Eddy Current Flashcards
What is the eddy current inspection method?
An electromagnetic system for detecting flaws on usually small parts.
In what situation would the eddy current inspection method be preferable over the magnetic particle or dye penetrant inspection
methods?
If the material is electrically conductive. Furthermore, if a material is porous and non ferrous, the eddy current inspection would be used.
What are eddy currents?
Eddy currents are induced electrical currents that flow in a circular path.
How are eddy currents utilized in the inspection procedure?
Eddy currents are induced in the material and analyzed. Any disturbance in the eddy currents will cause the equipment to signal an indication.
What is the inspection probe constructed of?
The probe is constructed of coil of very fine wire.
What are the different types of inspection probes available?
Flat-surface, spring-loaded, pencil, shielded pencil, right-angle pencil, or fastener hole probes.
What is impedance and what affect does it have in the eddy current inspection procedure?
Impedance is the total resistance to current flow through a coil. It is the sum of the resistance and inductive resistance. A change in the normal impedance detects the defects.
What frequencies are used for eddy current testing and what factors are considered when selecting a specific frequency?
The frequencies used for eddy current testing is 100 Hz to 4 MHz. Factors considered when selecting frequency are the depth of penetration of eddy currents, size and location of cracks, and conductivity.
What is used for displaying the results of the inspection?
Liquid Crystal Display and Cathode Ray Tube are used for displaying results
Why is the eddy current probe cable shielded?
It is shielded to restrict the spread of the magnetic field as well as to increase sensitivity.
Why are crack and conductivity standards used?
Conductivity standards are used to sort and identify materials by conductivity in case technicians have to determine the proper heat treat of a part. Crack standards are used to calibrate the eddy current instrument to measure crack depth.
What type of defects (location, type) can the eddy current inspection method detect?
The eddy current inspection method can detect corrosion, crack, stress crack, material loss on the surfaces and subsurfaces of conductive materials.
Explain the absolute and comparison method.
Absolute method: A test comparison is made with sound material of equal conductivity, mass and permeability. The meter is set to the null or middle position and when the probe is passed over a defect the needle moves indicating a change in the materials characteristics.
Comparison method: Two probes are used, one is held in contact with the reference specimen and the other with the inspection piece. A comparison is made with the wheatstone bridge circuit.
When inspecting honeycomb structures, what is the preferred N.D.T inspection method?
The preferred NDT inspection method when inspecting honeycomb structures is the coin tap test.
