ET Lvl 2 Flashcards
The coil to specimen impedance Z can be defined by (where Zc is coil impedance and Zs is specimen impedance)
a) (Zc X Zs) / (Zc + Zs)
b) (Zc + Zs) / (Zc X Zs)
c) Zc - Zs
d) none of the above
a) (Zc X Zs) / (Zc + Zs)
At high operating frequencies, the effective coil diameter (sensing diameter) is approximately equal to
a) 0.5 coil diameters
b) the actual coil diameter
c) 2 coil diameter
d) the skin depth
b) the actual coil diameter
A significant disadvantage of using a natural crack as a calibration standard is accurately sizing it. What is the only reliable direct sizing method to determine natural crack depth?
a) time of flight diffraction
b) x-ray
c) potential drop
d) cutting the specimen open and optically sizing it under a microscope
d) cutting the specimen open and optically sizing it under a microscope
For a given sized defect, what significant defect signal change occurs when testing a plate using the through transmission (send-receive) method and the defect occurs first 25% of the wall thickness from the transmit coil, then 50%, then 75%?
a) phase angel increases
b) amplitude increases
c) both a and b
d) none of the above, no significant change occurs to the defect signal
d) none of the above, no significant change occurs to the defect signal
Magnetic saturation techniques for EC testing that use DC saturation coils are limited to the amount of saturation achieved by
a) test frequency
b) heating of the saturation coil
c) the size of battery used
d) the voltage that can be safely used
b) heating of the saturation coil
During evaluation of an indication in a heat exchanger tube, the probe is moved back and forth over the defect. It is noted that the indication has changed position along the length of the tube. What is the likely source?
a) a magnetic deposit
b) a spiral fret
c) active cracking
d) a probe wire has loosened at the connector
a) a magnetic deposit
Why are eddy current coils not made using iron wire?
a) to avoid hysteresis effects
b) to make mathematical calculations easier
c) to prevent excessive heat build-up
d) for cathodic breakdown considerations
a) to avoid hysteresis effects
Eddy current flow and its associated magnetic flux are a function of position under the coil. The relationship could best be described as being proportional to (r = radial distance from coil centre)
a) r
b) 1/r
c) r^½
d) no relationship exists
a) r
By convention, the direction of a magnetic line of force is represented by an arrow on a line. The arrow would point in the direction
a) in which a unit north pole would be moved
b) in which a unit south pole would be moved
c) perpendicular to the plane of the line
d) indicated by the thumb in the left hand rule
a) in which a unit north pole would be moved
When both a primary (energizing) and secondary (pickup) coil are used as an encircling coil probe, the time varying flux in the test piece induces an AC voltage in
a) the energizing coil
b) the pickup coil
c) both a and b
d) the surrounding air
c) both a and b
The best method of measuring the effects of a specific discontinuity totally within a test specimen but at different depths and orientations is by using
a) suitably shaped insulators inside a mercury filled tube
b) suitably shaped conductors inside a water filled tube
c) saw cuts in the material to be tested
d) EDM notches in the material to be tested
a) suitably shaped insulators inside a mercury filled tube
Assuming resistance is negligible and probe inductance is 80 µhenries, for a cable with 5 X 10^-9 farads capacitance, what is resonance frequency?
a) 25 Hz
b) 250 Hz
c) 250 kHz
d) 250 MHz
c) 250 kHz
Multifrequency can discriminate signals at the same depth because
a) sensitivities are greater at lower frequencies
b) the phase will be different at different frequencies
c) ferrites are independent of frequency
d) sensitivity is the same for defects but reduced for geometry changes as frequency increases
b) the phase will be different at different frequencies
What is the standard depth of penetration for 301 stainless steel having been 25% cold worked (71µohm-cm, µrel = 10) tested at 10kHz?
a) 1.3mm
b) 2.3mm
c) 3.4mm
d) 4.6mm
a) 1.3mm
The only way to reduce or eliminate the edge effect is by
a) multifrequency techniques
b) magnetic focusing probes
c) spring loaded probes
d) using remote field eddy current techniques
b) magnetic focusing probes
The heating of a ferromagnetic part that occurs when the AC field works to align the magnetic domains into a preferred magnetic orientation is reduced by
a) performing eddy current tests under water
b) performing eddy current tests where air temperature is below 0øC.
c) “pre-aligning” the domains with D.C. saturation
d) multifrequency eddy currents
c) “pre-aligning” the domains with D.C. saturation
Resistivity of a material is a function of
a) a material’s cross-sectional area
b) a material’s length
c) overall resistance
d) none of the above
d) none of the above
The through transmission method has the advantage that
a) lift-off does not decrease sensitivity
b) conductivity and thickness can be measured simultaneously
c) temperature changes do not affect conductivity readings
d) all of the above
b) conductivity and thickness can be measured simultaneously
The “time constant of the circuit” is a ratio of inductance to resistance (L/R). This accounts for
a) generation of eddy currents
b) phase lag of induced currents
c) voltage amplitude
d) self inductance
b) phase lag of induced currents
Which of the following is the direct cause of eddy currents in a test piece placed in an encircling transducer?
a) induced voltages form the AC magnetic field
b) back emf within the transducer
c) resistivity of the test piece
d) the magnetic field opposing the transducer’s field
a) induced voltages form the AC magnetic field
In general, the edge effect seen as a probe is moved towards edge of a magnetic test piece as compared to a nonmagnetic test piece would be recognized by what feature?
a) its curved shape
b) its vertical direction of movement
c) both a and b
d) edge effect on the magnetic material would follow the lift-off trace exactly
c) both a and b
Given a brass tube 20 mm diameter (OD) with a 3mm wall and the resistivity of brass is 7.0 µohm-cm, what is the f90 for testing this tubing?
a) 1630 Hz
b) 2.3 kHz
c) 70 kHz
d) 128 kHz
b) 2.3 kHz
The vector sum quantity of resistance and reactance in an AC current is
a) impedance
b) resistance
c) reactance
d) reluctance
a) impedance
Ferromagnetic deposits and inclusions are usually
a) not detectable
b) nonrelevant or false indications
c) more critical than their signals indicate
d) eliminated by small saturating magnets within the coil
b) nonrelevant or false indications
Hall detectors are used to sense magnetic fields. They detect
a) magnitude of magnetic fields
b) direction of magnetic fields
c) magnitude and direction of electric fields
d) both a and b
d) both a and b
In resonant circuit crack detectors, lift-off adjustment is used to
a) minimize probe wobble effects
b) compensate for paint coatings
c) filter ferrite effects
d) change phase lag
a) minimize probe wobble effects
The through-transmission technique is used for testing of sheet and foil under certain conditions
a) when test surfaces are not excessively large
b) when both surfaces are accessible
c) when the sheet is not multi-layered
d) both a and
d) both a and
Voltage changes across the probe due to a defect in most eddy current inspections are on the order of
a) 1%
b) 10%
c) 100%
d) 1000%
a) 1%
Coil spacing on differential probes for general inspection purposes of tubing is usually
a) defect depth
b) wall thickness
c) both a and b
d) not important
c) both a and b
A test level above or below which test specimens are found to be unacceptable is called the
a) cut-off level
b) rejection level
c) acceptance threshold
d) both a and b
d) both a and b
When inspecting a rod with an encircling coil the eddy current density at the centre of the rod is zero for
a) rods with diameters greater than 2ë
b) rods with radius greater than 2ë
c) all conditions
d) no condition, a slight current density will always exist
c) all conditions
The purpose of multifrequency ECT techniques is
a) to increase frequency response of instruments
b) elimination of the effects of undesirable parameters
c) increase sensitivity to non-surface breaking defects
d) to allow inspection with phased array probes
b) elimination of the effects of undesirable parameters
When tube testing at f90 (internal absolute probe), if ID wall loss moves the operating point for an absolute coil in a negative X direction, a shallow OD defect would move the operating point
a) +X
b) -Y
c) +Y
d) both -X and -Y in equal proportions
c) +Y
In a through transmission test of sheet products, why might a metered output monitor the product of thickness and conductivity (absolute measurement method)?
