| Carbon fiber composites have been widely used in aerospace field due to light-weight,high-strength,design flexibility and impact resistance.The carbon fiber composites have good electrical property,whose reinforcement structure has a significant effect on electrical conductivity and potential distributions.However,the internal conductive mechanism is still not clear owing to the inhomogeneity and electrical anisotropy.Composite parts are often to encounter various external threats during service life.To track and monitor the initiation and propagation of damage in the composites is critical to optimize the service life and properties.Piezoresistivity of carbon fiber composites refers to the reversible change in electrical resistance with strain.Carbon fiber composites could be regarded as a sensor itself.It is of great importance to explore and reveal structural effects on the electrical conductivity and potential distributions in the composites,especially for electrical structure design and electrical-resistance-based structural health monitoring.This project aims to investigate the electrical conductivity and potential distributions of carbon fiber composites with different reinforcement structures.We investigated effects of reinforcement structures on the electrical conductivity and potential distributions.The conductive mechanism of carbon fiber composites has been revealed,and the electro-mechanical coupling behavior of threedimensional woven angle-interlock（AI）composites has been also explored,which can provide a theoretical and experimental reference for the electrical structure design and structural health monitoring of carbon fiber composites.We have conducted:（1）The electrical conductivity and potential field distributions of carbon fiber composites were characterized.The electrical conductivities along different directions were measured with two-probe method.Interfacial conductivity measurement was carried out to investigate effect of interface with different ply orientations on electrical contact conduction.Two-dimensional potential method was employed to test the potential field distributions under different current injection modes,probing depths and reinforcement structures.（2）Equivalent electrical conductivity tensor and finite element analysis models were established to further explore effects of different ply orientations,lay-up sequences,and surface fiber layers on the electric properties of unidirectional/multi-directional laminates.Based on the experimental results,equivalent electrical circuit model and electrical anisotropic conductivity linear model were proposed.Then the attenuation mechanism of potential field distributions along thickness direction in PW composites were discussed,and direction effects of surface potential field distributions in threedimensional woven AI composites were also revealed.（3）Short-beam shear（SBS）failure of three-dimensional woven AI composites along different orientations was characterized by electrical resistance measurement.Fourprobe method was conducted to measure electrical resistance change of composites under quasi-static and cycle loading.The electro-mechanical coupling behavior and shear failure mechanism of three-dimensional woven AI composites were also explored.We have found:（1）The unidirectional（UD）lamina has higher conductivity along carbon fiber direction than the perpendicular directions,and equipotential contours also show different gradients along the two directions.The cross-ply（CP）and quasi-isotropic（QI）composite laminates have the mixed effects of the UD lamina electrical conductivity and ply orientations,while the surface potential field distributions mostly depends on the surface lamina direction.The thickness conductivities of multidirectional laminates depend on each lamina and inter-laminar bonding.A finite element analysis（FEA）model was also developed to show the effect of ply orientation on potential field distribution.The CP and QI laminates with 0° surface ply have uniform potential distributions and isotropic electricity behaviors.（2）Yarn waviness and bridging region in the PW carbon fiber laminates lead to the decrease of electric conductivities along in-plane direction,and the increase along thickness direction.The symmetric equipotential contours orient along the current flow direction under one pair of electrodes.Two pairs of electrodes with vertical current flow paths lead to a positive superposition effect,and the resultant electric potential field increases.While a negative superposition effect comes from opposite current flow paths,and the resultant electric potential field of the PW laminate decreases significantly.The electric current along thickness direction decreases because of high inter‐laminar contact resistance.The highest and lowest potential values in three-dimensional equipotential contours eliminate at the electrical current effective penetration thickness due to the shunt effect caused by inter-laminar resistance along thickness direction.The PW laminates can be assumed as a homogenized isotropic electrical conductive media in‐plane direction.（3）Off-axis angle and yarn waviness of 3D carbon fiber / epoxy AI composites increase current conduction path,resulting in the in-plane anisotropic electrical conductivity.The surface electrical potential fields vary with current injection angles.When the angle is 0°（weft direction）,the equipotential lines are steep and distorted,and the potential value reduction is abrupt in the warp direction.The equipotential lines are smooth and parallel to the weft direction when the current is introduced in 90°（warp direction）.In case of 45°,the equipotential lines extend both along the weft and warp directions,leading to the potential values falling in between those of 0° and 90° current injection angles.Electrode position can significantly affect the potential values and field distributions.The resultant electric potential field decreases under two pairs of electrodes with two opposite current flow paths,resulting in a large range of diamondshaped low potential area at the composite panel center.While the resultant electric potential field increases under two same current flow paths,forming a higher density of equipotential lines and more uniform potential distributions than that of one pair mode.（4）The on-axis（0° and 90°）samples of three-dimensional woven AI composites have higher shear strength and failure resistance increase than other off-axis（30°,45°and 60°）samples.The load-displacement curves of on-axis samples exhibit quasi-brittle behavior,whereas those of the off-axis samples present elastic-plastic nonlinear behaviors,showing a ductile failure feature.The damage mainly occurs in delamination and yarn breakages in the on-axis specimens,whereas the main damage mechanisms of off-axis samples are inelastic deformation and localized delamination.Relative resistance changes of three-dimensional woven AI composites become larger during quasi-static loading and decrease under cyclic loading.The 45° and 60° samples show higher electrical resistance changes compared with other samples under cyclic loading.Relationship between shear strength and electrical resistance changes reveal that the electric-mechanical coupling behavior could provide an effective way for characterizing composites inner damages.These results on the above reveal the effects of different reinforcement structures on the electrical conductivity and potential field distributions of carbon fiber composites.The attenuation mechanism of composites’ potential field along thickness direction is further revealed by equivalent electrical circuit model and electrical current effective penetration thickness.Electro-mechanical coupling behaviors of three-dimensional carbon fiber woven composites are also discussed,and then the reliability and effectiveness of electrical resistance measurement are verified.We hope our investigations can provide a significant useful guide on electrical structure design and electrical-resistance-based structural health monitoring of carbon fiber composites. |
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