ASNT NDT Level III Electromagnetic / Eddy Current Testing (ET)

Which physical law explains why a changing magnetic field induces eddy currents in a nearby conductor?

• AOhm's law
• BFaraday's law of electromagnetic induction
• CCoulomb's law of electrostatics
• DKirchhoff's current law

What does Lenz's law tell you about the direction of eddy currents induced in a conductor by an approaching probe coil?

• ATheir direction depends only on temperature, not on probe motion
• BThey flow in the same direction as the probe current to reinforce the field
• CThey flow in a direction that opposes the change in the original magnetic flux
• DThey flow only along the surface in the direction of probe motion

Using the standard depth-of-penetration relationship delta = 503 / sqrt(sigma * mu_r * f), with sigma in S/m, what happens to skin depth when frequency is quadrupled and all other variables are held constant?

• ASkin depth is reduced by a factor of four
• BSkin depth is unchanged
• CSkin depth doubles
• DSkin depth is halved

A bobbin coil with inside diameter 14.0 mm inspects a tube whose outside diameter is 15.9 mm. What is the fill factor?

• AApproximately 1.13
• BApproximately 0.78
• CApproximately 0.88
• DApproximately 0.93

On a standard impedance-plane display for a surface coil on a nonferromagnetic conductor, which axis is typically shown vertical and which horizontal?

• AVertical = current, horizontal = voltage
• BVertical = resistive component, horizontal = inductive reactance
• CVertical = inductive reactance, horizontal = resistive component
• DVertical = phase angle, horizontal = lift-off only

On the conductivity locus of an impedance plane, where does a high-conductivity material like pure copper plot relative to a low-conductivity material like Inconel?

• AOff the locus, on the negative resistance side
• BAt the origin regardless of conductivity
• CHigher on the locus, near the top of the curve
• DLower on the locus, away from the top

Lift-off describes which physical condition during a surface-coil inspection?

• AThe probe-to-conductor stand-off distance through any nonconductive layer
• BVariation in coil inductance with temperature
• CChange in tube wall thickness during scanning
• DProbe motion parallel to the surface

Calculate the standard depth of penetration in 304 stainless steel (sigma about 1.39 x 10^6 S/m, mu_r about 1) at 100 kHz, using delta = 503 / sqrt(sigma * mu_r * f) with sigma in S/m.

• AApproximately 13 mm
• BApproximately 0.4 mm
• CApproximately 1.3 mm
• DApproximately 4.3 mm

Why does ferromagnetic material (mu_r much greater than 1) produce a very small skin depth at the same frequency compared with a nonmagnetic conductor of similar conductivity?

• ABecause Lenz's law does not apply to ferromagnetic material
• BBecause eddy currents only flow in nonmagnetic materials
• CBecause the impedance plane collapses to a single point
• DBecause permeability appears under the square root in the denominator of the skin depth f…

What is the approximate phase angle between the surface eddy current density and the current one skin depth below the surface?

• A57 degrees (1 radian)
• B90 degrees
• C0 degrees
• D45 degrees

Roughly how much does eddy current density decrease from the surface to one standard depth of penetration?

• AAbout 5% (drops to 0.95 of surface)
• BAbout 37% remaining (drops to 1/e of surface)
• CAbout 50% remaining
• DIt is unchanged at one standard depth

A Level III wants to detect a near-surface crack with maximum sensitivity. Which choice describes the best general strategy?

• AAlways use a multifrequency mix with low and very high frequencies on every job
• BUse the highest available frequency regardless of geometry
• CUse the lowest possible frequency to maximize penetration
• DUse a frequency that places the standard depth near the expected crack depth, then phase-…

How does increasing test frequency typically affect a surface-crack signal's phase angle on the impedance plane?

• APhase angle increases, rotating the defect signal away from the lift-off direction
• BPhase angle is unaffected by frequency changes
• CPhase angle inverts completely above 1 kHz
• DPhase angle decreases toward the lift-off direction

Edge effect in eddy current testing refers to which condition?

• ADrift caused by instrument warm-up time
• BSignal distortion from the probe approaching a geometric edge or boundary
• CLoss of signal from a corroded couplant interface
• DMechanical vibration of the probe cable

Which statement about effective permeability for a small surface coil on a flat conductor is correct?

