Preparation And Mechanical Properties Of Titanium Corrugated Sandwich Structure

The titanium corrugated sandwich structure is being widely used in the field of aerospace,marine,automobile,building,etc,for its light-weight,high specific strength,high specific rigidity,high temperature resistance,excellent corrosion resistance,high capacity of energy absorption and potential multi-functionality,and is regarded as the frame structure of a new heat insulation/loading bearing integration thermal protection system for Reusable Launch Vehicle(RLV).The heat insulation is realized by heat insulating material filled in the space of the corrugated core,while the titanium corrugated structure is the primary load-bearing component.It is therefore of great importance to investigate the mechanical properties of the titanium corrugated sandwich structure.The investigation on the preparation and mechanical properties of the titanium corrugated structure,the frame structure of a new heat insulation/loading bearing integration thermal protection system for Reusable Launch Vehicle,was conducted in this paper.Firstly,investigations were conducted on the preparation method of the titanium corrugated sandwich structure,including the FE simulation of the preparation of corrugated core and the optimization of the brazing process between the corrugated core and facesheets.Then,the equivalent elastic constants were computed by analytical and FE methods.Next,the static mechanical properties of titanium corrugated sandwich structure,including flat compression,in-plane shear and three-point bending,were studied experimentally and numerically,and the influence of the layer number and configuration of corrugated core on the the static mechanical properties were mainly discussed.Finaly,the dynamic response of the titanium corrugated sandwich structure was investigated when subjected to Split Hopkinson Pressure Bar impact and low-velocity falling impact.The main conclusions were as follows:1)Finite element simulation method was adopted to study and compare the advantages and disadvantages of bulging formation and cold rolling.In view of the uniformity and reduction of wall thickness,cold rolling method was demonstrated the best one to fabricate the titanium corrugated core.Additionally,the brazing process,a technique to connect the corrugated core and the blank,was optimized by varying the brazing time under the permanent temperature of 870℃.The influence of brazing time on the microstructure,micro-hardness,shear strength and failure mode of the brazed joint was studied at the brazing time of 5,10,20 and 30 minutes respectively.The results showed that the brazing time had little influence on the micro-hardness of brazed joint.When the brazing time was short,such as 5 min,a reaction layer appeared in the central area of the brazed joints,which would cause the debonding of the brazed joint under shear loading.Long brazing time would result in the fracture occurred within the base metal of CP Ti due to the excessive growth of grains of the base metal.As a consequence,the temperature of 870 C and time of 10 and 20 min were considered to be the most appropriate for reducing the brittle intermetallic compounds in the brazed joint,as well as for controlling the excessive growth of grains of base metal.2)An orthotropic representation for a Reissner-Mindlin formulation of the equivalent core is proposed,based on the geometric characteristics of the sinusoidal corrugation,including equivalent shear modulus Gxz,Gyz,Gxy,equivalent elastic modulus Ex,Ey and equivalent density ρeq.Meanwhile,a finite element method was also adopted to compute the equivalent shear modulus and equivalent elastic modulus of the titanium corrugated core.The effects of corrugated core thickness,amplitude of the corrugated core and the half-period length of the corrugated core on the equivalent shear modulus,equivalent elastic modulus and equivalent density were investigated using both methods.The results were compared and found to be in good agreement with each other.Two equivalent three-point bending finite element models were established based on the results obtained from analytical and numerical method respectively,and a 3D three-point bending finite element model was also set up with fully detailed 3D geometry represented.The bending rigidities derived from the two equivalent models are consistent with that derived from the 3D model,demonstrating the accuracy and rationality of the proposed equivalent theory.3)For flat compression,the failure mode of a single-layered titanium corrugated structure was firstly numerically investigated,the results was compared with that of experiment.Compression experiments were carried out on different titanium corrugated structures to explore the influence of corrugated core layers and configuration direction on the compression properties and failure modes.An initial imperfection factor was introduced to the numerical model of titanium corrugated structures with different layers and configuration directions of corrugated core.The load-displacement curves as well as failure modes obtained from the numerical model were found to be agreed well with that of experiments.For in-plane shear,the failure mode of single-layered titanium corrugated structure in MD and CD direction was acquired numerically.The in-plane shear tests were then conducted on the titanium corrugated structure in MD and CD direction.The failure mode of the titanium corrugated structure in MD direction obtained from experiment in line with numerical results,but the load-displacement curves showed great discrepancy.While the numerical and experimental results in both failure mode and load-displacement curve exhibited distinct difference,even after introducing initial imperfection factor to the numerical model.For three-point bending,the failure mode of single-layered titanium corrugated structure in MD and CD direction was investigated numerically,and the span was found to have great influence on the shear effect in MD direction.However,the shear effect of three-point bending in CD direction could be ignored.Experiments were conducted to study the influence of face sheet thickness,corrugated core layers and corrugated core direction on the three-point bending properties of titanium corrugated structures.In addition,the numerical method was employed to investigate the influence of face sheet thickness,corrugated core thickness and height-length ration of corrugated core on the maximum bending stiffness of the titanium corrugated structure.4)The compressive stress-strain behaviors of six different titanium corrugated structures under four high strain rates revealed obvious strain rate effect.The Perzyna empirical constitutive models were established to describe the stress-strain behaviors under high strain rates of six structures.The discreteness and predictive accuracy of the developed models were evaluated and compared with each other,of which the model of MD/CD structure showed greatest accuracy,while that of MD/MD-A structure was worst.The stress-strain behaviors of MD/CD and MD/CD/MD structures were analyzed and found not to conform to the Johnson-Cook model.The absorption energy per unit volume of different structures were compared with each other,and the results indicated that those structures,of which the adjacent corrugated cores laid out in perpendicular directions,had high energy absorption capacity.Drop-weight tests were performed into the MD/CD and MD/CD/MD structures under the velocity of 2,4,and 6m/s,the peak force and energy absorption of the two structures were evaluated.Furthermore,the dynamic response of MD,MD/CD,MD/CD/MD structures under 2,3,4,5,6m/s were numerically investigated.Energy absorption of different structures as well as each part under different impact velocities were evaluated and compared.The peak forces obtained from analytical and numerical method showed slight difference due to the ignorance of strain hardening effect in the analytical method.However,the maximum deflection predicted by analytical method agreed well with numerical results.Experimental results and numerical results exhibited obvious difference in peak forces while coincided well with each other in energy absorption.