Research on Low Velocity Impact Behavior and Damage Mechanism of CFRP Laminates

Author:Xiao Lin

Supervisor:zhang dong xing


Degree Year:2019





Carbon fiber reinforced polymers(CFRP)has excellent properties such as low density,high specific modulus,high specific strength,fatigue resistance,low thermal expansion coefficient and low thermal conductivity.It is widely used in aviation,aerospace,marine and so on.CFRP is made of unidirectional carbon fiber or woven fabric and resin matrix by lamination and heat pressing.Because the arrangement of carbon fibers is concentrated in all directions of the laminate,and the strength in the normal direction is almost equal to the strength of the resin matrix.The strength of CFRP in this direction is relatively low,and it is easy to cause damage when it is impacted,which leads to serious consequences.Therefore,it is important to study the mechanical response and damage mechanism of CFRP during impact.In this study,the interaction between the impactor and CFRP laminates during the impact process was analyzed.The correspondence between load,deformation and response time of laminates was explored.And then,the impact response and damage mechanism of unidirectional continuous CFRP laminates during low velocity impact in the impact energy range of 3 J to 12 J were studied experimentally.An improved three-dimensional Chang-Chang progressive failure criterion was proposed and a model of unidirectional continuous CFRP laminate was established.The finite element analysis results were compared with the experimental results to verify the failure criteria and the accuracy of the model.The results showed that as the impact energy increased,the damage of the unidirectional continuous CFRP laminate was more serious.The main damage modes of the laminate were the cracking of the matrix and delamination.Moreover,the impact damage shape was roughly irregular elliptical shape,and the damage extended in the fiber direction mainly.There were twice load drops of the impact response curve of the laminate under higher impact energy.The compression after impact(CAI)damage mode was mainly some penetrating longitudinal long cracks,and most of the cracks past through the impact damage area.The impact response,in-plane damage and interlaminar damage of CFRP laminates with[0/90]6ss and[45/-45/0/90]6s at different impact energies were studied by means of finite element simulation and experiment.Meanwhile,the energy absorption and transformation of laminates during impact were studied.Moreover,the effects of impact energy and stacking angle on the impact resistance and residual strength of CFRP laminates were discussed.Simulation and experimental results showed that the±45°layers could improve the impact resistance of CFRP laminates compared to orthogonal laminates.The CFRP laminate with[45/-45/0/90]6s had less damage and higher residual compressive strength.In the impact energy range of 3 J to 21 J,the impact energy was not completely absorbed by the laminate.The absorbed energy by the laminate was classified into elastic specific energy,damage absorbed energy and viscoelastic absorbed energy.The elastic specific energy absorbed by CFRP laminates increased with the increase of impact energy,and reached almost maximum absorption value at 12J.The damage absorbed energy of CFRP laminate always existed and was directly related to its residual performance.When the impact energy reached 12 J,the damage absorbed energy increased rapidly to more than 50%.The viscoelastic-absorbed energy was relatively small,but it was higher than the rebound kinetic energy at the higher impact energy.Two kinds of CFRP laminates did not exhibit obvious damage until the impact energy reached 12 J,the damage led to the decrease of the moduli of the laminates.Based on the purpose of improving the impact damage tolerance of composite,discontinuous CFRP laminate was designed and prepared.The impact behavior,tensile behavior and damage mechanism were studied by means of finite element simulation and experiment.The results showed that when the impact energy levels were 6 J and 9 J,the impact resistances of the discontinuous laminates were better than that of the continuous laminates.Short overlap laminates had the best impact resistance,and the finite element analysis results were consistent with the experimental results.The discontinuous laminates could restrain the crack propagation and still maintained high material properties after low velocity impact.The tensile strengths of the laminates gradually increased as the overlap length increased,but the pseudo ductility decreases.The damage mechanism of CFRP laminates with discontinuous layup structure was the pseudo-ductile fracture process under the tensile stress.By fitting the tensile strengths of CFRP laminates with different discontinuous layups,the tensile strength prediction model of the discontinuous laminates can be used to estimate the tensile strengths of laminates with different overlap lengths.In addition,a simple quantitative area method for estimating the damage area of discontinuous laminate was proposed.Compared with the calculated results of the image analysis software,the measurement error was less than 12%.It showed that the method was reasonable and feasible.