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17 Cards in this Set
- Front
- Back
What is EC testing normally used for?
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- surface crack detection
- tube and bar inspection - metal sorting - layer thickness measurement |
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Describe the main principles of the EC testing techniques
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- 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 |
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What factors impose limitations to the flow of AC current?
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- 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 |
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What are the equations related to impedance Z?
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abs(Z) = (R^2 + Xl^2)^0.5
Xl=inductive reactance=ωL=2πfL |
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What factors affect eddy currents?
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- lift-off
- electrical conductivity - magnetic permeability - edge effect - skin effect |
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What is the effect of lift-off on the impedance plane diagram and why?
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- 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) |
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What can lift-off used for to measure?
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To measure the thickness of non-conducting coatings on metals
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What effect does electrical conductivity have on the display spot and why?
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When conductivity is reduced the spot moves less on approaching the metal and at a different angle
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What factors affect the conductivity and permeability of a metal?
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- chemical composition
- heat treatment - age hardening - temperature - residual stress |
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What is the magentic permeability µ?
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- 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 |
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What is the edge effect and how can it be reduced?
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- 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 |
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What is the skin depth and what is it dependent on?
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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 |
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what is the trade-off between depth of penetration and sensitivity of flaws?
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the lower the frequency, the greater the depth byt sensitivity is reduced
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What is an absolute coil?
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if the same coil is used for both generating the primary field and detecting the second
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what is a differential coil and how does it work?
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- 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 |
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What are the advantages of a differential coil?
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- higher degree of amplification since blanced voltage is normally zero
- factors affecting both coils, such as temperature, lift-off etc., are automatically cancelled |
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Draw the bridge circuit diagram of a differential coil
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IMAGE
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