a) changes in either parameter results in the same change in transmission coefficient
b) conductivity of a sheet can be assumed to always be constant
c) thickness of a sheet can always be assumed to be constant
d) because it is not possible to arrange the frequency ratio to provide maximum sensitivity
a) changes in either parameter results in the same change in transmission coefficient
What test frequency has a standard depth of penetration of 1mm for a plate material with resistivity of 130 µohm-cm and relative magnetic permeability of 500?
a) 650 Hz
b) 1.20 kHz
c) 240 kHz
d) 1 MHz
a) 650 Hz
A parallel L-C circuit used in crack detectors has an inductive reactance of 150 ohms. The capacitive reactance would be about _______ under normal operating conditions.
a) 0.1 ohms
b) 75 ohms
c) 150 ohms
d) not possible to know
c) 150 ohms
Shielding obtained by eddy current skin effect differs from magnetic methods of shielding in which way?
a) skin effect methods amplify the magnetic fields
b) skin effect methods attenuate the field rather than change the path
c) magnetic methods only work on ferromagnetic test pieces
d) skin effect methods are the same as magnetic methods
b) skin effect methods attenuate the field rather than change the path
As conductivity of a material approaches infinity its resistive losses approach zero. What type of material exhibits such extremes?
a) insulators
b) semi-conductors
c) superconductors
d) gold
c) superconductors
The purpose of curved wear pieces (shoes) to guide surface probe assemblies is to
a) maintain constant lift-off
b) ensure the coil axis is perpendicular to the test surface
c) prevent the probe from scratching the test piece
d) shape the magnetizing field
b) ensure the coil axis is perpendicular to the test surface
The magnetic condition of a ferromagnetic part based on its previous exposures to magnetic fields is the part’s
a) flux density
b) flux leakage
c) magnetic history
d) permeability
c) magnetic history
The magnitude of the Hall voltage is
a) proportional to the external magnetic field
b) proportional to the control current in the element
c) both a and b
d) fixed only by the direction of the magnetic field
c) both a and b
Test levels used in ECT that establish the group into which a material under test belongs are termed
a) acceptance limits
b) reject level
c) test criteria
d) group level options
a) acceptance limits
Decrease in sensitivity resulting from increasing lift-off is more pronounced for
a) large diameter probes
b) small diameter probes
c) deeper defects
d) both b and c
d) both b and c
The principal cause of magnetism in a naturally magnetic substance is
a) hysteresis
b) the weak nuclear force
c) uncompensated electron spin
d) graviton concentration in the Domain wall
c) uncompensated electron spin
When a ferromagnetic material has a magnetizing force applied to it, the magnetic flux that builds within the material lags the applied force. The same lag occurs upon the reduction in magnetizing force. What is the lag called?
a) permeability
b) hysteresis
c) Barkhausen effect
d) phase shift
b) hysteresis
If the resistance in a 1 cm long wire is 2 ohms when it has 0.1cm diameter, what will the resistance be in a wire of the same length and material but only 0.05cm diameter?
a) 1 ohm
b) 2 ohms
c) 4 ohms
d) 8 ohms
d) 8 ohms
For a non-magnetic foil thickness D, conductivity å, the effective coil distance is found from Aeff = (253,000 / fg å D). Effective coil distance will decrease if
a) thickness decreases
b) resistivity increases
c) both a and b
d) none of the above
d) none of the above
When testing ferromagnetic bars with an encircling coil, the effects of changes in ________ are reduced or eliminated by DC magnetic saturation.
a) resistivity
b) diameter
c) relative magnetic permeability
d) fill factor
c) relative magnetic permeability
Permeability changes are of greater concern in eddy current testing because
a) they can cause parts to fail but cannot be detected
b) small changes in permeability cause large impedance changes
c) small changes in permeability can obscure other test variables
d) both b and c
d) both b and c
The relationship between electric current flow, electromotive force and resistance to electric current flow is described by
a) Lenz’s law
b) Ohm’s law
c) Faraday’s rule
d) the ampere-ohm equation
b) Ohm’s law
Which of the following is not a probe configuration used in eddy current testing?
a) self inductance (single coil)
b) send-receive (2 coils)
c) magnetic reaction (coil & Hall detector)
d) semi-conductor reaction (2 Hall detectors)
d) semi-conductor reaction (2 Hall detectors)
Given a coil with 2 ohms resistance and 20 µH inductance and operated at 20 kHz, what is the coil’s inductive reactance?
a) 1.59 ohms
b) 2.51 ohms
c) 6.3 ohms
d) 10 ohms
b) 2.51 ohms
On the normalized impedance plane showing the effects of changing conductivity (å) the coil’s normalized resistance is zero under what condition?
a) å is zero
b) å is infinite
c) both a and b
d) when it equals the normalized inductive reactance
c) both a and b
All other conditions being equal for a bar tested in an encircling coil system, an increase in relative permeability of the bar tested would result in
a) decreased secondary coil voltage
b) increased secondary coil voltage
c) no change in secondary coil voltage
d) none of the above, the premise of the question is incorrect as testing of bars with relative permeability over 1 is not possible
b) increased secondary coil voltage
To avoid rotating parts, probes or test piece, what system would be used to inspect round bar stock?
a) encircling probes
b) circumferential array of probes
c) Hall effect exciters
d) both a and b
d) both a and b
In eddy current inspections of chiller tubes, freeze cracks located at freeze bulges are often not possible to detect using conventional differential probes because
a) bulges and cracks have the same phase
b) the bulge signal is so large it masks the crack
c) the crack only occurs under the support plate
d) both a and c
b) the bulge signal is so large it masks the crack
Voltage and current will be in phase for _______ in an AC circuit.
