Eddy current tests are important test & widely used method within the broad field of Nondestructive materials & evaluation. This method is particularly well suited for the detection of service induced cracks usually caused either by fatigue or by stress corrosion. Eddy current inspection can be performed with a minimum of part preparation and a high degree of sensitivity.
Eddy currents are electrical currents induced in a conductor of electricity by reaction with alternating magnetic field. Eddy currents are circular & oriented perpendicular to the direction of the applied magnetic field.
The a) electrical conductivity
b) magnetic permeability
c) geometry and
d) homogeneity of the test object, all affects the induced currents.
The electrical conductivity & magnetic permeability of a material is influenced by its chemistry & heat treat condition. Mixed lots of materials or parts subjected to fire or excessive heat damage can be quickly & easily separated (conductivity testing).
Changes in the geometry & homogeneity of the test object will change the magnitude & distribution of the eddy currents. By monitoring these changes, the presence of cracks & other flaws can be detected.
The eddy current inspection system basically consists of five functions : a) Oscillator b) Test coil absolute or differential c) Bridge circuit d) Signal processing circuits e) Read out or display.
Equipment : Usually for aircraft eddy current inspection following test instruments are used
- 1) Meter display instrument - It comprises a graduated scale in milliampers of moveable meter needle. The amplitude of needle movement in proportional to the impedance of the test circuit.
2) Impedance plane display instrument - It features a 'flying dot' on a CRT, LCD or video display. The position of flying dot indicates the impedance of the test circuit, but also displays effect of both resistance & reactance presenting both phase and amplitude information.
3) Linear time base display instrument - It is usually used with rotating open hole probe scanners. The 'horizontal position' of the signal on the display indicates sensor clock pPosition in the hole & the 'vertical peak' of the signal indicates amplitude of response.
4) Bargraph display instrument - It features on LCD read out bar scale graduated in voltage sensitive increments. The position of the display indication is adjustable from one bar to full scale.
i) High frequency surface & bolt hole probes
ii) High frequency special probes (counter sink plug & shaped)
iii) Low frequency probe (spot encircling & shaped)
iv) Sliding probe (driver/receiver).
Applications : Eddy current test is used to detect surface & subsurface defects, corrosion in aircraft structures, fastener holes and bolt holes. Surface detects and conductivity testing by high frequency and sub-surface detects by low frequency methods.
Routine eddy current inspection is carried out on aircraft under carriage wheel hubs for cracks also used to detect cracks in different tubes, tublar components of aircraft & engine.
Key Points : Only applicable to conductive materials (ferrous, non ferrous & austenitic components). Calibration standards & trained operator required. Fast & portable. Spacial probes required for variation of materials and accessibility.