Basic principle of Eddy current test

Eddy current testing is using the principle of electromagnetic induction,The sine wave current excitation probe coil,When the probe is close to metal surface,The alternating magnetic field around the coil generates induced current in the metal surface,To plate metal, The flowing direction of induced current in roundness that have same center of circle with coil。Shaped like a spiral,called eddy current, eddy current produces magnetic field with same frequency ,Its direction and the coil magnetic field in the opposite direction。

The wastage resistance of the eddy current channel ，and The eddy current generates magnetic flux,Reflect to the probe coil,change the current strength of coil and its phase。Namely changed impedance of coil。Therefore, the probe is moving on the metal surface。If flaw or material or dimension is changed ,make eddy current magnetic field has different reaction ,cause coil impedance changed, by eddy current detector measures this change to identify whether there is flaw on surface or not,。There are many factors affect eddy current ,such as coupling degree of probe coil and detected material,material size and dimension、conductivity ,magnetic permeability,and flaw,so utilize eddy current principle can solve metal material flaw detection、thickness measurement、and sort。

Eddy current test is one of many NDT methods, it applies “electromagnetic ” basic theory as basis of conductor test。Generating Eddy current is derive from magnetic induction,when alternating current applies to conductor,such as copper conductor,magnetic field will be produced in conductor。Eddy current is induce generating current，it flows in a loop circuit,why it is called “eddy current”,is due to it has same flaw form in liquid or gas 。If put a conductor into changeable magnetic field,eddy current will be generated in this conductor,and eddy current will produce its own magnetic field,the magnetic field is extending with alternating current rising,blanking along with alternating current reducing 。So when conductor surface or close to surface has flaw or some of properties of detecting metal material are changed ,it will affect strength and distribution of eddy current,and then we can detect changeable condition of eddy current to indirectly know internal flaw and metal properties are changed or not。

Superiority of Eddy current test mainly include:

1. Sensitivity on small crack and other flaw

2. Quick speed and high sensitivity on Testing surface and close to surface   

3. Testing result is instant

4. Device connector is good

5. Little preparation work is needed

6. Testing probe does not need to touch object

7. Detect conductor with complex dimension and shape

What is eddy current?

When metal conductor is in magnetic field or moving in magnetic field,due to electromagnetic induction principle,generating spiral shaped current in conductor,called eddy current。

What is impedance plane display?

Eddy current test is by testing impedance changing value of eddy current sensor to realize。Spot impedance includes reactance and impedance,we use R as horizontal coordinates, X as longitudinal coordinates,generating rectangular coordinate system ,by changing impedance of eddy current testing sensor。

What factors affect impedance display?

Conductivity、magnetic conductivity ,appearance dimension,filling coefficient,lift off effect,margin effect

What is lift off effect?

When detecting coil has relative position change with detected material,eddy current density generated by testing coil on material will be changed,eddy current density is reducing along with increasing distance between detecting coil and material，and make Q mobile on displayed plane,this phenomenon is called lift off effect，using this principle can do metal surface nonmetal coating thickness measure 。

What is filling coefficient?

Coupling degree between Detecting probe and material ,filling coefficient is bigger,probe and material coupling better,magnetic induction effect is better,detecting sensitivity is higher。filling coefficient  can be showed by η=（d/D）2。

What is called margin and end effect?

Magnetic field in coil is extended to every direction,when coil up to margin,due to margin signal ,eddy current is changed,this is called margin effect，when detecting coil closes to beginning and ending terminal,usually called end effect。

What is called the skin effect of alternating current to flow,When direct current flows through a cylinder, the current density on the cross section are the same;but The alternating current through the cylinder,Current density is different in all parts of the cross section,surface current density is big,This phenomenon is called the skin effect。Metal conductor connects to alternating current,Alternating current density distribution on the conductor cross section is index law from the surface to the internal attenuation,The attenuation law of e­xpression as follows:Jx=Joе-αx ,x is depth from surface,Jx is current density in X depth in conductor,J0 is current density on conductor surface,α is attenuation coefficient,f is frequency ,μ is magnetic conductivity,σ is conductivity。Attenuation of Alternating current density in the conductor cross section is related to frequency of the alternating current,permeability and conductivity conductor.

