| In aerospace applications,honeycomb sandwich panels can be perforated in the honeycomb core wall to balance the internal pressure and external gas pressure,and provide a heat flow channel.In order to reduce the number of holes and make it easy to process,only the L-direction cell wall is opened.In this paper,the effect of honeycomb core opening on the mechanical properties of honeycomb sandwich panel was studied.A vented TC4 honeycomb sandwich panel was manufactured by high temperature brazing.Compared to the interaction between the face-sheets and the brazing filler,the diffusion and reaction between the honeycomb core and the brazing filler were stronger.The flatwise compression and bending mechanical properties of the vented and unvented honeycomb sandwich panels were investigated at 20,160,300,and 440 °C,respectively.With the increase of temperature,the compression and bending properties of unvented and vented honeycomb sandwich panels decrease.The compression performance of unvented honeycomb sandwich panel is similar to unvented honeycomb sandwich panel.There is no negative effect on the compression performance of honeycomb sandwich panel when the opening of cell wall is small.The compression performance of unvented honeycomb sandwich panel is similar to unvented honeycomb sandwich panel.There is no negative effect on the compression performance of honeycomb sandwich panel when the opening of cell wall is small.Moreover,the flexural strength of the L-direction sandwich panels was larger than that of the W-direction sandwich panels at the same temperature.The three-point bending fracture morphology of the specimen was observed by scanning electron microscope(SEM).The results show that the fracture modes of the panel,core and brazing zone are ductile fracture,quasi cleavage fracture and cleavage fracture respectively.Compared with the original properties,the compressive strength and elastic modulus of cell wall decreased by 0.7% and 5.32 times,respectively.After thermal exposure,the compression performance of vented honeycomb sandwich panel is better than unvented honeycomb sandwich panel.Because of the vacuum negative pressure,unvented honeycomb sandwich panel has more obvious plastic deformation at high temperature,which makes it more prone to instability and performance degradation in the flat compression test.The mechanical properties of specimens degraded from thermal exposure can not be recovered.After thermal exposure at 400 ?,500 ? and 600 ?,the flat compression properties of the specimens decrease slightly,and the failure mode is plastic buckling of honeycomb core.After thermal exposure at 700 ?,the compression performance of the specimens deteriorated obviously.The failure mode of the unvented honeycomb sandwich panel changed to double honeycomb wall debonding after 6 hours of thermal exposure.The failure mode of the unvented honeycomb sandwich panel changed to cell wall fracture after 9 hours of thermal exposure.After 3 hours thermal exposure at 800 ?,the specimens were destroyed directly.The opening of honeycomb core has no effect on the flat compression property of the specimen after thermal shock.With the increase of the number of thermal shock,the flat compression performance decreases.Compared with the original sample,the effect of the first 10 thermal shocks on the structural properties is higher than that of the last 20 thermal shocks.The effects of the size,location and number of openings on the flat compression performance of honeycomb sandwich panels were studied by flat compression test.The results show that the compression performance of the structure does not decrease when the hole diameter is small.With the increase of perforation diameter,the influence on the compression performance of the structure is accelerated.The hole in the center of the L-direction cell wall has the best compression performance.The closer to the upper and lower panel openings,the worse the compression performance of the structure.The strength of the specimen with a hole at the top is less than that with a hole at the bottom.When there are multiple holes in the L-direction cell wall,the flat compression performance of the structure is affected by the location of the opening.At the same time,the more the number of holes,the greater the impact on its stability.Through the finite element analysis,it is found that with the increase of the thickness of honeycomb core,the compressive strength of vented honeycomb sandwich panel increases linearly.With the increase of honeycomb core height,the the compressive strength of vented honeycomb sandwich panel gradually tends to be stable.The compressive strength of vented honeycomb sandwich panel with circular hole is slightly better than that with square hole,which is obviously better than that with triangular hole.The compressive strength of W-direction opening is significantly higher than that of L-direction opening.The cell wall with hole will affect the adjacent cell wall.The farther the opening distance is,the better the compression and shear properties are.The density formula and buckling stress formula of vented honeycomb sandwich panel are derived.The elastic and plastic buckling formulas of vented honeycomb sandwich panels with l-direction cell wall opening,w-direction cell wall opening and different angle of cell walls are given respectively.The theoretical value and experimental value of plastic buckling theory are compared,and the error analysis is carried out.According to the asymptotic homogenization finite element method,a program for predicting the macro equivalent performance of vented honeycomb sandwich panel is compiled.Comparing the calculated and experimental values,when the aperture is less than 2mm(the wall width ratio of the aperture is not more than0.62),the calculated value is very close to the experimental value.The equivalent compression modulus of vented honeycomb sandwich panel with different geometric parameters is calculated.The results show that with the increase of the thickness of the cell wall,the equivalent modulus increases linearly;With the increase of the height of the honeycomb core,the flat compression modulus of the vented honeycomb sandwich panel increases gradually,but the growth rate decreases gradually;The angle of cell wall increases from 15 ° to 60 ° and the compression modulus decreases and reaches the minimum value.With the increase of cell wall angle to 90 ° the flat compression modulus increases gradually. |
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