| Due to the superior structural properties,such as improved strength,fracture toughness,higher impact damage tolerance in the through-thickness direction,no delamination,fatigue resistance,net-shape,and structure integrity,three-dimensional(3-D)braided composites are being used in many industries including aeronautic structures,automotive structures,marine applications,and pressure piping,etc.In the paper,both the experimental and micro-structure finite element(FE)simulation methods were used to study the quasi-static and high strain rate axial compressive behavior and damage mechanism of 3-D circular braided composite tubes.The effect of braided parameters and temperature on the impact compressive behavior of 3-D circular braided composite tubes were also studied in this paper.The main contents are(1)The 3-D circular carbon fiber braided preform with different braiding parameters were manufactured with 4-step 1×1 braiding method;The braided angle was 25o,35o and 45o;The braided layers were 2,3,4 layers and the braided structure included 3-D four direction(3D4D)and 3-D five direction(3D5D).(2)The axial quasi-static and impact compressive behavior of the 3-D braided composite tubes with different braided structure parameters were investigated experimentally at room and low temperature field.The strain rate of quasi-static with compression speed of 2 mm/min and high strain rate compression were about 0.001/s and 416/s to 936/s,respectively.The environment temperature field was-100oC~23oC.The experimental results including stress-strain curves,compression strength,compression stiffness,special energy absorption and the macroscopic failure patterns were obtained.The internal and microscopic failure forms were observed by computed tomography(CT)imaging testing system and scanning electron microscope(SEM)for damaged samples.(3)The micro structure FE models of six types of 3D4 D and 3D5 D circular braided composite tube were in accordance with actual basic configurations of braided fabric obtaining from the yarn spatial distribution in the matrix of the composite tube.It is an accurate characterization to utilize this model to evaluate quasi-static and dynamic mechanics,due to build up the link micro structure geometric model and basic material properties.The quasi-static and impact compression process,the stress-strain curves and the final damage patterns can be obtained by the finite element calculation.(4)The rate and temperature-dependent constitutive model by experiments and formula derivation was developed.The heat flux density per unit volume was defined by the inelastic heat fraction.Then the thermo-mechanical coupling computation with FE method was performed based on the 3-D solid micro structure model of 3-D braided composite tube established at room temperature.The special node temperature rise time curves,compression deformation,progressive damage,final damage status stress distribution and temperature distribution were presented to analyze the impact compression thermo-mechanical coupling damage process and mechanism of braided composite tubes.The main conclusions are(1)The stress-strain curves of 3D4 D and 3D5 D circular braided composite tubes with different structure parameters were sensitive to strain rate at room temperature.The compression strength,stiffness and special energy absorption(SEA)increased with the increase of strain rate,which showed obvious strain rate dependence.The quasi-static and impact compression stress-strain curves both showed the elastic-plastic features.The compressive response of braided composites tubes under various strain rate was significantly influenced by braided angle and braided layers;3D5D circular braided composite tube had higher axial compression performance and impact damage tolerance than 3D4 D braided composite tube;The failure mode can be divided into shear failure,fiber pull,fiber fracture,resin cracking,compression deformation buckling failure and crashing damage.Under low temperature,the compression properties of braided composite tubes were superior to the room temperature;the compression stiffness,strength,and specific energy absorption were more sensitive to strain rates than temperature.The material presented brittle characteristic.The compression failure patterns included resin cracking,fiber pull,overall buckling deformation and fracture,resin and fiber bundle interfacial debonding,etc.(2)The FE calculation results of braided composite tubes under various strain rates showed good agreements with experimental.This showed the FE model can accurately predict the compression mechanical properties of braided tubes.The impact compression failure mechanism of 3-D braided composite tubes can further be revealed by the FE model from views of impact compression process,stress distribution and failure modes.The effects of strain rates,braided angle,braided layers and the axial yarns on axial compressive performance and damage under impact compression were studied in detail.From the simulated impact compression progress,the stress distribution of the braided fabric and the resin were obtained,respectively.The deform process of braided yarn,fiber breakage,fracture of resin matrix,interaction between components caused by high strain rate impact compression were reappeared.The load difference between reinforcement and resin during the compression was also studied.For 3D5 D braided composite tubes,five special points were chosen symmetrically on the inside and outside surface braided yarns and axial yarn along axial direction to characterize the regional stress distribution by the node points stress time curves.Three points were selected on the different sections of resin to present stress-time curves of resin along the axial and through thickness direction.The results showed that the stress wave propagated faster in axial yarns than braided yarns.The stress wave propagation was uniform in resin and resin plays the role of stress transfer in the composite.The nodes stress of axial yarns was higher than braided yarns and resin was far lower than that of yarns.These results which were rarely achieved from experiment will further reveal the different impact damage mechanism between 3D4 D and 3D5 D braided composite tubes.(3)Due to the extruding effect of resin from 3-D braided reinforcement under impact compressive deformation,stress concentration occurred easily and early at the position with maximum curvature in fiber tows under low temperature,which leaded to inelastic temperature rise in resin.Due to the influence of braided yarns orientation,the “scattered” point stress/temperature concentration was developed on the impact loading end of specimens.And the “zigzag” type shear band located along the direction of braided yarn on the middle part of specimens.The temperature rise in braided yarns was greater than resin and change fasterthan resin.The research provides the guidance for the impact-resistance design of structures under high speed loading.The strain rate effect and structure effect of braided tubes under impact loading are obvious.The micro-structure performance and dynamic response with accurate characterization can provide theoretical guidance for the impact-resistance design of composites.And under the low temperature,the study of thermal coupling response and failure mechanism provides reference for the materials in the field of aerospace industry application. |
PDF Full Text Request