Detecting coil and classify

Crossing coil 

Crossing coil is put detected workpiece into coil to detect coil, mainly used for inspection on tube ,bar,wire.due to magnetic generated by coil first to have effect on  wall outside the specimen。So it has good effect on detecting outer wall defect,The lining defect detection is to use the permeability of the magnetic field, normally,internal wall flaw sensitivity is lower than outer wall,The internal defects of thick wall pipe can't be use outside through the coil to detect。

Through the coil

Through the coil is coil that put into internal pipe,mainly used to check the thick wall pipe wall or the defects of borehole wall,it also can be used to detect tube quality,such as heat exchange type coil

Probe coil 

Probe coil is a coil that put onto spacimen surface, it is not only suitable for panel，slap,square billet, round billet,bar and tube with large diameter,but also applied to shape more complex mechanical parts inspection,compare to through coil ,it has little volume,little magnetic effect,so it suitable for detect surface flaw with little diameter。

What is multiple frequency eddy current and far field eddy current technique?

When detecting many complex component, component will produce  strong disturbing signal，Make it hard for single frequency eddy current to accurate to detect defects，for overcoming this interference,let detecting signal eliminate fake and keep true,

Improving detecting reliability,Using several frequency excitation coil at the same time,Multiple sets of data obtained at the same timeby detecting coil ,And mixingprocessing on collected data  to suppress interference signal,this is called multiple frequency eddy current technique。

The far field eddy current is a low-frequency eddy current technology can penetrate the wall of the ,It is mainly used when testing wear type probe ,The excitation coil and measuring the distance between coil is about two times the pipe diameter,connect to Low-frequency alternating current,Coil can be detected through the wall and back to excitation magnetic field signal,Defects of inside and outside wall and wall thickness thinning degreecan be effectively detected。

What is the leakage magnetic field

When using magnetizer magnetized ferromagnetic materials are measured,If the material of material is continuous and uniform,The material of magnetic induction line will be constrained in the material,Flux is parallel to the surface, nearly no magnetic induction line penetrate from surface。No magnetic field on the surface,But when there are cutting lines of magnetic force the defects in the material,Defects on the surface of the material or organization status change can make the permeability change,Due to the defects of permeability is very small, the magnetic resistance is very big, make magnetic flux have distortion in the magnetic circuit 。magnetic induction lineWill change the way ，In addition to the part of the magnetic flux directly through the defect or routing around the defects inside the material，And part of the magnetic flux will leave the material surface，Routing around the defects via air back into the material， in the material surface generates leakage magnetic field。

Whether magnetic flux leakage Can detect internal defects?

Magnetic flux leakage detection is mainly aimed at the surface defect detection,For internal defect detectionmainly depends on the distance between flaw and surface and magnetization strength of material。

If defects is deeper in material then there is no way to accurately detect。

What is magnetic mechanical effect and Magnetic memory effect?

Primary sources of mechanical parts and metal components damage,is all kinds of micro and macro mechanical stress concentration,In stress concentration area,Corrosion, fatigue and creep process of the development is the most intense。

Mechanical stress with the phenomenon of magnetization of ferromagnetic material and residual magnetic conditions have a direct link。Under the condition of magnetic effects,the permeability of defect decreases.leakage magnetic field on the surface of the workpiece .the properties of ferromagnetic material is called magnetism machinery effects.the existence of the magnetic mechanical effect makes the ferromagnetic metal workpiece metal workpiece surface magnetic field improved.

The main detecting target of magnetic memory.

Metal magnetic memory testing is mainly used for the early diagnosis of ferromagnetic material prevention,pipes ,containers,turbine blade,a plane,the aircraft landing gear,oil well pipe construction as well as a variety of different shapes,welding connector and so on.MMM detection doesn’t need surface treatment,detection is quick and simple.