a) pure inductance
b) pure resistance
c) all conditions
d) no conditions
b) pure resistance
Effects of temperature drift are reduced by using
a) differential probes
b) probe pre-heat
c) liquid nitrogen baths
d) gap probes
a) differential probes
An increase in electrical resistivity of a sample will move the operating point on the impedance curve
a) upward
b) downward
c) to a point inside the curve
d) to a point outside the curve
a) upward
What is a resistivity of 6.2 µohm-cm as a % IACS?
a) 27.7
b) 13.1
c) 9.8
d) 6.2
a) 27.7
Signals received in the remote field eddy current set-up give two responses off large defects, one occurs due to the receiver coil passes the defect. What causes the other signal?
a) the magnetic field due to skip off the opposite wall phase lagged
b) the electric field due to skip off the opposite wall phase lagged
c) the exciter passing the defect
d) mode conversion
c) the exciter passing the defect
The phase angle used to estimate defect depth is the angle between the
a) lift-off vector and the defect to “sound specimen” vector
b) total voltage vector and the resistive voltage vector
c) about double the phase lag
d) both a and c
d) both a and c
Degree of cold working of which material can be determined by eddy current methods monitoring for permeability changes instead of resistivity changes?
a) austenitic stainless steel
b) titanium
c) tungsten
d) annealed aluminium
a) austenitic stainless steel
In eddy current tests to determine non-conductive coating thicknesses, probe diameter and operating frequency are selected to minimize the effects of what parameter?
a) conductivity
b) density
c) lift-off
d) thermal coefficient of resistivity
a) conductivity
The effect of increasing coil diameter on the effective coil distance is
a) to increase effective coil distance
b) to decrease effective coil distance
c) unpredictable
d) not noticeable
a) to increase effective coil distance
The complex impedance plane presentation for testing a ferromagnetic bar should be changed in what way from the same test on a non-ferromagnetic bar?
a) the real and imaginary components are interchanged
b) the real component is rotated by 180°
c) the vertical and horizontal scales are increased by the magnitude of the relative permeability
d) the complex impedance plane presentation cannot be used when testing ferromagnetic material
c) the vertical and horizontal scales are increased by the magnitude of the relative permeability
Natural aging of aluminium alloys occurs at what temperature?
a) 50° below melting point
b) at TC (critical temperature)
c) room temperature
d) in a range from -50°C. to +15°C
c) room temperature
An instrumentation technique that discriminates between variables in the test piece by different phase-angle changes these variables produce in the test signal is
a) impedance analysis
b) phase analysis
c) modulation analysis
d) frequency analysis
b) phase analysis
Maximum test sensitivity is obtained at which point on the signal locus of the complex plane?
a) maximum displacement to the right
b) maximum displacement to the left
c) maximum vertical displacement
d) minimum vertical displacement
a) maximum displacement to the right
Eddy current flow in a test sample is accomplished indirectly by monitoring
a) current changes in the sample
b) resistivity changes in the sample
c) impedance changes in the coil
d) coil resonance
c) impedance changes in the coil
Eddy current tests using encircling coils would provide similar test coil impedances or voltage signals for tests on 100mm diameter aluminium rod and 2mm diameter steel wire if
a) f/fg ratios are equal
b) fill factors are equal
c) both a and b
d) none of the above, signals could never look the same for magnetic and nonmagnetic materials
c) both a and b
Given a coil with 20 ohms resistance and 60 microhenries inductance in air and operated at 50 kHz, when brought next to an inconel sample the probe impedance is 28.5 ohms and impedance phase is 45°, what is the probe’s inductive reactance?
a) 20.2 ohms
b) 20.2 microhenries
c) 44.7 ohms
d) not possible to determine with information given.
a) 20.2 ohms
The phase angle between applied voltage and resultant current in an AC circuit of pure inductance is
a) 0°
b) 45°
c) 90°
d) 180°
c) 90°
The impedance of probes used in eddy current testing can vary over a range. Instruments must be able to balance over this range. Most instruments can handle probe impedances between
a) 50 to 75 ohms
b) 0.1 to 100 kohms
c) 10 to 200 ohms
d) 5 to 500 ohms
c) 10 to 200 ohms
Problems with ferromagnetic indications occurring in material that is not ferromagnetic can be overcome by
a) using a saturating permanent magnet
b) retesting at a lower frequency
c) retesting at a higher frequency
d) both a and b
d) both a and b
Substitutional solid solutions and interstitial solid solutions of metals are forms of
a) nonmagnetic coatings applied to magnetic bases
b) alloys
c) superconductors
d) isotopic variations of the metal
b) alloys
Large D.C. saturation units for eddy current inspection of ferromagnetic tubing are often required. What technique can be used to avoid use of these heavy D.C. saturation units?
a) pulsed eddy current testing
b) remote field eddy current testing
c) multifrequency eddy current testing
d) both a and b
d) both a and b
Work hardened aluminium has a higher resistivity than annealed aluminium for what reason?
a) changes in alloy content
b) disruptions in lattice structure
c) excitation states of electrons are higher
d) none of the above, the premise of the question is wrong, resistivity is a constant for a given alloy content regardless of
worked state
b) disruptions in lattice structure
X-Y recorders
a) have very low response rates (8 Hz)
b) are usually used as a playback recording instrument for hardcopies of specific signals
c) do not have the ability to “locate” defects and provide length information about the defect
d) all of the above
d) all of the above
What is the effect of overaging on aluminium heat treatable alloys?
a) increased conductivity
b) decreased hardness
c) both a and b
d) no effect
c) both a and b
Differential coils are, in some areas, also called
a) ID coils
b) bucking coils
c) annular coils
d) tandem coils
b) bucking coils
If a plate material has a resistivity of 65 µohm-cm and relative magnetic permeability of 50, what test frequency should you use to achieve f90 at a depth of 0.2mm?
a) 100 Hz
b) 50 kHz
c) 250 khz
d) 320 kHz
b) 50 kHz
Compared to single frequency units, multifrequency eddy current instrument circuits are
a) the same except for signal separation circuitry
b) the same except for signal separation and combining circuitry
c) the same in every way
d) equipped with better filters and signal averaging circuits
b) the same except for signal separation and combining circuitry
Instrument frequency response is limited by
a) probe size
b) operating frequency of the probe
c) probe motion (inspection speed)
d) none of the above
d) none of the above
The higher the value of inductance for a given frequency the greater the degree of
a) balance ability
b) sensitivity
c) Q factor
d) capacitive reactance
b) sensitivity
The equation 2pifL =
a) inductive reactance
b) inductance
c) capacitive reactance
d) total impedance (electric)
a) inductive reactance
When determining resistivity of a sample of an aluminium alloy, why is it recommended you do not touch the sample with your fingers?
a) oil of the skin increases resistivity
b) oil of the skin decreases resistivity
c) sample temperature can be changed
d) oils on the test surface from the fingers will produce an unwanted lift-off
c) sample temperature can be changed
Eddy current test methods are more sensitive than x-rays for detection of ______________ in aircraft structures.
a) corrosion
b) missing fasteners
c) fatigue cracks
d) overloading cracks
c) fatigue cracks
Given an encircling coil with an average coil diameter of 10.5 mm and testing a tube 10mm OD with a 1mm thick wall, what is the fill factor of this setup?
