Research On Eddy Current Testing Of Fiber Waviness In Unidirectional Carbon Fiber Reinforced Polymer

Carbon fiber reinforced polymer(CFRP)is a kind of composites widely used in aerospace,construction and other fields.It not only has high specific strength,high specific stiffness,good resistance to fatigue and corrosion,but also can be designed according to the engineering needs.Fiber waviness is a common type of defect in CFRP and deteriorates the mechanical performance of CFRP structure.Therefore,nondestructive testing is of importance to ensure the safety and reliability of CFRP structure.Eddy current testing(ECT)is a prevailing technology for inspecting conductive materials,e.g.CFRP,for its advantages such as noncontact,low requirement of surface cleaning and feasibility of on-line automatic detection.ECT can be used to detect the CFRP product and monitor the quality of semi-product in CFRP production process.Although achievements on ECT of CFRP have been made,a method that is easy to operate,sensitive to variation of fiber direction,and suitable for CFRP on-line testing during the process of manufacturing and s is still in demand.Based on the research situation,this thesis conducts research on ECT of fiber waviness in CFRP.This thesis proposes a method for detection of fiber waviness in unidirectional CFRP.Firstly,the distribution of eddy current(EC)in unidirectional CFRP excited by rectangular coil is studied through three-dimensional finite element(FE)numerical simulation,which proves that EC flows predominantly along the fiber direction.Secondly,the angle between the probe direction and the fiber direction is defined as deviation angle,and the relation between the output voltage and deviation angle is computed.Then,The trend of the induced voltage when the probe scans CFRP with fiber waviness defect is obtained,which proves the feasibility of the probe in detecting the fiber waviness defect of unidirectional CFRP.Next,five kinds of probes for detection of fiber waviness are designed,including orthogonal probe with large reception coil,orthogonal probe with small reception coil,GMR sensor probe,TMR sensor probe and flux-gate sensor probe.The frequency characteristics and sensitivities to the fiber direction of five probes are compared,orthogonal probe with large reception coil is selected for the research on ECT of fiber waviness.This thesis also studies the detection of fiber waviness and reconstruction of fiber distribution in CFRP and carbon fiber prepreg.Firstly,the experiment of detecting fiber direction in CFRP is carried out,and the relation between the output voltage and the deviation angle is studied.The resolution of detecting fiber direction using the proposed probe is 0.50 through the experiment with finer step of changing deviation angle.Then the linear scanning experiment of CFRP with fiber waviness defect is carried out,which proves that the probe is able to detect the fiber waviness defect.Next,the C scanning in CFRP is carried out,and the fiber direction is calculated by interpolating the relation between the output voltage and the deviation angle.The reconstruction of fiber distribution is carried out.After that,the experiment of detecting fiber direction in carbon fiber prepreg is carried out,and the output voltage and the deviation angle is obtained.The resolution of detecting fiber direction in carbon fiber prepreg using the proposed probe is 0.50°through the experiment with finer step of changing deviation angle.The linear scanning experiment of carbon fiber prepreg with fiber waviness defect is carried out,which proves that the probe is able to detect the fiber waviness defect.Finally,C scanning in carbon fiber prepreg is carried out,and the fiber direction is calculated by interpolating the relation between the output voltage and the deviation angle.The reconstruction of fiber distribution is carried out,and the reconstruction result of fiber distribution is compared witih the actual image of fiber distribution,which proves the validity of the reconstruction method for fiber distribution.