| Composites structures with complex-curved shapes have attracted wide attentions in aerospace and industry transportation fields due to the advantages of high strength and stiffness ratio,light weight and easy manufacturing process in large areas.The mandrels for fabricating composites with complex-curved shapes were usually designed by the requirement of structures.Nowadays,the mandrels mainly included: multi-piece metal mandrel,water-soluble mandrel and inflation mandrel.Multi-piece metal mandrel owned high weigh,long work-cycle,unique assembly and disassembly sequence and complicated demoulding process.Water-soluble mandrel was one-time,the material need curing,drying,second-dealing before application.The mandrel need be washed after curing structures.As to inflation mandrel,the stiffness was low and the composites would be failure when the mandrel was leak.Considering these disadvantages of traditional mandrels,a kind of shape memory polymer(SMP)mandrel was developed and the feasibility was verified for future composite structures with complex-curved shapes.SMP mandrels for composites with complex-curved shapes were developed based on the good variable stiffness and shape memory properties.SMP mandrel was in the rubber state and the modulus was low under the temperature was above glass transition temperature(Tg).The mandrel was easily deformed into temporary shape under inner pressure,and the deformed shape can be hold with decreasing of temperature and removing of inner pressure.In addition,the mandrel was in the glass state and the modulus was high under room temperature.The mandrel can bear outer pressure effect for fabricating composite structures.When the structure was cured,the mandrel was heated to recover the initial shape to realize demoulding of mandrel.In order to verify the feasibility of SMP mandrels for composite structures with complex-curved shapes,the detailed work was carried out for thermo-mechanical properties characterization of SMP,design and assessment of SMP mandrels for composites structures.Firstly,the comprehensive thermo-mechanical properties of two types of styrene-based SMPs with different glass transition temperatures were characterized.The dynamic mechanical properties,static mechanical properties,deformation behavior for shape memory process,deformation behavior for isothermal condi tion and thermal strain were detailed studied by dynamical mechanical analyzer,static mechanical test machine and temperature controller systems,etc.The thermo-echanical properties,including glass transition temperature,storage modulus distribution for widely frequency range,elastic modulus and critical strains for different temperatures,coefficient of thermal expansion for different temperature,shape fixity and recovery ratios for tensile deformation,were all discussed in our work.Meantime,the effect of frequency/heating rate on glass transition temperatures,temperatures on shape recovery process,were all also studied.Moreover,SMP tubes were fabricated and the shape memory effect under tensile,compression,bending and torsion deformations were characterized.All the thermo-echanical experiments of SMP samples can provide some necessary material parameters for Finite Element Method(FEM)simulation and fabrication process of SMP mandrels.Then,the critical deformation ability of SMP mandrel was simulated based on low elastic modulus and good deformation property under high temperature.Meanwhile,the critical bearing load ability was also be investigated by analyzing the buckling damage of SMP mandrel model under low temperature.In addition,the master curve of storage modulus for two types of styrene-based SMPs in widely frequency range(f=10-3~ 104 Hz)were obtained based on different temperature/work frequency(f=0.1Hz~100Hz)scan curves and shifting of curves in horizon.The mechanical parameters of SMPs were get by fitting master curve of SMPs based on least square method.The SMP sample with 50% stretching ratio was used to verified the feasibility of Gerneralized Maxwell model for shape memory process simulation.In addition,the SMP mandrels for composite structures with complex-curved shapes were designed.The Gerneralized Maxwell model owning 11 relaxation modulus and times was put into finite element method(FEM)software ABAQUS 6.12 to simulate the shape memory process of three types of SMP mandrels(bottle-shaped mandrel,S-shaped mandrel and star-shaped mandrel).The simulated results can provide deformation temperature and pressure effect for fabrication process of SMP mandrels.Finally,three types of SMP mandrels for composite structures with complex-curved shapes were fabricated after charactering thermo-echanical properties and simulating the shape memory process of SMP mandrels.The high shape fixity ability,good surface accuracy and stability,easy demoulding and good reusable ability of SMP mandrel were analyzed.The bottle-shaped SMP mandrels with different diameters deformation(25%~100%)and S-shaped SMP mandrel with complex-curve shape were fabricated by inflating deformation,cooling and unloading for SMP tubes.The star-shaped SMP mandrel can be fabricated by mechanical mould pressing technology due to the complicated surface of mandrel.The laser measuring distance machine combined with temperature controller system was used to investigate the shape fixity ability of SMP mandrels under different temperature.Meanwhile,a moving-part support was added to analyze the surface accuracy and stability of SMP mandrel during 30 days.Moreover,the composite structures with bottle-shaped,S-shaped and star-shaped surface were fabricated by filament winding or curing technologies and the good demoulding behavior was demonstrated after curing and heating above Tg.In addition,the good reusable properties of SMP mandrels within deformation ranges were studied by inflating-recovering cycle test of SMP mandrels until SMP mandrels fracture.In sum,the work detailed characterized the thermo-echanical properties of two types of styrene-based SMPs,simulated the shape memory process of SMP mandrel by ABAQUS,fabricated three types of SMP mandrels for composites structures with complex-curved shapes.These results can provide a useful guide for smart manufacturing technique of composites structures with complex-curved shapes in the future. |
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