a) 0.735
b) 0.819
c) 0.907
d) 0.956
c) 0.907
Given copper at 20°C. with resistivity 5.9 µohm-cm and thermal coefficient of resistivity of 0.0039, what is the resistivity when the copper is warmed to 40°C.?
a) 2.90 µohm-cm
b) 5.80 µohm-cm
c) 6.25 µohm-cm
d) 11.60 µohm-cm
c) 6.25 µohm-cm
The phenomenon whereby depth of penetration decreases with increasing frequency is called
a) skin effect
b) Doppler shift
c) edge effect
d) phase shift
a) skin effect
In what way is eddy current testing more suitable to high speed production tests on hot metals than ultrasonics?
a) ultrasonic probes will depolarize test hot surfaces
b) no stream of water coupling is needed for ECT
c) UT cannot be done on steels above the curie temperature
d) UT mechanical waves cannot penetrate the surface scale
b) no stream of water coupling is needed for ECT
If temperature of a test piece increases what other eddy current parameter will likely increase?
a) conductivity
b) resistivity
c) frequency
d) lift-off
b) resistivity
Crack detector type ECT instruments based on resonant circuits detecting surface defects on low resistivity materials such as aluminium would have operating frequencies in the ______ range.
a) DC
b) 60-100 Hz
c) 10-100 kHz
d) 10-100 MHz
c) 10-100 kHz
During an eddy current inspection of heat exchanger tubing, what is the purpose of recording a calibration signal with each tube inspected?
a) monitor effects of any temperature changes
b) monitor instrument drift
c) monitor probe degradation
d) all of the above
d) all of the above
Given a standard depth of penetration of 1.3mm exists for a 10kHz test on navel-brass (6.63µohm-cm), what is the effective depth of penetration?
a) 1.3mm
b) 3.9mm
c) 5.2mm
d) 6.5mm
b) 3.9mm
What condition can be “eliminated” using multifrequency eddy current techniques?
a) denting and pilgering
b) magnetic deposits
c) support plates
d) all of the above
d) all of the above
If a sheet was composed of 2 metallic layers with thicknesses D1 and D2 and conductivities å1 and å2, what would the equivalent product be when tested by through transmission?
a) åD = å1D1 + å2D2
b) åD = å1å2 + D1D2
c) åD = (å1D1) (å2D2)
d) åD = (å1D1)² + (å2D2)²
a) åD = å1D1 + å2D2
Most eddy current instruments use some form of _______ for balancing but several options are available for lift-off compensation.
a) inductor
b) AC bridge
c) DC bridge
d) potentiometer
b) AC bridge
What would the effect on conductivity signal be as radius of curvature of the test piece is decreased?
a) signal amplitude increases
b) conductivity measured would decrease from the true value
c) conductivity measured would increase above the true value
d) no effect would be noticed
b) conductivity measured would decrease from the true value
Recording of eddy current signals from ECT instruments requires that the recording instrument
a) be impedance matched to the ECT instrument
b) be phase locked to the ECT instrument
c) have an equal or higher frequency response
d) have a lower frequency response than the ECT instrument
c) have an equal or higher frequency response
Holding a probe in your hand after it has been balanced in air will tend to warm the probe. This will move the operating point on the impedance curve. It will move
a) up the curve
b) down the curve
c) to a point inside the curve
d) to a point outside the curve
a) up the curve
If the electric current in a coil is doubled the magnetic flux density
a) halves
b) remains unchanged
c) doubles
d) quadruples
c) doubles
Phase lag of eddy currents in a sample is dependent on
a) depth into the sample
b) resistivity of the test piece
c) relative magnetic permeability of the sample
d) all of the above
d) all of the above
An increase in test frequency will move the operating point on the impedance curve
a) up
b) down
c) to a point inside the original curve
d) to a point outside the original curve
b) down
Which of the following will increase conductivity of an alloy?
a) solution heat treating
b) precipitation or aging
c) annealing
d) cold working
c) annealing
What is the standard depth of penetration for 304 stainless steel (68.96 µohm-cm) having 60% cold work applied (µrel = 2) and tested at 20kHz?
a) 1.1mm
b) 2.1mm
c) 3.1mm
d) 4.4mm
b) 2.1mm
Thickness determination of a non-conductive coating on a conductive (non-magnetic) material is done using a frequency
a) that is as low as possible
b) that is as high as is practical
c) giving only one skin depth of penetration
d) one half the resonant frequency
b) that is as high as is practical
In an R-L circuit _________ is plotted on the ordinate (vertical axis).
a) the imaginary component
b) inductive reactance
c) resistance
d) both a and b
d) both a and b
The reactive power of inductance and capacitance in a tuned L-C circuit are
a) equal
b) maximum
c) minimum
d) zero
a) equal
Sorting of materials by impedance values of an eddy current probe require
a) relative permeability of all parts be fixed at 1
b) specimen thickness exceeds depth of eddy current penetration
c) conductivity of all parts tested be within 10% of each other
d) use of the characteristic frequency for test frequency
b) specimen thickness exceeds depth of eddy current penetration
Eddy current testing of hot billets (1,100øC.) can be done provided what precautions are taken?
a) the probe is water cooled
b) the probe is set back from the test surface at least 3 cm
c) extra windings and diameter are used in probe construction
d) both b and c
a) the probe is water cooled
When ECT is used to test thickness of coatings having a tolerance range, what is the minimum number of calibration specimens required to calibrate the instrument?
a) 2
b) 3
c) 4
d) 2 or 4 depending on if the coating is conductive or not
b) 3
Impedance diagrams for cylinders are not the simple semi circular shapes used for plate. This is a result of
a) skin effect
b) phase lag
c) leakage fields
d) both a and b
d) both a and b
Increasing which of the following parameters will move the operating point up on the impedance curve?
a) resistivity
b) thickness(of tube or plate)
c) frequency
d) diameter of a surface probe
a) resistivity
When gap between two plates of the same material is being measured, the probe should be placed on the thinner of the two plates when possible. Why?
a) the frequency needed would be too low otherwise
b) to minimize depth of penetration problems
c) so results are linear
d) to increase signal to noise ratio
b) to minimize depth of penetration problems
Generally in multifrequency techniques for in-situ boiler tube inspections, high frequencies are used to suppress ___________while low frequencies are used to suppress ______________ .
a) wobble, electrical noise
b) dimensional variables, material variables
c) internal variables, external variables
d) external variables, internal variables
c) internal variables, external variables
Given a tube with a 15mm OD and 1.5mm wall, what size (average diameter) coil is used to obtain an 85% fill factor for an internal inspection?
