Strain And Damage Evolution Self-sensing Properties Of Composites Modified By Carbon Nanofibers

Epoxy based composites have been widely used in aerospace,civil engineering and other fields due to their high specific strength,high specific modulus,fatigue resistance and corrosion resistance.However,these composite are often suffer fatigue load,impact load,environmental erosion and other factors during long-term service,thereby it is easy to form different degrees of damage.Among them,the generation and expansion of micron-scale damage in epoxy matrix is one of the important factors affecting the reliability of these composite.Under the conditions of environment and load,tiny damage gradually evolves into larger cracks,thus affecting the safety and service life of the epoxy based composite.Therefore,monitoring of strain and damage evolution in composite during the service is of great significance.There is still a lack of efficient monitoring of micron-scale damage initiation and expansion.The traditional non-destructive monitoring method not only consumes a lot of time,but also needs to predict the location of the damage in advance,so its application in the health monitoring of composite is greatly limited.In recent years,carbon nanofibers(CNFs)have been widely used in epoxy based composite structures due to their excellent mechanical and electrical properties,providing a broader application prospect for in-situ electrical resistance measurements.In this paper,the relationship between electrical resistivity and microstructure change of composites modified by CNFs was investigated for the health monitoring of composites.The in-situ electrical resistance measurement based on the conductive networks was used to conduct strain and damage self-sensing of composite.The damage evolution process of composites modified by CNFs under tensile loading was also analyzed.The main contents and conclusions are as follows:(1)Firsitly,the mechanical,electrical and piezoresistive properties of the composites filled with different contents of CNFs were investigated.On this basis,the CNFs/epoxy sensors were embedded into concrete cylinders to monitor their compressive strains under monotonic and cyclic loadings,thereby assessing practical applications of the CNFs/epoxy sensors as strain sensors for monitoring concrete structures.The experimental results show that adding CNFs can effectively enhance the compressive strengths and elastic moduli of the composites.The calibration and monitoring curves exhibited a consistent variation trend when the concerte cylinders embedded with sensors were subjected to monotonic and cyclic loadings.This demonstrates that the CNFs/epoxy sensors have considerable potential to be used as embedded strain sensors for structural health monitoring of concrete structures.(2)Conductive carbon nanofibers(CNFs)were dispersed into epoxy resin matrix and then infused into glass fiber fabric to fabricate CNFs/glass fiber reinforced polymer(GFRP)laminates.The electrical resistance and strain of CNFs/GFRP laminates were measured simultaneously during monotonic,constant amplitude cyclic and incremental amplitude cyclic tensile loadings to investigate in-situ strain and damage monitoring capability of CNFs/GFRP laminates.The results indicate that resistance response during monotonic tensile loading could be classified into three stages corresponding to different damage mechanisms,which demonstrated a good ability of in-situ damage monitoring of the CNFs/GFRP laminates.In addition,the capacity of in-situ strain monitoring of the laminates during small strain stage was also confirmed according to the synchronous and reversible resistance responses to strain under constant cyclic tensile loading.Moreover,the analysis of the resistance responses during incremental amplitude cyclic tensile loading with the maximum strain of 1.5% suggested that in-situ strain and damage monitoring of the CNFs/GFRP laminates were feasible and stable.(3)This study investigated the strain and damage self-sensing performances of basalt fiber reinforced polymer(BFRP)laminates fabricated with carbon nanofibers(CNFs)/epoxy composites subjected to tensile loadings.The conduction mechanisms based on the tunnel and percolation effects as well as the damage evolution were also explored.A compensation circuit was proposed to eliminate the influence of temperature on sensing signals of the composites.The results indicated the resistivity of the CNFs/BFRP laminates and CNFs/epoxy composites exhibited similar change rule,indicating that the conductive networks of CNFs/BFRP laminates were governed by CNFs/epoxy composites.With the increase of strain under monotonic tensile loading,the electrical resistance response could be classified into three stages corresponding to different damage modes.This confirmed CNFs/BFRP laminates have excellent self-sensing abilities to monitor their internal damages.Moreover,stable and repeatable strain self-sensing capacity of the CNFs/BFRP laminates was verified under cyclic tensile loading because the electrical resistance varied synchronously with the applied strain.(4)1.0 wt% of carbon nanofibers(CNFs)were dispersed into epoxy matrix and then infused into flax fiber fabric to prepare CNFs/flax fiber reinforced polymer(FFRP)laminates.The strain and damage self-sensing behaviors of CNFs/FFRP laminates were investigated under tension by measuring the change of electrical resistance.The acoustic emission(AE)signals were simultaneously recorded to verify the feasibility of in situ electrical resistance measurements.The results indicated that both the electrical resistance and AE responses during monotonic tensile loading could be classified into three stages corresponding to different damage mechanisms,such as matrix cracking,interfacial debonding,delamination and fiber breakage,which confirmed that CNFs/FFRP possess a good damage self-sensing ability.Moreover,the synchronous and reversible resistance responses to strain proved the stable and repeatable strain monitoring capability of CNFs/FFRP laminates.Therefore,this study concludes that adding a small account of CNFs to form an electrically conduction network in non-conductive FFRP composites is an effective method to in situ monitor the strain and damage development.