Nonlinear Dynamic Constitutive And Impact Damage Of Carbon Fiber Reinforced Polymer Composites

For the high strength,corrosion resistance and designability,fiber reinforced composite material was used in aeroplanes as the form of laminates,sandwich plates and stiffened plates,playing the role of load bearing,energy absorbing and protecting.During the service process of aircraft structure,composite material would suffer from impact loading by rock,hail,bird and so on.This will lead to damage and failure of the material and affect the load bearing ability of the structure.As a result,the damage and failure behavior of fiber reinforced composite material under dynamic loading have attracted much attentions.This paper built a nonlinear dynamic constitutive equation based on the quasi-static and dynamic mechanical behavior of carbon fiber reinforced epoxy composite material.By experimental and numerical study,the damage and failure behavior of composite structure under impact loading were revealed.The major study can be summarized as follow:The quasi-static and dynamic mechanical behavior of fiber reinforced composite were studied.Through tensile and compressive tests with various strain rates for composite laminates under layer design,the stress-strain relationships were obtained along different directions.Dynamic digital image correlation technique was applied in the tests to investigate the strain distribution and failure process of the specimens.The results show that for tension and compression along the fiber direction,as well as tension perpendicular to the fiber direction,the material behaves as linear elastic.For compression perpendicular to the fiber direction and in-plane shear,the material presents nonlinear behaviors.As the increase of strain rate,the modulus and strength of material arise while the failure strain remains unchanged.Based on the experimental results,a nonlinear dynamic constitutive model was built for fiber reinforced composite material.Considering different engineering applications,constitutive models were built for plane stress condition and three-dimensional stress condition respectively.Through the loading-unloading tests,the nonlinear behavior was attributed to the material stiffness reduction caused by damage.Two damage factors were introduced to describe the damage behavior perpendicular to the fiber direction and in-plane shear.The damage factors were expressed as a quadratic function and an exponential function of the strain along the two directions respectively.The parameters of the two functions were found unchanged under various strain rates,indicating the strain rate independence behavior of the damage process.The damage initiation criterion and failure criterion were proposed based on the strain invariants of transversely isotropic hypothesis.In the constitutive model of three-dimensional stress condition,fiber reinforced composite was regarded as transversely isotropic material.Thus,the damage and failure state of the material were considered to be invariants as the revolving of the reference coordinate around the axis of elastic symmetry.Additionally,the damage initiation and failure strains were found to be constants through the study of quasi-static and dynamic mechanical behavior.Combining the hypothesis and test investigation above,the damage initiation and failure criterion were expressed as the function of strain invariants in first and second orders.Employing the user subroutines in Abaqus,the nonlinear dynamic constitutive model with failure criterion were implemented into finite element analysis.The constitutive model was validated through numerical simulations of quasi-static and dynamic tests.The damage and failure behavior of composite laminates and sandwich structures under low-velocity impact were investigated.Drop weight tests were carried out on composite laminates and foam core sandwich laminates.Delamination regions among the structures were revealed through ultrasonic C-scan.According to the tests,delamination was found distributes symmetrically around the impact point,forming two triangles,while the region was related to the fiber directions of adjacent layers.A prediction method was proposed to determine the position and shape of delamination which would provide reference for layer optimization of composite laminates.The delamination behavior of upper faceplate in sandwich structure under low-velocity impact was similar with that in composite laminates.Large impact energy and high foam density will lead to large delamination area,but the area stop growing after the upper faceplate was penetrated.When penetration occurred,the failure region of the sandwich structure can be divided as crush region and fracture region with different failure modes.The nonlinear dynamic constitutive model for fiber reinforced composite was used to simulate the damage and failure behavior of composite laminates and sandwich structures under low-velocity impact.Since the strain rate effect was not obvious under low deformation rate,the model was degenerated into a rate-independent mode only considering the nonlinear,damage and failure behavior.The simulation result was found to coincide with test observations,while different failure modes were presented by the model,validating the effectiveness of the constitutive model.The damage and failure of composite wing structure under bird strike was studied.Bird impact tests were carried out on a aircraft structure composed by composite laminates.High speed camera was used to capture the deformation process of bird and structure during the impact.A 3D dynamic digital image correlation system was involved to obtain the displacement field and strain field distribution of the structure near the impact point.Based on the nonlinear dynamic constitutive model proposed in this paper,a numerical model was built to simulate the bird impact problem.The numerical results show good agreement with the test results both in the deformation process and damage appearance.The nonlinear dynamic constitutive model was also compared with the traditional model.The results showed that the model proposed in this paper has the advantage in modeling the response of fiber reinforced composite material under high speed loading.This paper proposed a nonlinear dynamic constitutive model for fiber reinforced composite based on the quasi-static and dynamic mechanical behavior of material.The model was applied to the simulation of laminated composites under low velocity impact and bird strike.The damage and failure characteritics were revealed for composite structures under impact loading.The work makes contribution to the design of optimization and verification of impact resistance for composite structures.