Research On Mechanical Behavior Of Volcanic Rock Particle/Magnesium Syntactic Foams And Their Sandwich Structures

With the increasing prominence of environmental pollution and energy shortage,countries around the world regard high-performance lightweight materials as an important solution in the selection of engineering materials.Metal foam sandwich is a "sandwich" structure composed of metal foam core and high-performance face sheets.This sandwich structure can be used in aircraft wings,car roof covers,high-speed rail carriages and other equipment components,which can not only reduce weight,but also absorb energy.In recent years,high-end equipment components in aerospace,rail transit,automobile industry and other fields have higher requirements for sandwich structure properties.In the selection of metal foam core,it is required to have higher specific strength and specific energy absorption capacity under large loads.Magnesium matrix syntactic foams have the advantages of light weight,high specific strength,high specific stiffness,good damping capacity and excellent energy absorption capacity,which has great development potential.However,magnesium matrix syntactic foams usually use cenospheres with "thin-walled hollow structure" as filler particles,which often results in magnesium melt filling hollow cenospheres during preparation,leading to the reduction of porosity and energy absorption characteristics of syntactic foams.It has become an urgent problem and a hot spot for research at home and abroad.To solve the above problems,two types of porous volcanic rock particles(scoria particles and pumice particles)were selected as filler particles,and scoria/Mg syntactic foam(SMSF)and pumice/Mg syntactic foam(PMSF)were prepared by pressureinfiltration method.The problem of melt-filled particles is effectively solved by using the barrier effect of the multilayer pore walls of the volcanic rock particles on the magnesium melt.The mechanical properties of the two types of syntactic foams were investigated,and subsequently they were used as core materials to combine with carbon fiber reinforced plastics(CFRP)skins to prepare new magnesium syntactic foam sandwich structures.The internal relationship between specimen structure and properties is given by exploring the core material structure,the nickel layer deposited on the surface of pumice particles,and the stacking sequence of CFRP skins.The composition of CFRP/magnesium syntactic foam sandwich structures was optimized through systematic experiments,and the mechanism and scientific problems were discussed.The main results are as follows:1.The microstructures of two types of volcanic rock particles and volcanic rock particle/magnesium(VR/Mg)syntactic foams was investigated.The results show that both scoria and pumice particles are porous structures.Compared with scoria particles,pumice particles have higher porosity,smaller pore size and lower crushing strength.The multilayer pore walls of volcanic rock particles can effectively prevent magnesium melt from filling the pores inside the particles.Both types of VR/Mg syntactic foams have good structural integrity and high porosity.The porosity of scoria/Mg syntactic foam and pumice/Mg syntactic foam is 25.5% and 49.5% respectively.2.The effects of particle properties and test temperature on the compressive properties and energy absorption characteristics of VR/Mg syntactic foams were revealed.High strength particles are beneficial to the compressive strength and energy absorption capacity of syntactic foam.As the test temperature increases,the compressive strength and energy absorption capacity of syntactic foams gradually decrease.The deformation behavior and deformation mechanism of syntactic foams at room temperature and elevated temperatures were revealed.At room temperature,scoria/Mg syntactic foam exhibits shear failure mode,and pumice/Mg syntactic foam exhibits mixed failure mode.As the temperature increases,the two types of syntactic foams gradually tend to undergo plastic deformation.The theoretical formula is derived to predict the compressive strength of VR/Mg syntactic foams,and the predicted results are well in agreement with the experimental results.3.CFRP/Mg syntactic foam sandwich structures were prepared.The effects of core structures and properties on mechanical properties of foam-cored sandwich structure was investigated,and the failure mechanism was discussed.Compared with the traditional metal foam core,the magnesium syntactic foam cores have numerous micropores,which increase the contact area between the cores and the skins and is conducive to improving the interface bonding strength.It is clarified that the structure and properties of the core materials can significantly affect the initial failure mode,the severity of interfacial debonding and the path of crack propagation at the interface,thus affecting the flexural properties of sandwich structures.When pumice/Mg syntactic foam is used as the core material,the interfacial debonding of the sandwich structure is inhibited and the specimen exhibits higher specific flexural strength and specific energy absorption capacity.4.The effects of nickel coatings with different thicknesses deposited on the surface of pumice particles on syntactic foams and their sandwich structures were studied,and the failure mechanism of sandwich structures was also discussed.The nickel coating on the surface of the pumice particles exhibits a typical "cauliflower" morphology,and the phase composition is a mixture of microcrystalline and amorphous nickel.There is a close bond between coatings and particles,and the thickness of the coating increases with the increase of plating time.Depositing nickel coating on the surface of pumice particles can effectively improve the mechanical properties of syntactic foams and their sandwich structures,and the mechanical properties gradually increase with the increase of coating thickness.However,the deposition of nickel coating on the surface of pumice particles will not change the failure mode of sandwich structures.5.Based on the selection of nickel-coated pumice/Mg syntactic foam as the core material,the effect of stacking sequence of CFRP skins on the flexural properties of sandwich structures was investigated,and the corresponding failure mechanism was discussed.Compared with unidirectional(0° single angle)CFRP laminates,multidirectional CFRP laminates with different ply angles(30° and 45°)not only have higher flexural properties,but also effectively reduce the anisotropy.CFRP skins with different stacking sequences have a significant influence on the flexural properties of sandwich structures.Generally,the flexural properties of sandwich structures are positively correlated with those of CFRP skins.