Eddy Current

Front 1

What is EC testing normally used for?

Back 1

- surface crack detection
- tube and bar inspection
- metal sorting
- layer thickness measurement

Front 2

Describe the main principles of the EC testing techniques

Back 2

- the AC coil produces an alternating magentic field oriented perpendicular to the direction of the current
- when this magentic field inetersects an adjacent conductor, eddy currents are induced in the specimen flowing normal to the primary field
- the eddy currents in turn generate their own magnetic field which is opposite to the primary field and modifies it
- this in turn causes a decrease in magnetic flux through the primary coil modifying the primary current in both phase and amplitude and thus changing the coil impedance
- EC instruments are based on teh voltage change across the coil which is proportional to the impedance change

Front 3

What factors impose limitations to the flow of AC current?

Back 3

- the ohmic resitance R: resistance to the flow of electricity and proportional to energy dissipated per cycle.
- inductance L: proportional to energy stored in magnetic field, and returned to generator in each cycle
- capacitance C: proportional to the electric charge stored

Front 4

What are the equations related to impedance Z?

Back 4

abs(Z) = (R^2 + Xl^2)^0.5
Xl=inductive reactance=ωL=2πfL

Front 5

What factors affect eddy currents?

Back 5

- lift-off
- electrical conductivity
- magnetic permeability
- edge effect
- skin effect

Front 6

What is the effect of lift-off on the impedance plane diagram and why?

Back 6

- effect is point moves down and right
- because as coil is moved towards specimen, eddy currents and a secondary field are generated, causing an apparent increase in resistnace (due to losses in metal) and a decrease in the inductance of the coil (due to reduction in primary field)

Front 7

What can lift-off used for to measure?

Back 7

To measure the thickness of non-conducting coatings on metals

Front 8

What effect does electrical conductivity have on the display spot and why?

Back 8

When conductivity is reduced the spot moves less on approaching the metal and at a different angle

Front 9

What factors affect the conductivity and permeability of a metal?

Back 9

- chemical composition
- heat treatment
- age hardening
- temperature
- residual stress

Front 10

What is the magentic permeability µ?

Back 10

- it is the ability to concentrate the magnetic flux
- the ratio betyween flux density and field strength
- in a material with high permeability a larger density of magnetic field lines will be created form a given source and the lines will tend to concentrate in the material

Front 11

What is the edge effect and how can it be reduced?

Back 11

- when a coil approaches the end or edge of a part, currents are distorted
- geometry and large edge effects can be reduced by probe design

Front 12

What is the skin depth and what is it dependent on?

Back 12

The depth at which the density of eddy currents has fallen to 37% (1/e) of the density at the surface

δ=1/(πfµσ) σ=conductivity

Front 13

what is the trade-off between depth of penetration and sensitivity of flaws?

Back 13

the lower the frequency, the greater the depth byt sensitivity is reduced

Front 14

What is an absolute coil?

Back 14

if the same coil is used for both generating the primary field and detecting the second

Front 15

what is a differential coil and how does it work?

Back 15

- two similar coils are connected in a series-opposing arrangement
- the impedance changes from each coil are subtracted producing a zero output if both coils are influenced by the same condition
- if only one coil is affected an output occurs

Front 16

What are the advantages of a differential coil?

Back 16

- higher degree of amplification since blanced voltage is normally zero
- factors affecting both coils, such as temperature, lift-off etc., are automatically cancelled

Front 17

Draw the bridge circuit diagram of a differential coil

Back 17

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