Natural Crack Detection For Complex Structures Based On Region Electromagnetic Sensing Thermography

Nondestructive inspection for fatigue cracks is a critical and challenge task in both science and industry fields.During manufacture and in-service of materials,fatigue cracks will originate and development in enduring intense stress concentration and under long-term continuous alternating loading.The conventional testing methods are Magnetic Particle Testing(MT),Penetration Testing(PT)and Ray Testing(RT).However,MT is complicated and it only works for ferromagnetic material,the penetrant process requires to be cleaned and it produces pollution to the environment,and the radiation is harmful to human body.Recently,the electromagnetic thermography has attracted increasingly attention as it enables to conduct a rapid inspection over a large area with high spatial resolution and sensitivity.In addition,there are none consumables and pollution.It is widely used in the detection of damages including corrosion in steel,rolling contact fatigue(RCF)cracks in rail and welding cracks in pressure vessels,boilers and pipelines.However,the problem of in-homogenous heating for nature micro-crack of complicated structure can not be ignored.It is difficult to extract the crack signal from big noise.Thus,this paper investigates the region sensing effect for the distribution of the electric,magnetic and thermal fields.Different lift-off distance and region effect between sensing-defect are considered.Increasing the vertical and horizontal lift-off distance between sensing-defect,the eddy current intensity will increase while the uniformity will drop.There is one region where the eddy current uniformity and intensity are balanced.These studies enable us to build the physic context connected between the local and the global electromagnetic thermal region sensing model which benefits the detection on natural cracks.It provides a possibility to dramatically increasing the thermal contrast between the defect and non-defect areas for weak thermal signal detection.In this paper,firstly,the eddy current distribution on the surface of test sample for electromagnetic test has been analyzed.The study indicates the exits of region sensing.Secondly,through finite element analysis,the change of eddy current intensity and distribution of non-defect sample when increasing the vertical and horizontal lift-off distance between sensing-defect has been discussed.The study also analyzed the thermal contrast because of increasing lift-off distance.The simulation consists of surface crack and sub-surface crack,the size is set at several hundred micron,which is generally accepted for evaluating micro-cracks.Thirdly,verification experiment has been implement using the welding cracks in pressure vessels.The welding crack is small,almost cannot be seen by human eyes.The effect on crack signal extracting because of vertical and horizontal lift-off distance increasing has been discussed.The study of the proposed region electromagnetic sensing thermography beyond the common thermography algorithms confirms that without obtaining the optimal region electromagnetic thermal field,it is difficult to detect the cracks correctly.Finally,this paper resumed region electromagnetic sensing thermography under speed-effect.Considering slow speed electromagnetic thermography,as the optimal test region is bond to appear in this process,and the temperature will superpose,the slow speed electromagnetic thermography is benefit for crack detect.The theoretical analysis and experiment indicates that region-sensing effect is exits in the electromagnetic thermography.As the vertical and horizontal lift-off distance increasing,the eddy current intensity drop,the uniformity increased.By choosing appropriate distance,the intensity and uniformity can be balanced,the thermal contrast can be enhanced and it is good for extracting the crack signal.Using sensing-region effect,the slow speed electromagnetic thermography is also benefit for crack detect as the temperature will superpose.