Mechanical Behavior Of 4D Printed Shape Memory Composites And Their Honeycomb Structures

Continuous fiber-reinforced shape memory polymer composites(SMPCs)are a class of smart composites that combine high mechanical properties and shape memory effects.They are prepared by doping continuous fibers into shape memory polymers(SMPs)and have promising applications in smart drives,flexible robots,and space deployable structures.However,specific metal molds and expensive process equipment are required for fabricating three-dimensional(3D)fiber-reinforced SMPC components with complex geometric configurations by conventional fabrication processes such as hand lay-up,resin transfer molding,and filament winding.Therefore,the development of an automated,low-cost,mold-free process for preparing continuous fiber-reinforced SMPCs is a current research hotspot in the field of smart composites.In this context,this paper proposes a4D printing method for continuous fiber-reinforced SMPC to provide a new way for integrated customization of high-performance SMPC.Anisotropic thermodynamic constitutive models are developed to predict the mechanical properties and intelligent deformation behavior of 4D printed SMPC and its complex structures.The mechanical design and integrated fabrication of multifunctional SMPC structures with high mechanical properties,high stretchability,lightweight characteristics and shape memory effect are also presented in the paper to provide guidance for practical applications of 4D printed continuous fiber-reinforced SMPC structures.Combining the fiber-matrix in-situ impregnating co-extrusion 3D printing process and thermoplastic SMP,a 4D printing method for continuous fiber-reinforced thermoplastic SMPC was proposed and used to prepare SMPC specimens with various fiber lay-up orientations and carbon fiber volume fractions.The 3D morphology of the SMPC specimens was characterized by X-rayμ-CT scanning,and it was observed that there were connected pores along the fiber direction inside the specimens,which could affect the mechanical properties of the SMPC specimens.Uniaxial tensile tests,compression tests,and three-point bending tests were performed to characterize the mechanical properties of 4D printed SMPCs.The results showed that the doping of continuous carbon fibers significantly improved the mechanical properties of SMPC.In particular,the SMPC possesses extremely high tensile modulus/strength,compressive modulus/strength and bending modulus/strength at a fiber lay-up angle of 0°.To characterize the shape memory effect of SMPC,thermo-mechanical cycling tests and electrically driven shape recovery tests were performed,and the results showed that 4D printed SMPC not only possesses thermally induced shape memory effect,but also the continuous carbon fibers in SMPC can serve as electrically heated elements to achieve their self-driven shape recovery under the applied electric field.To predict the anisotropic thermodynamic properties and shape memory behavior of SMPC,3D linear viscoelastic constitutive model and nonlinear thermodynamic constitutive model of SMPC are proposed from the perspective of micromechanics and continuum mechanics,respectively.The linear viscoelastic model is established by combining the multi-branching constitutive theory of SMP and the Mori-Tanaka method,and the imperfection of the fiber-matrix interface is considered by using the modified Eshelby inclusion principle.In addition to the two interface parameters,a damage parameter D_p is included in the linear model,which is able to integrate the effects of porosity and interface defects on the viscoelastic stiffness of SMPC.The nonlinear thermodynamic constitutive model is developed in the framework of the second law of thermodynamics,incorporating the three-phase assumption of SMP and proposing a new equivalent phase evolution equation that includes viscoplastic and rate-dependent effects.Both models were applied in the commercial finite element software Abaqus by writing the user material subroutine Umat,thus providing a convenient theoretical simulation tool.The linear viscoelastic model can effectively predict the tensile modulus and shape memory cycle of 4D printed SMPC over a linear viscoelastic range.The nonlinear thermodynamic model extends the prediction range from linear viscoelasticity to nonlinear viscoplasticity and is able to reproduce the rate-dependent and viscoplastic yielding phenomena of SMPC.However,the nonlinear model takes into account the viscoplastic flow of SMP,so the computational process requires multiple iterations,resulting in lower operational efficiency than the linear model.Based on the proposed 4D printing method for continuous fiber-reinforced SMPC,SMPC honeycomb structures with both high mechanical properties and high stretchability were designed and prepared.The fiber cross-layering strategy was used to eliminate the out-of-plane deformation of the honeycomb structures during in-plane stretching,which can meet the demand of stretchable substrates with high mechanical properties and smooth deformation surfaces for flexible electronic devices.By customizing the fiber crossing angle,it is able to achieve tunable tensile modulus and stretchability.The uniaxial stretching and shape memory behaviors of stretchable SMPC honeycomb structures were simulated using the established anisotropic constitutive model of SMPC,and the results showed that the model can effectively predict the thermomechanical properties of the structures,thus providing theoretical support for the mechanical design of 4D printed complex SMPC structures.A variety of SMPC lightweight honeycomb structures with different cell configurations were designed and fabricated by 4D printing,and carbon fibers were continuously distributed along the honeycomb wall to enhance the structural mechanical properties.An analytical model for the mechanical properties of SMPC honeycomb structures under in-plane bending was derived,two-dimensional failure mechanism diagrams were drawn,and several sets of typical three-point bending specimens were designed in conjunction with failure mechanism diagrams,and failure modes such as face yielding,face buckling and core shear failure were observed from three-point bending tests.The out-of-plane/in-plane compression tests of SMPC honeycomb structures with different relative densities and cell inclination angles were carried out to obtain the compression strength and quasi-static energy absorption characteristics.Finally,the shape memory effect of SMPC honeycomb structures was evaluated by thermal-mechanical cycling tests,and the results showed that 4D printed SMPC honeycomb structures possessed high shape fixity ratio and shape recovery ratio,and thus demonstrated great application prospects in intelligent lightweight structures with active deformation capability and high mechanical property requirements.