On Line Structural Health Monitoring Of Composites Based On Electromechanical Response Of Carbon Nanotube

Composite materials possess the advantages of light weight,high strength and strong designability,and are widely used in modern industry and life.However,the properties of matrix and reinforced fiber and the interface between matrix and reinforcing phase make them have anisotropic and excellent structure and functional characteristics,but also are susceptible to small damage.Therefore,the composite materials often appear matrix damage,fiber debonding or fracture,interlayer cracking and other internal damage under the action of load stress and strain environment during service.These internal damage will continue to expand and eventually lead to catastrophic damage of composite materials.Therefore,it is of great academic significance and engineering value to develop the detection technology of structural damage of composite materials to ensure the safety of composite materials.In recent years,as a kind of precise monitoring technology for deformation and damage of composite materials,composite structural health monitoring can provide early warning for the failure of composite structure,and provide real-time detection means for the safe use of composite materials,which has been paid more and more attention by worldwide researchers.The design and development of monitoring technology which can detect the health status of composite structure on line has become an important research direction in the field of composite materials.However,the existing damage detection methods of composite structures,such as fiber optic sensors and adding conductive materials,still have some problems,including the difficulty of locating the damage position online,affecting the properties of composite materials and increasing the forming process difficulty.In this work,aiming at the problem that the on-line detection and location of the health status of composite structures,light and high strength carbon nanotubes(CNT)films with controllable network structure were prepared,and their electromechanical response characteristics were used to sensing the changes in its network,as a result,the sensitive online monitoring of the structural health state of composite structures is realized.The influence of the structure of CNT films on the electromechanical response characteristics and its mechanism are explored.On this basis,the quantitative and location analysis method of damage of composite materials under the mode of through-hole and interlayer cracking is studied.Then,the method is extended from static damage monitoring to dynamic process monitoring of resin flow.The on-line monitoring of resin infiltration,flow and curing in resin transfer molding process of composite materials is realized.Furthermore,on the basis of the realization of structural health monitoring of composite materials,a composite material with self-monitoring and self-repair is developed.The relationship between the structure and electrical parameters of composite materials in initial,damaged and repaired state is studied.The equivalent criterion method of damage and repair monitoring based on mechanical and electrical properties of composite materials is proposed,which provides a reliable evaluation basis for online damage and repair state monitoring of composite material.The specific contents of the study are as follows:The preparation technology of CNT film,the influence of CNT dispersion state and additives on its mechanical and electrical properties were studied.The relationship between the structure and mechanical/electrical properties of CNT film was established.The CNT films with high strength,high toughness and good conductivity were prepared by adjusting the additive types,and the mechanism of strengthening and toughening the films was revealed by the molecular dynamics simulation.Based on the above,the relationship between the structure of CNT film and its sensitivity of electromechanical response is studied,which lays a foundation for the application of CNT film in the structural health monitoring of composite structure.The CNT film and composite were formed by co-curing method,and the sensing unit was integrated with composites without reducing the mechanical properties of the composites.The number of electrodes and the arrangement mode were simulated and their influence on the damage monitoring accuracy was studied and optimized.The highprecision online monitoring of composite damage was realized in the damage mode of through-hole and interlayer crack.Based on the electrical resistance chromatography technology,the CNT film is used as the sensor unit to develop the on-line monitoring technology for the structural health of composite materials.The above static damage monitoring technology based on CNT film is extended to the monitoring of the dynamic preparation process of resin based composite.The influence of resin infiltration,flow and solidification on the network structure and electrical properties of carbon pipe film is studied.The dynamic monitoring of resin flow in vacuum assisted resin transfer process is realized,and the flow position and position of resin and its position are realized Accurate monitoring of shape.The real-time online monitoring of the preparation process of composite materials can provide a new idea for improving the preparation process of composite materials.In addition,after the composite is formed,the electrode can be reserved for damage monitoring,and provides a method for the health monitoring of the composite structure from production to bearing service.Based on the study of the electrical resistance tomography technology of CNT films,the law of structure and resistance change of CNTs films is revealed.A thermal plastic composite film are prepared based on thermoplastic polyurethane and carbon nanotubes.The electrical properties of the three states of carbon nanotubes/polyurethane composite films were studied by resistance method and electrical resistance tomography,revealing the relationship between the structure of the films and the resistance changes before and after the damage repair.The relationship between mechanical and electrical properties of composite film is established,and the equivalent criterion for evaluating the damage repair effect of composite materials is proposed.The results verified the application of this method in damage detection and location imaging of composite materials,and realized the judgment of online repair efficiency of composite materials.This approach provides a new method for online structural health monitoring of composite materials.