A Novel Eddy Current Testing Technology Based On The Electromagnetic Acoustic Effect

In recent years,China's avation industry has made succesive achievements,keeping the new breakthroughs and new progerss.At the same time,in order to ensure the stable development of China's avation industry,the non-destructive testing(NDT)technique is facing the higher requirement.The eddy current testing(ECT)technique is one of the earliest and mature NDT technologies in avation field.However,most of existing ECT utilize the impedance change of pick-up coil to obtain eddy current disturbance imformation and then to realize defect detection.Then,to enrich ECT technology and improve NDT level,this thesis studies a novel eddy current disturbance acquisition method base on the basic of the existing ECT principle.Therefore,a novel eddy current testing technology based on the electromagnetic acoustic effect was proposed in this thesis.In this technology,a new meander eddy current coil is designed according to the prototype of transmitting and receiving ?-?eddy current array probe.Unlike the existing ECT technology,utilizing the electromagnetic field effect the eddy current signal is converted to the ultrasonic signal by setting a bias magnetic field.In this new technology,the novelty is that the ultrasonic wave is used to replace the traditional pickup coil as a means of effect signal acquisition.By this way,the impendance change of pickup coil is transformed into the change of ultrasonic amplitude,and the spatial distribution of eddy current disturbance is transformed into the time-domain disturbance of ultrasound.All in all,the method proposed in this thesis can not only defect defects,but also analyze the spatial distribution of eddy current disturbances by using ultrasonic signals,so as to locate defects.Firstly,the electromagnetic field of the new meander eddy current coil was modeled and analyzed,so that the vertex factor method based on the second order vector potential was proposed.The electromagnetic field model of the coil was established by using the vertex factor method,and the distribution of the induced eddy current field of the coil was obtained by the model.On this basic,the working principle of the new eddy current coil was discussed.Then,to solve the problem existing in the new eddy current coil,the coil expansion research was carried out,and the straight wire eddy current "coil" and corres eddy current coil were respectively designed.Further,the working principle of two extended coil was analyzed and the characteristics of the extended coil were discussed.Finally,the experimental platform for a new current testing based on the electromagnetic acoustic effect was built.At the same time,a series of experiments were carried out to defect surface defects of aluminum alloy conductors with new eddy coil,straight wire eddy current "coil",and cross eddy current coil,respectively.The above experiments results showed that:? the new eddy current coil can resplace the ?-? type eddy current array probe,which can not only defect with a width of more than 6 mm,but also locate the surface defects;?the straight wire"coil" have the characteristics of large scanning area and without the ultrasonic signal overlapping interference,and this type coil can defect long defects wih a width of more than 1 mm;?the cross eddy current coil can realize simultaneous detection in two orthogonal directions.So,it can not only defect the long strip defect,but also locate the circular defect whose diameter is more than 8 mm accurately.