Damage Mechanisms And Braiding Angle Effects Of 3-D Braided Carbon Fiber/epoxy Resin Composite Under Multiple Impact Compressions

Three-dimensional(3-D)braided composites have extensive applications to aerospace,aircraft and high-speed vehicles.They have shown the excellent performance in delamination-resistant and damage tolerance under impact.Stress waves will propagate and reflect in the composite samples between different surfaces.The 3-D braided composites always are under multiple compressions during their service life.We focused on the damage development and mechanisms of 3-D braided carbon fiber/epoxy resin composite under multiple impact compressions in this project.We have conducted:(1)Dynamic tests of 3-D braided composites on split Hopkinson pressure bar(SHPB).Damage development of samples under impact were monitored with a high-speed camera system.Damage development and mechanisms of the samples within every single impact were analyzed.(2)Establishment of meso-scale finite element analysis(FEA).A meso-scale finite element model was developed to analyze damage development and mechanisms of 3-D braided composites under impact.Failure criterion was defined to simulate damage status and distribution of yarns,resin and interface.Energy absorption of each component was calculated within every single impact to figure out its energy absorption rules.(3)Comparisons of the impact damage behaviors of 3-D braided composite with different braiding angles.The influence of braiding angles on damage development and mechanisms of 3-D braided composite has been analyzed.The effect of braiding angles on stress-strain response,failure modes and energy absorption were compared between experimental and the FEA results.We also conducted the in-plane and out-of-plane compressions for finding the directional effect.We have found:(1)Braided composite showed different failure modes and damage distributions among every single impact.The plastic deformation mainly occurred in the first impact.The stiffness degradation mainly occurred in the second impact.The stiffness degradation of elements initiated in the first impact for the resin and the second impact for the yarns.The proportion of energy absorption by yarns arises from the first impact to the third impact.The principal component of energy absorption gradually transformed from the resin at the first impact to the yarns at the third impact.(2)Braiding angle has significant effect on impact damage behavior of composite under out-of-plane impact.Composite with smaller braiding angle(15°)exhibited brittle failure behavior.The failure of composite was relatively concentrated in local area.Composites with larger braiding angles(26° and 37°)showed the ductile failure behavior.The failure distributed more evenly in the composite.The proportion of energy absorption by yarns changed when the braiding angle changes.The yarns of the 37° sample absorbed the highest proportion of impact energy in the first impact,while it is the yarns of 15° and 26° samples absorbed the highest proportion of impact energy in the second and third impact,respectively.(3)The braiding angle influences impact damage behavior of composites under in-plane impact.For composite with smaller braiding angle(15°),the failure is resulted from the yarns and interface damage.For composites with larger braiding angles(26° and 37°),the resin and interface damage lead to composites failure.For the 15° sample,the failure mode of yarns is breakage and dislocation deformation;For the 26° sample,the failure mode of yarns is breakage and buckling;For the 37° sample,the main failure mode of yarns is inclined deformation.The stress undertaken by yarns in the 15°sample is much higher than that in the 26° and the 37° sample.The principal component of energy absorption for the 15° sample is yarns;while it transformed to resin in the 37° sample.The braiding angle influences damage distribution.The failure area of composite is relatively concentrated for the 15°sample compared to larger braiding angle samples(the 26° and the 37° sample).(4)Damage development and mechanisms under in-plane impact is different with that under out-of-plane impact.Taking the 15° sample as an example: under in-plane impact,the sample was most damaged during the first impact,and the main failure mode is yarn and interface damage.Under out-of-plane impact,the sample was most damaged during the second impact.The main failure mode is the resin and interface damage.Under the in-plane impact,the yarns absorbed the most of impact energies during every single impact,and the braided structure broken down to single yarns.When under out-of-plane impact,the yarns only absorbed the most of impact energies during the last two impacts,the braided structure still kept integrity.This project presented impact damage behavior of braided composite under multiple impact compressions.Damage morphology,failure mode and energy absorption in every single impact have been analyzed.The influence of braiding angle on dynamic damage behavior of composites has been revealed.The results are expected to be used for optimizing 3-D braided composite design under multiple impact compressions.