a) 14mm
b) 13mm
c) 12mm
d) 11mm
c) 12mm
The f90 frequency has been found empirically from the ratio of thickness and skin depth. For testing tubing this ratio is
a) 0.08
b) 0.9
c) 1.1
d) 3
c) 1.1
Using a shielded ferrite coil and the pulsed eddy current technique, penetration of measurable currents in a metal sample can be increased to ________ (where ë is the standard depth of penetration)
a) 2ë
b) 3ë
c) 5ë
d) 10ë
d) 10ë
If testing a material and you have set up acceptable conditions for phase separation of 90 degrees for 1mm sample depth when relative magnetic permeability is 1, what depth would the 90ø separation occur at if relative magnetic permeability changed to 20?
a) 20mm
b) 10mm
c) 0.1mm
d) 0.05mm
d) 0.05mm
Given a probe with 50 ohms resistance and 40µH inductance, when operated next to a copper sample at 20 kHz the probe impedance is 55 ohms and impedance phase is 40°, what is the inductive reactance of the probe when operating on the sample?
a) 55 ohms
b) 42.1 ohms
c) 35.3 ohms
d) 5 ohms
c) 35.3 ohms
Measurement of the thickness of a non conductive coating would utilize the _______ effect.
a) skin
b) lift-off
c) Hall
d) Bessel
b) lift-off
A differential transducer with the two windings around perpendicular to each other used to detect both longitudinal and
transverse cracks is called a(n)
a) orthogonal winding transducer
b) multi-pancake probe
c) zig-zag probe
d) transverse compensating probe
a) orthogonal winding transducer
When using an external encircling coil the frequency ratio f/fg to obtain maximum sensitivity to all test variables will be greatest
for which variety of heavy wall tube?
a) solid bars
b) wall thickness to outside tube radius = 0.5
c) wall thickness to outside tube radius = 0.01
d) none of the above, f/g is constant for all encircling coil tests
c) wall thickness to outside tube radius = 0.01
An “absolute” probe requires _______ interact(s) with the test material.
a) one coil
b) two coils
c) more than two coils
d) a DC magnetic field
a) one coil
Low frequency eddy current (100Hz to 5kHz) is commonly used in aircraft inspections for
a) paint thickness determinations
b) subsurface corrosion detection in multilayer structures
c) conductivity determination for alloy sorting
d) all of the above
b) subsurface corrosion detection in multilayer structures
How does the differential (or auto-comparator) coil provide insensitivity to gradual changes?
a) coils are wound in opposition to each other
b) coils are operated at canceling frequencies
c) one coil has an air core and the other has an iron core
d) a subtractive circuit is incorporated into the eddy current instrument
a) coils are wound in opposition to each other
Which of the following is not a means of suppressing an undesired eddy current test signal?
a) varying phase rotation
b) reducing receiver gain
c) varying bridge balance point
d) tuning reactive components in the probe bridge circuit
b) reducing receiver gain
An increase in which of the following would result in the increase of magnetic flux density (B) in a solenoid?
a) core permeability
b) number of coil turns
c) current in the coil
d) all of the above
d) all of the above
What is used to regulate the consistency of the manufacturing of calibration standards?
a) eddy current instruments calibrated to national standards
b) codes and specifications
c) licensed metrology labs
d) level 3 technicians
b) codes and specifications
In the 1960’s a non-storage type oscilloscope was used for eddy current tests. The defect free specimen gave a horizontal line. A defective specimen gave a(n)
a) negative Y spike
b) positive Y spike
c) ellipse
d) angulated line (ie. not horizontal)
c) ellipse
When performing an eddy current test and you encountered a signal that could be a crack, permeability change or resistivity change, you would
a) change frequency and test again
b) rotate the phase to put the lift-off vertical
c) increase gain and look for roughness of signal
d) use MPI instead of eddy current testing
a) change frequency and test again
Maximum response to defects detected by eddy currents are obtained when
a) eddy current flow is parallel to the maximum dimension of the defect
b) eddy current flow is perpendicular to the maximum dimension of the defect
c) the defect causes probe resonance
d) lift-off is eliminated
b) eddy current flow is perpendicular to the maximum dimension of the defect
A practical f/fg ratio for thickness measurements would be in the range of 1 to 7.5. This would provide maximum sensitivity to
a) conductivity
b) resistivity
c) lift-off
d) both a and b
c) lift-off
An increase in tube wall or plate thickness will move the operating point on the impedance curve
a) up
b) down
c) inside the curve
d) outside the curve
b) down
Which equation would be used to calculate the resistance of a length of conductor at room temperature other than standard temperature?
a) R = Ro + T
b) R = Ro + dT
c) R = Ro (1 + à dT)
d) none of the above
c) R = Ro (1 + à dT)
Electric “fields” are the same as
a) potential differences
b) eddy currents
c) electron flow
d) strawberry fields
a) potential differences
The ideal signal voltage in a Hall detector element in the absence of a magnetic field is
a) zero
b) maximum
c) a local minimum
d) determined by ambient temperature
a) zero
Which of the following series of stainless steels is not likely to exhibit an increase in relative permeability with increasing cold working?
a) 301
b) 302
c) 304
d) 316
d) 316
A circuit block that uses an analogue voltage as an input and outputs a proportional binary value is a(n)
a) D/A converter
b) A/D converter
c) motherboard
d) parallel interface
b) A/D converter
Phase adjustment on simple conductivity meter instruments is especially useful for what conditions?
a) thin samples
b) thick samples
c) rough surfaces
d) ferromagnetic samples
c) rough surfaces
Resistivity measurements made on bulk material and without reference standards
a) cannot be made by any methods known
b) use very high frequency eddy current probes
c) do not use eddy current methods
d) use magnetostrictive effects
c) do not use eddy current methods
If given total impedance of a probe operating on a test sample and knowing the impedance phase angle, what equation is used to determine the inductive reactance of the probe?
a) Xp = 2pifL
b) Xp = Zp cos é
c) Xp = Zp tan é
d) Xp = Zp sin é
d) Xp = Zp sin é
Strictly speaking, the standard skin depth equation; J/Jo = (e^-þ) sin (wt-þ), is true for only
a) thick material and planar magnetic fields
b) tubular products
c) thin plate inspection
d) all of the above
a) thick material and planar magnetic fields
Compensation for undesirable material and coupling variations can be achieved by
a) using a resonance test frequency
b) multiple coil probes
c) testing under liquid nitrogen
d) none of the above
b) multiple coil probes
What is not one of the advantages of drilled holes being used as reference standard?
a) simplicity
b) economic
c) behaves like a crack
d) provided a good indication of sensitivity
c) behaves like a crack
When a metal sheet is inserted into a through transmission probe arrangement, the transmission coefficient phasor
a) remains unchanged
b) changes in magnitude and phase
c) changes real and imaginary values
d) both b and c
d) both b and c
What limits the scanning speed when using meter display eddy current instruments?
a) operator response
b) meter movement (rise time)
c) operating frequency
d) defect type or coating thickness
b) meter movement (rise time)
What degree of accuracy can be expected when using eddy currents to determine paint thicknesses 10µm thick?
