The Influence Of Thermal Environment On The Mechanical Properties Of Composite Pyramidal Lattice Sandwich Structure

Carbon fiber/resin matrix composite lattice sandwich structures could be for multi-functional integration while reducing the structural weight,contributing to promising application in the fields of aerospace,aircraft,ships and automobiles,etc.However,with the temperature sensitivity,the resin matrix characteristics would vary by physical and chemical reaction,leading to impact on the performance of the whole structures.In recent years,domestic and foreign scholars have conducted numerous researches on mechanical properties of composite lattice sandwich structure at room temperature and an atmospheric pressure,but there are few studies focus on mechanical behaviors under different temperature conditions,which is,although,a research subject with great scientific significance.This dissertation deals with the design,fabrication and investigation of the mechanical behaviors of carbon fiber/epoxy matrix composite lattice sandwich structures under various temperature conditions,including room temperature,high and low temperature,after thermal exposure as well as after vacuum thermal cycling,utilizing theoretical prediction and experimental methods.Aiming at increasing the bonding strength between the skins and the cores,and sequentially,enhancing the shear performance,this dissertation designs and fabricates a node-strengthened carbon fiber/epoxy matrix composite pyramidal lattice sandwich structures with water-cutting technique and inter-locking assembly method.The out-of-plane compressive modulus and strength are derived through theoretical analysis,and afterwards,through experimental verification on four relative density sandwich cores.During testing,the main failure modes of pyramidal lattice sandwich structures are buckling and crushing of truss struts.The shear properties including stiffness and strength are-investigated both theoretically and experimentally,respectively.The lattice materials failure models including strut buckling,strut crushing and skin delamination are obtained in both theory and testing.An experimental test is conducted to compare the shear properties of the nodestrengthened design with the conventional configuration on the specimens at the same relative density of 1.8%,and the results show that the node-strengthened design concept could increase shear load capability considerably by 136%.Besides,the outof-plane compressive and shear performance of the composite sandwich structures are coordinated and present better bearing capacity when compared with the materials fabricated by other techniques.In order to investigate the influence of temperature on the failure mechanism and mechanical properties of lattice sandwich structures.The out-of-plane compressive and shear performance,including modulus and strength,with temperature of the carbon fiber composites pyramidal lattice sandwich structures is predicted by theoretical model and confirmed by experiments,respectively.According to the comparison between predicted curves and testing results,it is found that the theoretical model conform to the experimental response,and could reflect the regulation of property variation of the carbon fiber composites pyramidal lattice sandwich structures under different temperature.The effect of thermal exposure temperature and time on the out-of-plane compressive and shear behavior of carbon fiber/epoxy matrix composite pyramidal lattice sandwich structures are theoretically analyzed and confirmed on the basis of experimental tests on the post-exposure composite structures.The stiffness,strength as well as failure mechanism variation with exposure temperature and time for both compressive and shear are revealed.The studies show that fracture failure of the struts under compressive or shear loads would occur when the thermal exposure temperature is lower than 180℃,and the struts fail by buckling as well as delamination when the thermal exposure temperature reaches 230℃.The out-of-plane compressive modulus and strength,shear modulus and strength of the composite pyramidal lattice sandwich structures after vacuum thermal cycle are studied.The changing regulation with the vacuum thermal cycle increasing of the structural mechanical characteristics are derived by theoretical formulas and the validity of the analytical model is proved by experiments.The mass loss of the composite material after different vacuum thermal cycle times are measured to discuss the effect of high vacuum environment.And the results reflects that the outof-plane compressive and shear properties of the composite pyramidal lattice sandwich structures firstly increase and then decrease as the number of vacuum thermal cycle increases.