• AEffective permeability is irrelevant once the probe is calibrated
• BEffective permeability equals free-space permeability for all materials
• CEffective permeability accounts for the combined influence of conductivity, frequency, an…
• DEffective permeability changes only with applied DC bias

Why does a strong DC magnetic saturation field improve eddy current inspection of ferromagnetic tubing?

• AIt drives mu_r toward 1 so that eddy current penetration matches that of nonmagnetic mate…
• BIt removes lift-off variation entirely
• CIt eliminates the need for any reference standard
• DIt increases the tube's electrical conductivity directly

Which best describes the operating point's behavior as a surface coil is lifted slowly off a nonferromagnetic conductor?

• AIt stays fixed at the conductivity locus
• BIt moves toward the air point along a characteristic lift-off direction
• CIt moves vertically only on the impedance plane
• DIt moves randomly without a repeatable direction

Why are conductive films such as graphite or aluminum paint problematic for surface eddy current inspection?

• AThey magnetize the substrate
• BThey block all coil radiation
• CThey produce extra induced currents and act as a low-resistance shunt that masks subsurfa…
• DThey eliminate lift-off entirely

Which probe configuration is most appropriate for inspecting an accessible flat surface for fatigue cracks at a fastener hole?

• ASend-receive remote-field probe
• BBobbin probe
• CInternal encircling coil
• DSurface (pencil or shielded) probe

A bobbin probe is used inside a heat-exchanger tube. Which probe attribute is fixed by the tube's inside diameter?

• ATest frequency
• BFill factor (and thus coil diameter)
• CInstrument gain
• DStrip-chart speed

What is the chief advantage of an external encircling coil over a bobbin coil for inspecting solid bar stock?

• AIt works only on ferromagnetic alloys
• BIt produces no eddy currents
• CIt can scan around the full circumference at once without inserting a probe into the bar
• DIt eliminates the need for any reference standard

An absolute coil and a differential coil are run over identical short cracks. Which response is expected from the differential probe?

• ANo signal at all
• BA constant DC offset
• CA single large positive deflection
• DA pair of opposite-polarity lobes as the gradient between coils changes

Which type of indication will a bobbin differential coil most likely under-respond to?

• AA gradual, uniform wall thinning over many inches
• BA small ID corrosion cluster
• CA sharp local pit
• DA short circumferential notch

In a reflection (driver/pickup) probe arrangement, which coil's signal is normally analyzed?

• AThe bridge balance resistor
• BThe driver coil's primary current
• CThe pickup coil's induced voltage
• DThe shield coil's leakage

A send-receive probe with axially separated coils is the basis for which technique often used on ferromagnetic heat-exchanger tubing?

• APenetrant testing
• BMagnetic flux leakage
• CConventional bobbin testing
• DRemote-field testing (RFT)

What is the primary benefit of an eddy current array (ECA) probe over a single surface coil for in-service surface scanning?

• AIt covers a wide footprint with electronically multiplexed coils for faster, more repeata…
• BIt removes the need for any frequency selection
• CIt is unaffected by lift-off
• DIt eliminates calibration entirely

A shielded surface coil differs from an unshielded one because the shield does what?

• AReplaces the need for a reference standard
• BConcentrates the magnetic field beneath the coil for better edge response and resolution
• CEliminates skin-depth limits
• DIncreases coil DC resistance to lower noise

Which probe is the standard choice for in-service inspection of nonferromagnetic heat-exchanger tubing from the inside?

• APenetrant applicator
• BMagnetic yoke
• CExternal encircling coil
• DBobbin probe with absolute and differential channels

A motorized rotating pancake coil (RPC) inside a tube is most useful for which inspection goal?

• AHigh-resolution detection and sizing of localized ID flaws such as pitting and tight crac…
• BConductivity sorting of bars
• CCoating thickness on flat panels
• DBulk wall-thickness gauging on plate stock

Why does an absolute coil drift more than a differential coil when temperature changes during a long tubing run?

• AIt has more coils, so it heats faster
• BThere is no reference coil to cancel changes in conductivity, dimensions, and coil resist…
• CIt only operates above 1 MHz
• DIt is always operated with no reference standard

What benefit does a +Point or saddle-style flexible array probe offer over a flat pancake coil at a weld toe?

• AIt increases skin depth proportionally
• BIt is insensitive to all weld geometry
• CIts small crossed-coil footprint detects tight cracks at variable weld profiles with less…
• DIt eliminates the need for any couplant

Why does a probe with too low a fill factor on a tube give weak indications?

