Study On Mechanical Properties And Damage Mechanism Of Crimped Fiber Reinforced Composites

With the development and improvement of composite mechanics and damage theory,preparation technology,test method and numerical calculation model,composite materials have been widely used in construction machinery structure or bionic structure.In order to satisfy the complex working and various load conditions,the more refined researches on the mechanical properties and damage mechanism of the composites are required.Due to the special structure and distribution of the crimped fibers,the composite reinforced by the crimped fibers can absorb tensile stress under tensile load,and dissipate strain energy when unloading for reducing the effect of creep.Meanwhile,the spatial crimp fibers widely found in blood vessels,connective tissue,bamboo and woven composites,which is an important factor to resist fatigue,impact and multiaxial load.According to the different mechanical properties and application background of the fibers,the composite materials are regarded as nonlinear elasticity,anisotropic and transversely isotropic hyperelastic body in the finite strain range.In order to take into account the macro and micro mechanical properties of the materilas,damage mechanism and computational efficiency,an effective unit cell model for this multiphase composite structure is established and the corresponding periodic boundary conditions are applied.Compared with the homogenization and damage theory of the laminates,the proposed unit cell model can be used to reveal the relationship between the microstructure and damage peoperties and interfical strengths and the macroscopic damage mechanical properties of the composite.Firstly,according to the morphological and structural characteristics of the collagen fibers,the crimped fibers were divided into planar and spatial crimp structures and characterized by mathematical parameter equations.Secondly,based on the construction theory of unit cell model,the unit cell model for finite element analysis of the multiphase structure of the crimped fiber reinforced composites is established and the influence of fiber morphology on the macro mechanical properties and local stress field is investigated.Furthermore,considering the hyperelastic damage mechanical properties and interface interaction of the collagen fiber reinforced composites in biological tissue,the effects of fiber morphology,damage properties and interface properties on the macroscopic damage mechanical properties and fiber damage modes of the composites are discussed,and the relationship between macro and micro damage mechanics of this kind of hyperelastic composites is revealed.Finally,considering the influence of curing temperature on the crimp structure of kevlar fibers,kevlar fiber reinforced epoxy resin composites with different mirco-crimp structure and interfacial strength are prepared by controling the curing temperature.The interaction mechanism between macro damage mechanical response and micro damage of the composites is studied by tensile test and micro damage morphology analysis.The main researches of this paper are as follows:(1)Based on the morphological and structural characteristics of collagen fiber and braided fiber,the mathematical parameter equation of the crimp structure and the constitutive model related to mechanics and damage of the crimp fiber reinforced composites are given.Firstly,the mathematical parameter equations of the central curve of the planar and spatial crimp fiber are established respectively and the theory for producing three-dimensional crimp fiber is given.Secondly,according to the multi-scale mechanical and structural properties of the crimp fiber reinforced composites,the hyperelastic constitutive theory and damage constitutive theory are introduced in detail.(2)Taking the planar crimped fiber reinforced composite as the research object,a three-dimensional finite element unit cell model with controllable structural parameters for the numerical analysis was constructed.Based on the established finite element unit cell model and periodic boundary conditions,the macro mechanical response of the composite unit cell model is investigated and compared with the experimental results which has verified tthe applicability and representativeness of the cell model.Furthermore,based on the above cell model,the influence of single and combined geometric parameters on the overall mechanical properties and local stress field of the composite unit cell model is discussed.The results reveal that the stiffness of the unit cell model is mainly related to the straightness of the fibers.The size and distribution of the local stress filed are determined by the parameters H,? and ? and dominated by the parameter of ?.This research provides the basic data reference for the structural design of this kind of composite materials.(3)Taking spatially crimped fibers as the research object,the correlation between the nonlinear damage mechanical properties of composite unit cell model and the microstructure and damage characteristics is studied.First of all,the VUANISOHYPER?INV user subroutine of hyperelastic damage constitutive model is written and embedded into the finite element program ABAQUS/Explicit.And the correctness and realizability of the damage constitutive model is verified by studying the effect of single damage parameter.Then,the effects of fiber morphology and structure parameters on the overall damage mechanical properties and fiber damage mode of the composite unit cell model are studied.Furthermore,according to the damage initiation and evolution of the crimp fibers with different damage parameters,the interaction mechanism between the overall damage mechanical properties of the composite unit cell model and damage properties of the fibers is revealed.Finally,the interface properties on the damage mechanical properties and fiber damage modes of the unit cell model of the soft matrix composites are discussed under the same fiber morphology and damage parameter.The results show that the frequency,amplitude and initial damage state of the nominal stress-strain curve are related to the damage property and damage mode of the fiber.Meanwhile,the different interface properties would induce the fiber failure in different damage modes which provide a certain reference value for the interface design of this type composite.(4)In view of the similar skin-core structure and transversely anisotropic thermodynamic properties of the aramid fiber and collagen fiber,the aramid fiber reinforced composite with different degree of micro crimped structure is fabricated by changing the curing temperature.The nonlinear damage mechanical properties of the micro crimped aramid/epoxy composites prepared at different curing temperatures are studied by off-axis tensile test coupled with digital speckle correlation.The mechanism of the non-monotonic relationship between the nonlinear damage mechanical properties and the micro crimped structure is revealed by combining the SEM micro damage morphology and DIC strain field distribution.The results show that when the curing temperature is between 60°C and 120°C,the composite prepared at 80°C has the best strength and stiffness,and has the best energy absorption characteristics prepared at120°C.The research in this section is helpful for us to design the preparation process of aramid/epoxy composite laminates and interface modification.