| Composites are promising candidates for realizing light-weight and high-speed design of aircraft.They possess high specific mechanical properties,such as stiffness,strength,resistance to fatigue and chemical corrosions.These noteworthy mechanical properties come from properties and arrangements of inhomogeneities within the composites.Micromechanical study can predictive mechanical properties of composites from properties and shape of their constituents.Interphase is a region between inhomogeneities and matrix,furthermore strain within this region is often severely changed.In this way,its deformation and damages are important to micromechanical study on composites.However with developments of material and manufacturing,lots of composites are modified in interphase or much smaller scale to improve their properties.When modeling these kinds of composites,inhomogeneities in different scales cannot coexist in a uniform model.Furthermore the functional graded interphase is not efficiently expressed in models.When solving these models,explicit expression of effective properties cannot be obtained.Meanwhile most of the micromechanical models aimed at linear elastic properties of composites,however the nonlinear responses of composites are essential to safety of aircraft.To this end,investigation of trans-scale micromechanical model on composites with functional graded interphase is a significant but challenging area in manufacturing of aircraft.To deal with the above difficulties,this paper aimed at establishing a novel,accurate and efficient trans-scale modeling and homogenization scheme.Begin from study on functionally graded interphase,the novel method can take influences between functionally graded interphase and all other inhomogeneities into consideration.Then the scheme was extended to non-linear domain to study the rate-dependent and visco-plastic properties of composites.Following researches were done and corresponding conclusions were gotten:1)A trans-scale sequentially homogenization scheme was established.Trans-domain between matrix and reinforcements processing functionally graded properties was called interphase.During modeling,the interphase was divided into numerous thin homogeneous shells.With help of the Green’s function,it was found that the outer shell did not bring disturbance strain to the inner ones.In this way,a sequentially homogenization scheme,where inhomogeneities were added sequentially from outside to inside of the composites,was established.Effective properties of the composites were gotten with help of Self-Consistency and Mori-Tanaka schemes.Accuracy and efficiency of the present method was verified by FEM and experiments and effects of the functionally graded interphase on overall properties of the composites were studied.2)Effective properties of CNT grafted fiber reinforced composite(CG-FRP)were gotten by the sequentially homogenization scheme.CNTs and fibers were modeled separately in two scales.The CNT region was modeled as cylindrical inclusion embedded into isotropic material.With decrement of local volume fraction of CNT along radical direction of fiber,the CNT region was homogenized as functionally graded interphase.Then the CG-FRP was modeled by cylindrical inhomogeneities with functionally graded interphase embedded in transversely isotropic material.The results were verified by FEM and experiments.Based on analysis,CNTs could enhance the transverse modulus of fiber reinforce composites.In tiny volume fraction of CNT,there existed an optimum length of CNTs which obtained the maximum effective properties.On the other hand,in most cases,effective properties of composite would always increase with length of the CNTs.3)The trans-scale sequentially homogenization scheme was used in Laplace domain and rate-dependent mechanical responses were studied.After assuming the matrix and interphase to be viscoelastic and the reinforcement to be linear elastic,the sequentially homogenization scheme was applied in Laplace domain and effective viscoelastic modulus of composites were gotten.Rated-dependent responses were obtained by inverse Laplace transformation.Furthermore randomly distributed particle reinforced representative volume elements were built by combining of MATLAB and ANSYS.Validation of these two methods were verified by existing theories and experiments.After studying stress relaxation,various strain rate loadings and harmonic loadings of composites,it was proved that effective strain-stress of composites was highly influenced by strain rate and frequency of loadings.Increment of reinforcement could enhance the complex modulus and decrease the loss factor of composite.Interphase could increase the loss factors of composite.4)An incremental trans-scale sequentially homogenization scheme was built and the viscoplastic response of composite were obtained.Constitutive properties of matrix/ interphase/reinforcement were assumed as linear elastic and visco-plastic,respectively.Secant modulus of each phase in each load step were linearized.Effective properties of composites were sequentially homogenized and the strain-stress relationships were gotten.Results coincided with experimental data and strain status of each phase was studied.Based on analysis,bearing capacity of the composite depended on elastic deformation of each phase.In this way,bearing capacity of particle reinforced composite was highly influenced by yield stress of matrix,however fiber reinforced composite can enhance its bearing capacity in length direction.Furthermore,interphase could reduce the strain concentration between matrix and reinforcements.Consequently,plastic deformation in reinforcement was reduced and the load capacity of composites was enhanced. |
