Buckling Behavior Of Shape Memory Polymer Composites And Their Structures

At present,shape memory polymer composite(SMPC)has been widely used in the field of aerospace because of its shape memory effect and excellent mechanical properties.However,it is difficult to manufacture continuous fiber-reinforced SMPC laminate with highly parallel fibers and adjustable layering angles.And the micro and macro-buckling behaviors of SMPC and its structures during the compressive process have not been analyzed systematically.In addition,the application of SMPC tends to function components,and the research on bearing structure is not in-depth.In this context,the unidirectional fiber reinforced shape memory epoxy prepreg is manufactured by the technology of two-step automated tape laying technology first of all.And the unidirectional carbon fiber and glass fiber reinforced SMPC laminates are prepared by autoclave forming process.The compressive moduli and viscoelastic parameters of SMP at different temperatures are determined through uniaxial compression and stress relaxation experiments;the elastic constants of SMPC laminates at different temperatures are obtained by uniaxial tensile test.Besides,the glass and carbon fibers reinforced interlayer hybrid SMPC laminates with different hybrid configurations are manufactured.The mechanical properties and shape memory performance can be adjusted by controlling the hybrid ratios and fiber stacking configurations instead of modifying the resin matrix,which has certain guiding significance for the design of SMPC laminate.The above research provides the data basis for the subsequent finite element simulation and theoretical calculation,and also provides the material basis for the preparation of SMPC cylindrical shells.Secondly,the micro-buckling models of single-fiber and multi-fiber reinforced SMPCs under uniaxial compressive load are established,respectively.The effects of temperature(modulus ratio),fiber diameter,fiber volume fraction and Poisson’s ratio of resin matrix on the critical buckling wavelength,critical buckling strain and nonlinear buckling stress-strain response are investigated.Besides,the classical elastic-viscoelastic correspondence principle is established to solve the evolvement rule of the micro-buckling wavelength of single carbon fiber and carbon nanofiber reinforced SMPCs with temperature and time.On the basis of the above research,a design idea for pseudo-ductile composites based on continuous “micro-buckling(wavy)shaped” brittle fiber-reinforced polymer composite is proposed,and the equivalent mechanical properties of this type of composite material are calculated using the energy method.The 2D and 3D damage mode maps of this material set are obtained,and the expressions of the key parameters,such as the optimum thickness,maximum pseudo-ductile strain and maximum yield stress are calculated based on these damage mode maps.The mechanical models in this chapter explain the nonlinear mechanical response of SMPC during axial compressive process from the perspective of fiber micro-buckling and expand their practical application.Next,the macroscopic buckling model of SMPC laminate with low fiber volume fraction under initial biaxial pre-strains is established.The influences of biaxiality of the pre-strains,temperature and fiber laying angle on the buckling wavelength,critical buckling pre-strain and buckling amplitude are calculated.In addition,the global buckling and wrinkling behaviors of the SMPC sandwich structure under axial compressive load at different temperatures are investigated.During the analysis,the attenuation function of displacement in the inner-core is obtained by Euler-Lagrange equation.The variations of critical values(the critical buckling stress and buckling wavelength)and the associated buckling modes(global buckling and wrinkling)with modulus ratio,thickness ratio,fiber ply angle and temperature are obtained by the principle of minimum energy,and the accuracy of the theoretical model is verified by finite element analysis.These models are used to investigate the buckling behavior of SMPC laminate with low fiber volume fraction and SMPC sandwich structures at the macro-buckling level.Finally,the geometric imperfection sensitivity of SMPC cylindrical shell is systematically studied,and the effects of initial geometric imperfections on the buckling loads and post-buckling patterns of SMPC cylindrical shells at different temperatures are analyzed by three numerical simulation techniques.The results demonstrate that the buckling loads are sensitive to temperature,while the knockdown factors are insensitive to temperature.The SMPC cylindrical shells with the fiber stacking configuration of and the fiber winding angle of are manufactured by autoclave forming process and wet fiber winding forming process,respectively,which are used in the auxiliary vibration reduction structure and smart releasing device.The locking capacity,long-term bearing capacity,and shape recovery property are systematically tested.The above research can provide guidance for the preliminary design of the buckling loads and practical applications of the SMPC cylindrical shell bearing structures.