a) 0.01µm
b) 0.1µm
c) 1.0µm
d) 5.0µm
c) 1.0µm
The time required for a test system to return to its original state after it has received a signal is the
a) dead time
b) recovery time
c) recoil time
d) system delay
b) recovery time
Small eddy current sensors in the vicinity of cracks could be used for
a) precise crack length determinations
b) crack extension rate determination
c) crack width determination
d) both a and b
d) both a and b
Although specimen and standard may be within the recommended 5°C. temperature difference for resistivity measurement, why might the value determined still be incorrect?
a) measurement temperature and the temperature the standard was originally established at are different
b) a different probe is use than was used to establish the standard
c) test frequency is too high
d) the specimen is work hardened
a) measurement temperature and the temperature the standard was originally established at are different
What f/fg ratio is recommended for testing thin wall nonmagnetic tubing for cracks, alloy variations, or wall thickness variations?
a) 0.1
b) 1.0
c) 3.6
d) 10
b) 1.0
In an eddy current test setup, magnetic lines of flux from the probe which fail to couple to the test piece
a) carry no information
b) cause self inductance in the magnetizing coil
c) are responsive to the spacing of coil and test piece
d) both b and c
d) both b and c
When a single channel strip chart recorder is used with eddy current testing of bolt holes using spinning probes, what 2
parameters are recorded?
a) Y (vertical) output of signal vs. depth along hole axis
b) X (horizontal) output of signal vs. depth along hole axis
c) Y (vertical) output of signal vs. time
d) phase of signal vs. position of probe along the hole axis
c) Y (vertical) output of signal vs. time
As a general rule, probe diameter should be selected so that it is
a) greater than or equal to the expected defect length
b) less than or equal to the expected defect length
c) less than or equal to the expected defect depth
d) twice the minimum allowable defect length
b) less than or equal to the expected defect length
Alloying metals added to pure base metals result in decreasing conductivity of the initial value of the pure base metal. Why does this occur even with alloying metals having higher conductivity than the base metal?
a) altered lattice structure inhibits electron flow
b) electrons move to lower energy states when alloyed
c) electrons move to neutral energy states when alloyed
d) both a and b
a) altered lattice structure inhibits electron flow
Another term for voltage is
a) electromotive force
b) magnetomotive force
c) potential drop
d) both a and c
d) both a and c
“Balancing” is required in the eddy current instrument to
a) allow resonance
b) avoid resonance
c) set meter type instruments to zero
d) eliminate the voltage difference between two coils
d) eliminate the voltage difference between two coils
Which of the following will have the largest resistivity change with change in temperature (à = thermal coefficient)
a) brass à = 0.0046
b) copper à = 0.0050
c) titanium à = 0.0400
d) platinum à = 0.0040
c) titanium à = 0.0400
What are the main limiting parameters for a single coil probes dimensions?
a) coil width
b) number of turns
c) intended operating frequency
d) both a and b
d) both a and b
n-type semiconductors use what form of charge carrier?
a) electrons
b) positrons
c) holes
d) quarks
a) electrons
The equation 1/2pifC =
a) inductive reactance
b) capacitance
c) capacitive reactance
d) total impedance (electrical)
c) capacitive reactance
Which of the following is not a mandatory component in a “basic” eddy current test apparatus?
a) an AC source
b) a coil (probe)
c) an impedance plane
d) a volt meter
c) an impedance plane
When eddy currents are used for sorting techniques it is usual to establish impedance values from
a) probe characteristics
b) samples of known materials
c) published information
d) trial and error methods
b) samples of known materials
To discern very shallow cracks using a surface coil you would use a relatively high frequency-conductivity product (åf). Which of the following would then be true?
a) angle between crack direction and lift-off effect increases
b) magnitude of crack effect decreases
c) lift-off effect increases
d) all of the above
d) all of the above
When given a plate sample for resistivity determination, test frequency should be selected such that skin depth is at least
a) equal to the plate thickness
b) one third the plate thickness
c) one tenth the plate thickness
d) twice the plate thickness
b) one third the plate thickness
A network that passes electromagnetic wave energy over a described range of frequencies and attenuates energy at all other frequencies is a(n)
a) filter
b) gate
c) inductor
d) grate
a) filter
Long gradual defects can be missed by using _________ probes.
a) encircling
b) differential
c) bobbin
d) absolute
b) differential
Which of the following is not a method used to manufacture notches in calibration standards used for eddy current tests of tubing?
a) electric discharge machining
b) electrophoresis
c) milling
d) saw cuts
b) electrophoresis
Which of the following noise sources can be filtered with the appropriate electronics in an eddy current instrument?
a) external, stray magnetic and electric fields
b) electrical noise generated within the EC instrument
c) mechanical vibrations of test coil or material
d) all of the above
d) all of the above
In order to use a galvanometer (which normally measures currents in the range of milliamps) as an ammeter measuring 10 to
20 amps you would put a _________ in ___________ with the galvanometer.
a) resistor, series
b) resistor, parallel
c) capacitor, series
d) capacitor, parallel
b) resistor, parallel
When using surface coils for crack detection, shallow cracks and lift-off cannot be separated unless
a) lift-off compensating probes are used
b) frequency is high enough
c) frequency is low enough
d) both a and b
d) both a and b
Encircling probes (or internal probes) are likely to be replaced by surface probes for tubing with a diameter greater than 50mm.
The reason for this is
a) encircling probes cannot be made bigger
b) fill factor becomes too difficult to regulate for large encircling probes
c) higher defect sensitivity can be achieved using surface probes
d) both b and c
c) higher defect sensitivity can be achieved using surface probes
Determining plating thickness of a conducting nonmagnetic material on another conducting nonmagnetic material requires
a) a difference in conductivities between the two materials
b) use of a lift-off compensating probe
c) a resonance circuit be used
d) all of the above
a) a difference in conductivities between the two materials
Test frequency for solid cylinders, maximum sensitivity to defects, resistivity and dimensions is obtained when f/fg =
a) 2
b) 6
c) 100
d) 400
b) 6
Remote field eddy current testing is a technique commonly used on
a) ferromagnetic tubes
b) laminates sheets of tungsten carbide
c) paint coatings on aluminium boat hulls
d) riveted joints on aircraft fuselage
a) ferromagnetic tubes
Increasing temperature of a dielectric (insulating) material has what effect?
a) increased resistivity
b) increased conductivity
c) destabilization of isotopes
d) no effect on any properties (that’s why they are called insulators)
b) increased conductivity
What is the most significant drawback of dot matrix displays of EC signals compared to CRT displays?
a) image stability
b) resolution
c) cost
d) size and power consumption
b) resolution
Most impedance eddy current instruments will not operate at resonance. This situation is remedied by
a) operating at about 80% of the resonant frequency
b) using a lower inductance probe
c) reduce cable length
d) any or all of the above
d) any or all of the above
The amount of “reverse” magnetizing force required to eliminate the residual magnetic flux in a ferromagnetic material is
a) 5.5 kilogauss
b) the coercive force
c) the de-saturating force
d) hysteresis
b) the coercive force
The curve traced on the X-Y storage monitor as an active coil is brought up to a sample of 1100 aluminium (100% pure) is called the
a) reference curve
b) coil lift-off locus
c) aluminium standard arc (ASA)
d) eddy current curve
b) coil lift-off locus
Conductance is an electrical quantity which can also be defined as the reciprocal of
a) inductance
b) resistance
c) resistivity
d) reluctance
b) resistance
An encircling coil is used on a 12mm diameter solid rod. What is the fill-factor if the average coil diameter is 13mm?
