycomb,which have a negative impact on its mechanical and electrical properties.In response to these problems,this thesis prepared a polyurethane foam-filled absorbent honeycomb composite structure based on polyurethane foaming technology,established a defect detection microwave near-field test system,identified several honeycomb structural defects,and studied the impact of structural defects on the mechanical properties of filled honeycomb.The main research contents and conclusions of this thesis are as follows:(1)Based on the polyurethane foaming process proposed by our unit,polyurethane rigid foam materials with different epoxy resin addition ratios and carbon-based particle addition ratios,as well as composite structures of polyurethane foam-filled honeycomb cells,were experimentally prepared.It was found that when the mass fraction of epoxy resin and its curing agent is greater than 1%wt,the polyurethane rigid foam is prone to deep yellow brittle agglomeration,low foam closed-cell rate,and low adhesion between the foam and honeycomb wall.The addition of carbon fibers affects the foaming effect,and there are many large air holes in the foam,resulting in a lack of surface smoothness.The foam-filled honeycomb cells have structural defects such as uneven foam density and uneven filling of honeycomb holes.(2)Using a microwave near-field testing system,near-field detection of three typical defects in honeycomb structures was conducted.The test showed that the near-field testing system can detect and characterize honeycomb surface absorbers with conductive materials above 3mm,uneven impregnation of absorber blocks in absorbing honeycombs,and uneven distribution of foam in honeycomb cells filled with foam.(3)A defect detection and structural mechanics performance study was conducted on foam-filled honeycomb structures.Based on the geometric center symmetry of the honeycomb face,single-hole,double-hole,and triple-hole defects were designed.From the perspective of experiments and simulations,the influence of foam-filled honeycomb pore defects on its in-plane compressive behavior was analyzed: foam detachment and deformation collapse of honeycomb pores would occur in the central region of the filled honeycomb,forming compression strip bands that gradually took on an "X" shape along the diagonal direction(W)and a vertical arc-shaped compression strip band along the longitudinal direction(L).Pore defects located on the compression strip band would fail first,while those located on the horizontal centerline would delay the honeycomb’s lateral expansion and deformation,and the greater the distance from the horizontal centerline,the more pronounced this delayed expansion would be.The location of the pore defect affects the stress distribution during the compression of the filled honeycomb,with the maximum stress occurring at the diagonal "X" of the fully filled honeycomb,and the pore defect exhibiting similar characteristics,enhancing stress around the diagonal and reducing it at the top,bottom,left,and right.The location of the pore defect has little effect on the elastic-plastic parameters of the filled honeycomb,and an increase in the number of pore defects will increase the elastic modulus of the filled honeycomb but decrease its yield strength.