Study On Self-sensing Properties Of Strain And Damage For FRP Modified By Carbon Nanofibers Under Uniaxial Tensile Loads

Fiber Reinforced Polymer(FRP)has played an important role in a variety of fields due to its high stiffness-to-weight ratios,especially in aerospace and structural rehabilitation.Despite its superior properties,FRP is very sensitive to the deformation and micro-damage in structure.The FRP materials usually exhibit perplexed and complicated damage modes during service,and all of micro-damages are invisible to the naked eye.Thus,the nondestructive sensing technologies for monitoring complex damage modes of FRP composites in real-time are direly needed.Electrical Resistance Technique(ERT)is based on the principle of the inevitable association between the resistance change of specimen and the deformation or fracture of structure,which provides a new method for self-monitoring of Carbon Fiber Reinforced Polymer(CFRP).In fact,damage of FRP composite originates in the matrix,while the matrix deformation and damage can't be recognized only monitor the resistance change of conductive carbon fiber.At the same time,this method can not be applied to the real-time monitoring of insulated FRP materials,such as Glass Fiber Reinforced Polymer(GFRP)and Basalt Fiber Reinforced Polymer(BFRP).The rapid development of conductive nano-filler provides a new way to solve this problem.The conductive fillers are evenly divided into FRP matrix,and it can provide stable electric conductivity without reducing the mechanical properties of material,so that the FRP composite can be self-sensed in real time by ERT.In this paper,Carbon Nanofibers(CNFs),which are cost-effective and easy to disperse,were dispersed in the epoxy resin as the conductive enhancement phase,and the CNFs/EP composites with various amount of CNFs were prepared.The resistance of composites were monitored under constant amplitude cyclic tensile loading,quasi-static tension loading and incremental cyclic loading,the self-sensing properties of CNFs EP under tensile loading were studied.Based on this,the CNFs/EP was embedded to FRP,and the CFRP,GFRP and BFRP laminates modified with different contents CNFs were manufactured with hand lay-up method.The tensile mechanical tests and the in situ resistance measurements for FRP laminates were finished to realize their self-sensing of mechanical strain and damage in matrix.The resistance of FRP composites modified with CNFs were monitored in the temperature cycling at-30-60? for three times,and their thermosensitive were studied.The results show that the piezoresistive properties of CNFs/EP composites under various tensile loads are stable and repeatable,and their deformation and damage can be recognized successfully,which satisfy the requirements to act as smart matrix.The correlation between strain and conductivity of FRP laminate specimens is stable,the CNFs dispersed in matrix can act as sensors,which can provide deformation and damage information over the whole testing range.ERT can achieve the self-sensing ability of FRP laminates,including evaluating the material deformation,identifying the elasticity and plasticity of matrix and early warning material failure.After repeated temperature cycling,the thermosensitive characteristics of FRP modified with CNFs are stable,which can provide evidence for the temperature compensation during their actual use.