Research On Mechanical Properties Of Shape Memory Alloy Bump

Adaptive morphing structure technology,which is considered as a key technology for nextgeneration aircraft has the potential to dramatically improve the aerodynamic performance of an aircraft over its flight envelope and expand the flight capability for different missions.Taking advantage of the adaptive morphing structure technology,an aircraft can adaptively change its aerodynamic shape according to flight conditions,so that the aircraft can always keep the best aerodynamic performance.As one kind of functional materials with both sensing and driving capacities,shape memory alloys(SMAs)can tranform heat energy into mechanical energy,which are the ideal materials for the adaptive morphing structures.The utilization of SMAs to change the aerodynamic shape of the aircraft has become an important research topic.Differs from the usual application as an driver in the adaptive structures,this thesis perfers to directly use the 3-D shape memory alloy bump as an adaptive morphing structure.Thanks to the two-way shape memory effect(TWSME)of SMAs,this kind of adaptive structure has the ability to spontaneously realize the deformation between the high temperature configuration and the low temperature configuration under the changing ambient temperature.In this thesis,the thermodynamic responses of the SMA adaptive bump structures are analyzed from the aspects of mathematical modeling,numerical simulation,structural preparation and experimental test.The mian works and innovations of the thesis are summarized as follows:1.Phenomenological modeling on thermomechanical response of SMA strip.(1)Based on the energy dissipation and the change of internal energies during phase tranformation in SMAs,the energy dussipation equation of SMAs is developed,and then the relationship between the heat flux excitation and the temperature during phase tranformation in SMAs is obtained.(2)Transformation expansion tensor is defined to describe the relationship between transformation-induced deformation and the temperature.With this definition,the constitutive relationship of SMAs can be simplified and has a form similar to the thermal expansion effect.(3)Based on the principle of virtual work,a 3-D 8-node isoparametric SMA element is developed and the finite element(FE)analysis of the thermo-mechanical coupling problem of SMA structure is realized.(4)The developed subroutine enables the numerical simulations both of the thermal responses and the deformation of SMA actuators by using finte element method(FEM).Experimental results are provided and used to verify the proposed model.2.Modification of the 3-D SMA constitutive model and the implementation as the user material subroutine.(1)Based on the thermodynamic potential function of SMA,the deduction of the 3-D SMA constitutive model proposed by Byod-Lagoudas is revisited.(2)Combined with the basic properties of thermoelastic martensitic phase transformation,the constraint condition of the phase transformation tensor of SMAs is given in this thesis.The form of the maximum transformation strain for 3-D SMAs is also redefined.(3)Based on the displacement increment method,the continuum tangent modulis tensor of SMAs is deduced.Then the modified constitutive model of SMA is implemented into ABAQUS by using user material subroutine.3.Thermomechanical training of TWSME of the SMA bump under constraint boundary condition.(1)Based on the traditional training methods and the characteristics of the bump's deformation,a constant deformation cycling training method for 3-D SMA structure is proposed.(2)A set of control devices for the thermo-mechanical training of 3-D SMA bump is designed and developed.The devices are composed of loading-constraint part and the temperature control system,which dramatically reduces the difficulties of the bump's training and shortens the training time.(3)With the help of the proposed training method and the developed control devices,the TWSME thermo-mechanical training process of the SMA bump is completed and an SMA bump,which has a flat shape in high temperature and swells up under cooling,is obtained.4.Modeling of the maximum transformation strain and the study on the thermo-mechanical response of SMA bump under uniform/non-uniform distributed temperature field.(1)Based on the FE anslysis results and experimental results,the mathematical model of the maximum transformation strain of SMA is established.(2)FE analysis on thermo-mechanical behavior of SMA bump is carried out.The model was validated with the experimental results and a good agreement was obtained.(3)The ability of the two-way shape memory deformation of the trained SMA bump is tested and the influences of the external loading on the thermo-mechanical response of the trained SMA bump are also studied.(4)With the help of the low-speed wind tunnel,the influence of the low-speed flow field on the temperature distribution on the SMA bump was studied.The thermo-mechanical response of the SMA bump under the complex temperature fields is also studied numerically.5.Focusing on the local shock wave drag redcution of the supercritical airfoil,a 3-D adaptive shock control bump(SCB)based on SMA is proposed.(1)A 3-D SMA based adaptive SCB is designed for a supercritical airfoil.(2)FEM is used to study the thermo-mechanical responses of the designed SMA bump when thermally activated.Thanks to the initial prestrain of the SMA material and the negative pressure condition of the shock region,the SMA bump can swell up to form a bump during austenitic phase transformation.(3)The dependence of aerodynamic characterisitcs of the airfoil on the height of the SMA bump in certain range of angles of attack(AOAs)is investigated using computational fluid dynamics(CFD)method.(4)To optimize the bump height and the corresponding driving temperature under variable AOAs,FEM-CFD co-simulation method is adopted.The aerodynamic performances of the adaptive SMA bump in the case of variable AOAs are discussed and compared with those of the datum airfoil.