Circle-Ferrite Inductor Induction Thermography

Nondestructive testing(NDT)is an important means of reliability guaranteeing and structural health monitoring.As an emerging nondestructive testing technology,induction thermography(IT)combines the advantages of eddy current testing and infrared thermography testing.And it has many advantages,such as sensitive to the near surface defects,high spatial resolution,fixing the defect position,non-contact testing,convenient for data storage and so on.IT,which developed rapidly in recent years,has been widely used in the fields of iron smelting,electric power,nuclear industry,military,transportation,petrochemical industry,etc.However,it still has the limitations of small single detectable area,poor robustness to inclination defect detection,and small allowable lift-off distance.In order to overcome the problems,this paper proposes an induction thermography system based on circle-ferrite inductor which can improve the detection capability of the proposed system obviously.The proposed structure,which collects the spatial magnetic flux and magnetic energy and injects them into sample,forms a high-strength uniform magnetic field and a circular eddy current field in sample.In this paper,firstly,the induction heating model based on magnetic circuit theory and the thermal image acquisition model based on infrared radiation of the proposed system were modeled.And the advantages of the system was analyzed and interpreted theoretically.Secondly,the electromagnetic features of the inductor and the electromagnetism-thermal field in the sample was introduced.Thirdly,the detection capability of the proposed system was estimated by finite element numerical experiments and verification experiments.The numerical experiments,were compared with the line inductor testing,consist of omnidirectional defect group,single detectable area group and free-surface sample group.And the samples for numerical experiments includes ferromagnetic and non-ferromagnetic material.Verification experiments of circle-ferrite thermography consist of artificial defect group and nature defect group and are compared with line thermography.The artificial sample,which includes ferromagnetic and non-ferromagnetic material,was set the same as in numerical experiments to verify numerical experiments.The nature sample which including rail sample and nuclear intergranular corrosion 316 stainless steel sample is used for examining the reality detecting capability of the system.The theoretical analysis and experiments indicate that the induction thermography system based on circle-ferrite inductor has good detection capability for ferromagnetic and non-ferromagnetic components.The advantage of the proposed system over the traditional line inductor is that it is sensitive to omnidirectional defects,it has a large single detection area of more than 6000 mm~2,it allows higher lift-off distance and the signal to noise ratio of the detection results is larger than that of the line inductor more than 3 times.Finally,two reference rules for inductor design were summarized by analyzing the induction thermography system based on circle-ferrite inductor.And the future work was introduced.