a) 0.80
b) 0.85
c) 0.92
d) 1.08
b) 0.85
Tubes with a diameter of more than about 50mm are more effectively tested using ____________ than encircling probes.
a) internal axial coils
b) surface probes or arrays
c) differential probes
d) forked probes
b) surface probes or arrays
Ferromagnetic inclusions on or in normally non magnetic aluminium will arise due to
a) tooling or handling equipment
b) impurities in the melt
c) working below the curie temperature
d) oxidation
a) tooling or handling equipment
The equation f90 = 3 p/t² applies to
a) internal coil inspection of tubing
b) external coil inspection of tubing
c) pancake coil inspection of plate
d) both a and b
d) both a and b
Which of the following conditions is not necessary for eddy current testing?
a) electrical contact
b) specimen conductivity
c) an alternating magnetic field
d) induced electrical current
a) electrical contact
What is the advantage of eddy current testing over the potential drop method for sizing surface cracks?
a) accuracy
b) it is non-contacting
c) cost
d) there is no advantage
b) it is non-contacting
The transmit-receive or transformer style probe provides
a) improved s/n ratio
b) increased sensitivity to deeper defects
c) both a and b
d) no advantage over single coil probes
c) both a and b
What is the effect of ferromagnetic materials on the inductance of an eddy current test coil?
a) a minute increase
b) a significant increase
c) a decrease
d) no change
b) a significant increase
Multifrequency instruments have the same controls and functions as general purpose ECT instruments with the addition of
a) mixing modules
b) filters
c) phasors
d) frequency selectors
a) mixing modules
Lenz’s Law states
a) an alternating magnetic field induces an alternating voltage
b) the magnitude of induced current is a function of magnetic flux through a circuit
c) the induced EMF is opposite to the change causing it
d) I = B A cosþ where B= flux density, A = circuit area and þ = the angle between B and the circuit area A
c) the induced EMF is opposite to the change causing it
Separation of diameter and conductivity effects is better carried out at frequency ratios greater than 4 because
a) greater penetration afforded permits better determination of bulk properties
b) the angle between diameter and conductivity locii is greater
c) the angle between diameter and conductivity locii is 90ø
d) none of the above, frequency ratio should be less than 4 for such work
b) the angle between diameter and conductivity locii is greater
In order to do computer modeling of eddy current fields you must provide
a) physics of a testing media
b) characteristics of the test object
c) geometry of the test
d) all of the above
d) all of the above
Current flow that is time constant in both direction and amplitude is
a) direct current
b) eddy current
c) induced current
d) boring
a) direct current
Some CRT display eddy current instruments allow X and Y gains to be adjusted independently. Increasing Y gain and reducing X (eg. Y = 0.2 V/div, X = 2.0 V/div) accomplishes what?
a) elimination of frequency dependent phase angle
b) improved crack detection by suppressing lift-off output
c) increased signal to noise ratio
d) frequency control without affecting balance
b) improved crack detection by suppressing lift-off output
The result of using a longer encircling test coil to test a spherical object as compared to a short coil or hemispherical coil would be
a) increased sensitivity
b) reduced fill factor
c) improved phase discrimination of cracks and conductivity changes
d) all of the above
b) reduced fill factor
When an eddy current is balanced for surface testing for flaws, where is the probe placed?
a) over the shallowest notch
b) over the deepest notch
c) over a defect free area
d) in air away from the test piece
c) over a defect free area
In the impedance method of eddy current testing the impedance phase (in degrees) is calculated from (w is angular frequency, L is inductance R is resistance)
a) é= Arcsin (wL/R)
b) é= Arccos (R/Lw)
c) é= Arctan (wL/R)
d) é=Arcsin (R²+L²)^½
c) é= Arctan (wL/R)
In field applications, specific conductivity values are not used, instead a range of conductivities can be expected from a finished product. Why is this so?
a) variations in alloy
b) variations in heat treatment time/temperature
c) variations in fabrication stresses
d) all of the above
d) all of the above
In plate testing, to minimize effects of lift-off variations you would
a) increase coil to part spacing
b) increase coil diameter
c) decrease coil to part spacing
d) decrease coil diameter
b) increase coil diameter
The result of operating an eddy current test instrument at a point other than balance point is
a) nonlinear voltage output with change in probe impedance
b) increased sensitivity
c) reduced lift-off effects
d) all of the above
a) nonlinear voltage output with change in probe impedance
What would a polar coordinate based phase-gate look like?
a) a single line
b) a box
c) a pie-slice
d) a sinusoid
c) a pie-slice
In the L-C circuit used by simple meter crack detectors, the circuit is operated
a) independent of operating frequency
b) at the resonance frequency
c) very near resonance frequency
d) both b and c
c) very near resonance frequency
The most troublesome parameter in eddy current testing is
a) resistivity
b) lift-off
c) resonance
d) permeability
b) lift-off
On a normalized impedance curve which of the following parameters would move the operating point “up” the curve when increased?
a) resistivity of sample
b) operating frequency
c) sample conductivity
d) lift off
a) resistivity of sample
Eddy current density in a sample is
a) proportional to the conductivity of the sample
b) proportional to the permeability of the sample
c) inversely proportional to the depth from the surface of the sample
d) all of the above
d) all of the above
When tube testing (internal absolute probe) at f90 and setting OD wall loss to move +Y on the scope, what is the probable source of a +X moving signal?
a) ID wall loss
b) through hole
c) dent
d) support plate
c) dent
In the range of about 3mm to 13mm wall thickness, what frequency range would be used for low frequency remote field eddy current testing of ferromagnetic tubing?
a) 10 Hz to 300 Hz
b) 500 Hz to 2 kHz
c) 2 kHz to 10 kHz
d) 10 kHz to 50 kHz
a) 10 Hz to 300 Hz
To increase sensitivity to near surface defects using a bobbin style probe coil length and thickness are reduced. This however
results in
a) reduced frequency range
b) increased probe-cable capacitance
c) decreasing sensitivity to the far surface defects
d) bobbin breakdown
c) decreasing sensitivity to the far surface defects
Eddy current test systems using Hall detectors can accomplish differential tests by
a) using two Hall detectors
b) using two superimposed excitation frequencies
c) having the excitation coil double as a pickup coil
d) no method presently known
a) using two Hall detectors
You are given 2 plates of identical size (50 x 50 x 10 mm) both painted with a thin coating of black acrylic paint of the same thickness. Eddy current tests indicate both have a conductivity of 37% IACS, yet one is nearly twice as heavy as the other.
