| As a new type of structure-function integration composites developed in recent years,aluminum foam-polymer composites exhibit an extensive application prospect in the field of aerospace,transportation,civil engineering and military defense due to their good performances in high specific strength,high specific stiffness,and excellent energy absorbing capacity.The majority of research are reported on the composites under monotonic loading,there are quite few studies on the mechanical behavior of the composites under cyclic loading at present.In addition,the deformation ability of the composites under compression loading is obviously superior to that under tensile loading.Therefore,the composites will be more suitable to work in the compression status than in the tension status.However,up to now,the investigation on the compression properties of aluminum foampolymer composites is still in its infancy,and there are many problems to be solved.Given this,aluminum foam-polymer composites were manufactured by pressureless infiltration technique in this thesis,using open-cell aluminum foam as the matrix and two types of polymers,namely polyurethane and epoxy resin.The monotonic and cyclic compressive properties of the fabricated aluminum foampolymer composites were studied.The energy dissipation mechanism of aluminum foam-polymer composites was explored,and the phenomenological constitutive relationship of aluminum foampolymer composites was established.Also,the axial and transverse compression performances of thin-walled square steel tube filled with aluminum foam-polymer composites(composites steel tube members for short)were examined by experimental investigation and numerical simulation.The main work and results are as follows:(1)Experimental study on the monotonic compression properties of aluminum foam-polymer composites.The effects of polymer types,polymer coating layer and matrix density on the monotonic compression performances of the composites were discussed,and the deformation failure mechanism of the composites was revealed.The results show that the filling of polymer in the pores has changed the collapse failure mechanism layer by layer of aluminum foam.The aluminum foam-polyurethane composites exhibit shear damage failure mechanism,while the aluminum foam-epoxy resin composites exhibit brittle failure behavior.The monotonic compression process of the two kinds of composites can be divided into three stages: the elastic stage,the plastic plateau stage with a small fluctuation of stress and the densification stage.As the compressive strength of epoxy resin under the monotonic loading is significantly higher than that of polyurethane,it can withstand greater stress in the aluminum foam matrix.Thus,the enhancement effect of the epoxy resin on the plateau stress,energy absorption and specific energy absorption of the aluminum foam is greater than that of the polyurethane.Compared with pure aluminum foam,aluminum foam-polyurethane composites are more suitable for energy absorption applications under low stress and large strain conditions,while aluminum foam-epoxy resin composites are suitable for energy absorption under high stress and large strain conditions.The maximum values of energy absorption(ε=0.5)of the two composites were 9.88MJ/m3 and 46.05 MJ/m3,respectively,and the maximum values of specific energy absorption were6.41 J/g and 31.09 J/g separately.(2)Experimental study on the cyclic compression properties of aluminum foam-polymer composites.The influences of polymer types,polymer coating layer and matrix density on the deformation recovery and reversible energy dissipation capacities of the composites were investigated.The results show that both polyurethane and epoxy resin can improve the deformation recovery and reversible energy dissipation properties of aluminum foam.Polyurethane coating on the test surface perpendicular to the expected loading direction can significantly improve the deformation recovery and reversible energy dissipation performances of the composites in the elastic stage and the initial plastic plateau stage.The obvious enhancement of energy dissipation capacity is mainly related to the introduction of a new energy dissipation mechanism,namely polyurethane recoverable extrusion.However,epoxy resin failed to produce the same beneficial effects as polyurethane.The damping value of the composites increases with the increase of the thickness of polyurethane coating,and the maximum value is 0.333.In addition,the composites specimens can maintain good structural integrity after the cyclic compression test.(3)Experimental study on the low cycle behavior of aluminum foam-polymer composites.The aluminum foam-polymer composites with polyurethane coating layer on the top is taken as the research object.The impacts of cycle numbers and strain amplitude on the low cycle characteristics were analyzed.The results show that the recovery strain,tangent modulus,and peak stress of the composites decrease first,and then tend to be stable with the increase of the number of cycles,while the variation of the loss factor is affected by the strain amplitude and the type of polymer in the pores.The high damping and high stiffness are simultaneously preserved in the composites after 50 loadingunloading cycles,the maximum loss factor in the steady-state can reach 0.354.(4)Analysis of energy dissipation mechanism of aluminum foam-polymer composites.Based on the experimental data and the cyclic load-unloading deformation process of the composites,the energy dissipation mechanism of the composites was analyzed.Experimental analysis results show that the energy dissipation of aluminum foam-polyurethane composites is the outcome of combined action of many kinds of energy dissipation mechanism,including the intrinsic damping of aluminum foam and polyurethane,interfacial slip damping,energy consumption caused by the polyurethane in and out of the connection holes,dislocation damping,energy dissipation due to staggered friction at the macro-crack interface,and polyurethane recoverable extrusion,etc.The contribution of various energy dissipation mechanisms of the aluminum foam-polyurethane composites is related to the interface bonding state of the two-phase material,the thermal expansion coefficient of the two-phase material,the presence of polyurethane coating on the surface of the sample perpendicular to the loading direction,the damage degree of the aluminum matrix,the strain amplitude and other factors.The energy dissipation of aluminum foam-epoxy resin composites is also the result of multiple damping mechanisms,including the inherent damping of aluminum foam and epoxy resin,dislocation damping,and friction energy dissipation between independent blocks,etc.The contribution of various energy losses to the energy dissipation of aluminum foam-epoxy resin composites is affected by the thermal expansion coefficient of the two-phase materials,the degree of material damage,strain amplitude and other factors.(5)Study on the phenomenological constitutive relation of aluminum foam-polymer composites.Based on Lemaitre’s strain equivalence assumption,a dimensional damage phenomenological constitutive model for the composites is established by combining the two-parameter Weibull distribution function with the phenomenological constitutive model characterizing the three stage characteristics of the porous materials under monotonic compression,and the general expression of the model is presented.Then,the constitutive equation of the composites under uniaxial cyclic compression is proposed on the basis of the cyclic compression experimental data.At last,a simplified hysteretic model is proposed according to the displayed characteristics of the hysteresis curves of the composites.The comparisons between the predicted results and the experimental results show that the proposed models are in good agreement with the experimental results.(6)Experimental and numerical study of the composites steel tube members under monotonic compression.The axial and transverse compressive properties of composites steel tube members were investigated by combining experimental and numerical simulation.The deformation failure mechanisms,load-displacement curves,and crashworthiness indexes of the members under axial and transverse compression were analyzed.The results show that a mixed deformation mode comprised of axisymmetric shrinkage and splitting failure were overserved for the composites steel tube members under axial compression,while the deformation failure model of the outer square steel tube under transverse compression was changed from the “dumbbell” shape to “runway” shape due to the filling of the composites.The mean crushing force of the composites steel tube members under the axial compression is obviously enhanced as compared with that in the empty steel tube members,the energy absorption(ε=0.5)increases by about 13.99%～83.42%,reaching(9.34～15.03)k J,and the average value of the crushing force efficiency reaches to 0.582～0.718,increasing by approximately19.02%～46.83%.When compared with the empty steel tube members,the mean crushing force and the energy absorption of the composites steel tube under the transverse compression improve by almost 1.43～6.81 times,the specific energy absorption is increased by about 0.35 ～ 3.39 times,the maximum value is 16.52 J/g,and the average value of the crushing force efficiency is 0.651 ～ 0.792,increasing by about 32.86% ～ 61.63%.The accuracy of the finite element model is verified by comparing the load-displacement curve,deformation failure mode,and performance index of the test and finite element simulation results. |
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