Damage Resistance Of Low Velocity Impacted On Foam-core Sandwich Structure Composites

Foam-core sandwich structure composites are widely used in the industry ofaerospace due to their low density and high mechanical strength. The main drawback ofthe high performance structures is their poor resistance to localized. And there may haveno apparent damages on the surface but in fact serious damages may occur within thestructures, and it is a severe threat to flight safety. Therefore, it is significant to carry outthe research on the damage resistant characteristics of foam-core sandwich structurecomposites.In this paper, the equivalence of the low-velocity impact tests and quasi-staticindentation tests of PMI foam-core sandwich structure composites was verificatied. Theparameters which characterized the equivalence were also put forward. Thecharacteristic of damage resistance of low-velocity impact on foam-core sandwichstructure composites were analyzed by conducting quasi-static indentation tests. Thecommon problems with molding process which affected the damage resistance weresolved. The influences of different boundary conditions (clamping around and simplysupporting around), different foam-core heights and different toughness of skin on thelow-velocity impacting damage resistance were analyzed. The damage mechanism ofquasi-static indentation was analyzed by carrying out hot exposing layer experiment. Toprove the effectiveness of using finite element method in the research on the damageresistance of low-velocity impact, the quasi-static indentation process was simulatedwith this method. The results are as follows:Firstly, the load change trend of low-velocity impact tests was identical with that ofquasi-static indentation tests. Their section damage morphology are similar to each other.The inflection points of impact energy-indentation depth and quasi-static indentationdepth dimpling force all appeared neared the depth of0.27mm of the pit. The feasibilityof replacing the low-velocity impact tests by quasi-static indentation tests was verifiedto some extents by the above three results. Dent depth was suitable to characterize thedamage in order to establish the equivalence of low-velocity impact tests andquasi-static indentation tests. The quasi-static indentation force can be approximated asthe maximum load under the same impact energy on the condition that the impact energy was low(less than12J).Secondly, the carbon fiber might be broken up or distorted in the resin transfermolding process, and this can be improved by letting aluminum plates pin the edges ofpresetting body. The damage resistance characteristics of simply supported were similarto the clamped under the condition of small dimpling displacement. And they bothcould be used to characterize the damage resistance of foam-core sandwich structurecomposites. The damage resistance of foam-core sandwich structure compositesincluding damage process and form can be affected by different foam-core heights.They have both similarities and differences. The similarities are as follows:①foam-core collapsing,②foam-core cracking,③skin delaminating,④skin-coredebonding. The similarities are as follows: the damage form is that the cracks is±45°inthe direction of foam-core thickness when the core is thick, and90°when the core isthin. The damage resistance can be improved by adopting ‘ex-situ’ tougheningtechnique. The morphologies of delaminating plies in the skin are similar to"dumbbell-shaped" and the axis of dumbbell-shaped is in the direction of the ply whichis near the interface. The smaller relative angle of the interfaces is, the smaller area ofdelaminating is. The breakage of fiber in the skin lead to a sudden decreasing of thevalue of the maximum load.Thirdly, simulated results of the curves of displacement and load agree well withthe experimental results, which prove the effectiveness of using finite element methodto study the damage resistance of foam-core sandwich structure composites.