Macroscopic Mechanical Properties Of Shape Memory Alloy And Reinforced Composites

Due to the unique shape memory effect and remarkable super-elastic property,shape memory alloy(SMA)has been widely used in engineering fields,such as aerospace,transportation,marine structure,and biomedical.With the development of manufacturing technologies and processing,SMA is usually embedded into various matrixes in the shape of particle,fiber,trips,slice,and so on.The overall strength,toughness,and damage impact resistance of composite can be significantly enhanced,thus it has been considered as shape memory alloy composite(SMAC)or intelligent composite.However,the weak interface between SMA-fiber and matrix will affect the macroscopic mechanical properties of composite due to the discrepancy in compatibility of material.Therefore,the macroscopic mechanical properties of SMAC,particularly the effect of interphase between SMA fiber and matrix,are investigated in present thesis using the theoretical analyses,experimental tests,and finite element analysis methods.The main contents is briefly described as follows(1)The thermo-mechanical behavior of traditional SMA(Ni-Ti alloy)subjected to cyclic loading is investigated,and a new concept of shape memory residual factor is proposed.The shape memory residual factor evolution of super-elastic Ni-Ti alloy fiber is studied through the uniaxial cyclic loading tests.The effects of strain-rate,ambient temperature,and number of cycles are discussed on residual strain,elastic modulus and phase transformation critical stress.A one-dimension macroscopic constitute model is constructed to predict the cyclic mechanical behavior of traditional SMA.(2)The shape memory behavior of magnetic SMA(Ni-Mn-Ga single crystals)is investigated under the stress-temperature-magnetic coupled loading.Based on the thermodynamic approach,a one-dimension constitute model is derived considering the effect of residual magnetic shape memory strain on martensite variant volume fraction.The effect of temperature is discussed on the evolution of magnetic domain volume fraction,magnetic rotation angle,and magnetization behavior.In addition,the effect of temperature on the magnetic-field-induced shape memory behavior of Ni-Mn-Ga single crystals is studied through the experimental tests.The validation of present model is verified by the comparison between theoretical and experimental results.(3)A three-phase-cylindrical pull-out model is proposed considering the effect of interphase between fiber and matrix.Stress function method and principle of minimum complementary energy are utilized to obtain the stress distribution within present model,and the critical energy release rate of interphase is derived through fracture method.The pull-out tests of single Ni-Ti fiber reinforced epoxy matrix are conducted to determine interfacial debonding critical distance.The experimental results are compared with the theoretical results calculated by present model and finite element simulation results in literature.(4)Based on the interfacial stress transfer theory and modified mixture rule,the macroscopic mechanical constitutive model and effective modulus of SMA-short-fiber reinforced elastic-plastic matrix composite are obtained.Through the uniaxial tensile tests,the overall strength and fracture elongation of unidirectional random distribution Ni-Ti short fiber reinforced epoxy matrix composite are studied.The validation of proposed model is verified by the comparison between theoretical,experimental,and finite element simulation results.(5)Based on three-phase-cylindrical shear lag model,the macroscopic mechanical constitutive model and effective modulus of SMA-long-fiber reinforced hybrid composite is obtained.The effects of material parameters on the macroscopic mechanical behavior of composite are discussed,including interfacial thickness,fiber slenderness ratio,and fiber volume fraction.Long Ni-Ti fiber reinforced glass/resin hybrid specimens with various embedded number are fabricated by vacuum assisted resin injection(VARI).Through uniaxial tensile tests and observing by scanning electron microscopy,the overall strength,fracture elongation,and failure morphology of composite are investigated.Finite element method is utilized to evaluate the effect of interphase on the macroscopic mechanical properties of composite.The simulation results are compared with theoretical and experimental results.