• ABecause lower fill factor reduces electromagnetic coupling between the coil and the tube…
• BBecause lower fill factor cancels Lenz's law
• CBecause the impedance plane disappears
• DBecause the probe overheats

What advantage does a differential bobbin probe offer for through-wall pit detection at low scan speed?

• AIt cannot detect through-wall flaws
• BDrift and gradual temperature/dimension changes are suppressed, isolating localized signa…
• CIt removes the need for any reference
• DIt cancels short-circumferential cracks

Which is the best general statement about probe selection for inspecting ferromagnetic carbon steel tubing in a feedwater heater?

• AOnly ECA flexible arrays can be used
• BStandard bobbin probes work as well as on austenitic stainless tubing
• CConventional bobbin probes are limited; magnetic saturation, RFT, or NFT probes are typic…
• DPenetrant testing is the only acceptable method

What is the primary purpose of a balance or null on an eddy current instrument before scanning?

• AIt selects the inspection frequency automatically
• BIt increases skin depth
• CIt compensates for instrument warm-up over the next hour
• DIt zeros the impedance plane operating point so changes in coil state are displayed clear…

Phase rotation on an eddy current instrument is used to do which of the following?

• AIncrease the inspection frequency by a factor of two
• BRotate the impedance plane display so lift-off appears along a chosen axis (typically hor…
• CEliminate any need for calibration
• DConvert eddy current signals to ultrasonic A-scans

On an impedance-plane display, where does the operating point usually settle when a surface coil is held in air, well away from any conductor?

• AAt the resistive axis far from the locus
• BAt the origin
• CAt the air point near the top of the conductivity locus
• DAt the high-conductivity end of the locus

Why does a Level III routinely set the inspection frequency by experiment on a calibration standard, rather than just relying on a formula?

• ABecause skin-depth formulas cannot be trusted
• BBecause instrument frequencies are random
• CBecause the formula is only valid in air
• DBecause real materials, geometry, and probe geometry mean the optimum frequency is verifi…

Which display is most informative for sizing short tubing flaws with phase-angle techniques?

• AImpedance-plane vector (Lissajous) display
• BA-scan only
• CDC voltmeter readout
• DStrip-chart only

Strip-chart presentation on tubing inspections is used primarily for what purpose?

• ATo compute skin depth automatically
• BTo replace the impedance plane entirely
• CTo track probe position landmarks (support plates, tubesheet) and identify indications al…
• DTo measure conductivity in %IACS

Multifrequency eddy current testing is most commonly used to do which of the following?

• AAvoid the need for any reference standard
• BEliminate all lift-off effects regardless of geometry
• CReplace the inspection frequency calculation
• DCombine data from two or more frequencies to mix out unwanted signals such as support-pla…

When mixing out a carbon-steel support-plate signal on stainless tubing, which frequency choice is generally most effective for the auxiliary mix channel?

• AA frequency much lower than the primary inspection frequency, where the support-plate sig…
• BThe same frequency as the primary
• CDC only
• DA frequency much higher than the primary inspection frequency

Vertical and horizontal gain controls on a typical eddy current instrument do which of the following?

• ASet the inspection frequency
• BIndependently scale the vertical (Y) and horizontal (X) channel amplitudes of the vector…
• CAdjust the AC line voltage
• DIncrease skin depth

Why must instrument warm-up and operational checks be performed before a calibration run?

• ATo verify the candidate's certification
• BTo shorten the test report
• CTo preheat the calibration standard
• DTo stabilize coil and electronics drift so the calibration reflects the actual operating…

Why are calibration verification checks (e.g., re-running the standard) required at procedure-specified intervals during long scans?

• ATo confirm that drift, probe wear, or temperature has not changed the calibrated response…
• BTo re-license the technician
• CTo regenerate the conductivity locus
• DTo recharge the probe coil

Which of the following is a typical built-in feature of modern multifrequency eddy current instruments used for tubing?

• AHand-cranked galvanometer only
• BDigital filtering, simultaneous multifrequency drive, and configurable mix channels
• CMandatory radioactive source
• DOnly analog strip-chart output

On the impedance plane, what happens to a localized ID flaw response on a stainless tube as inspection frequency is increased?

• AIts phase angle remains the same as an OD flaw
• BIt disappears entirely
• CIts phase angle increases relative to the OD response
• DIts phase angle decreases until it lies along the lift-off direction