How is this possible?
a) the paint is conductive
b) edge effects are causing wrong readings
c) different metals are beneath the paint
d) the lighter one has a void defect
c) different metals are beneath the paint
The effect of sample and test parameters can be illustrated using
a) magnetographs
b) impedance diagrams
c) polar projections
d) polarized light
b) impedance diagrams
For two separate objects with different relative permeabilities and resistivities, equivalent eddy current tests can be performed by adjusting test frequencies. This is explained by
a) the similarity law for eddy current testing
b) Maxwell’s Law
c) Lenz’s Law
d) Newton’s First Law of Electromagnetics
a) the similarity law for eddy current testing
Which of the following is not an advantage of the eddy current test method?
a) 100% volumetric inspection is possible (within limits)
b) speed
c) clean smooth surfaces not required
d) no couplant required
c) clean smooth surfaces not required
Complex numbers are often used in the analysis of eddy current test systems. Complex numbers have 2 components, they are
a) real and imaginary
b) whole and natural
c) absolute and integer
d) real and unreal
a) real and imaginary
A tube being tested by an internal probe has an ID to OD ratio of 0.8. Under what conditions does this appear to be a thin wall tube?
a) higher operating frequency
b) lower operating frequency
c) when fill factor is 1
d) when fill factor is reduced
b) lower operating frequency
Multifrequency instruments may be one of two types; simultaneous frequency or alternate frequency. Which is not an advantage of the simultaneous frequency systems?
a) no unnecessary saturation in separation stages
b) wide passband of X and Y outputs
c) low cost of equipment
d) permits high inspection speeds
c) low cost of equipment
The “reversal swirl” that is observed on a normalized impedance graph showing the effects of decreasing thickness is a result of
a) skin depth and phase lag effects
b) resonance effect
c) test specimen capacitance effect
d) all of the above
a) skin depth and phase lag effects
A probe whose operating impedance is not between 20 and 200 ohms will most likely result in
a) decreased signal to noise ratio
b) decreased signal amplitude
c) both a and b
d) none of the above, probe impedance matching to instrument impedance is not important
c) both a and b
The purpose of small diameter and high frequency probes for determining thickness of thin coatings on conducting substrates is to
a) minimize the eddy current field in the substrate
b) maximize the eddy current field in the substrate
c) maximize the field in the nonconductive coating
d) minimize lift-off effects
a) minimize the eddy current field in the substrate
When the eddy current test system is represented by the transformer the sample can be considered the secondary winding with
a) a single turn
b) 10 turns
c) zero turns
d) none of the above, it is not possible to determine
a) a single turn
The back EMF opposing the inducing EMF is a result of
a) the Hall effect
b) eddy current flow
c) geo-magnetic reversals
d) weak nuclear forces
b) eddy current flow
The “send-receive” method of eddy current testing uses
a) changes in voltage across the primary coil
b) changes in current across the primary coil
c) two separate coils
d) a single “multiplexed” coil
c) two separate coils
Conductivity changes for annealed copper (100 IACS) as a function of temperature change are
a) linear
b) logarithmic
c) exponential
d) zero, that is why it is chosen as the standard
a) linear
When using Hall detectors, how are sensitivities to relatively great depths achieved?
a) increasing Hall detector size
b) increasing test frequency
c) increasing excitation coil size
d) both a and b
c) increasing excitation coil size
When performing thickness or gap testing, what should the operating frequency be?
a) 1 kHz
b) 1 standard depth of penetration (ë)
c) 3 standard depths of penetration (3ë)
d) it is not possible to estimate
c) 3 standard depths of penetration (3ë)
For practical applications of surface probes on curved surfaces
a) curvature should be small within the region directly below the cross-sections of the coil
b) frequency of operation should be as low as possible
c) perspex supports should be arranged to fit the curvature inspected
d) all of the above
a) curvature should be small within the region directly below the cross-sections of the coil
Surface coil eddy current transducers are
a) always used in the absolute mode
b) always flat
c) always used on flat surfaces
d) none of the above
d) none of the above
When interpreting eddy current signals by quadrature components on strip charts the X channel information is used for
a) analysing defect type
b) analysing defect depth
c) an analysis threshold
d) both a and b
d) both a and b
When two similar coils on the AC bridge of the eddy current instrument sense with the test material the probe is a(n)
a) absolute probe
b) differential probe
c) spring probe
d) pencil probe
b) differential probe
If the magnetic flux density for a given location and orientation near a current carrying conductor is 5 Wb/m², what is it when the current is cut by half?
a) 25T
b) 5T
c) 2.5T
d) 2.25Wb/m²
c) 2.5T
When online testing of ERW welds in steel pipe using eddy current testing, what problem occurs if the inspection is performed too far from the induction heating coils used for normalizing the weld?
a) noise results as the metal cools below the curie point
b) the induction coils cannot be used as primary coils in the inspection
c) too much warpage occurs and lift-off is excessive
d) near surface defects are masked by the spherodizing effects within the grain structure
a) noise results as the metal cools below the curie point
What is the effect on eddy current determined properties of aluminium alloys that have been annealed for an excessive amount of time?
a) increased resistivity
b) decreased permeability
c) both a and b
d) none of the above (no effect)
d) none of the above (no effect)
The magnetic flux density around an empty test coil is reduced by increases in __________ when testing nonmagnetic materials.
a) test piece conductivity and thickness
b) test frequency
c) proximity of coil to test piece
d) all of the above
d) all of the above
What is the advantage of artificial defects made by the EDM process?
a) speed
b) cost
c) accuracy
d) none of the above, EDM is not used to make artificial defects
c) accuracy
What is the main advantage of foil calibration standards over affixed coatings calibration pieces?
a) robustness\
b) accuracy
c) resilience
d) calibration on curved surfaces
d) calibration on curved surfaces
How many thresholds must be set on the CRT display of an eddy current instrument in a box gate alarm system?
a) 2
b) 3
c) 4
d) 8
c) 4
A part with a length to diameter ratio of 1 tested in an encircling coil
a) cannot be tested by eddy current methods
b) results in demagnetization effects
c) gives greatly reduced apparent magnetic permeability
d) both b and c
b) results in demagnetization effects
A part with a length to diameter ratio of 1 tested in an encircling coil
a) cannot be tested by eddy current methods
b) results in demagnetization effects
c) gives greatly reduced apparent magnetic permeability
d) both b and c
d) both b and c
What is the purpose of calibration reference standard?
a) establish acceptance criteria
b) verify accuracy of a test
c) provide traceability back to the national standard
d) all of the above
b) verify accuracy of a test
Eddy current generation to determine material properties use detection of variations in
a) heat
b) magnetic field strength
c) mechanical force or torque
d) all of the above
d) all of the above
Which defect parameter will not affect the probe frequency you select to locate a defect?
a) depth
b) size
c) orientation
d) none of the above (ie. all are frequency dependant)
c) orientation
The sense or direction of a magnetic field around a conductor is most commonly determined using
a) Lenz’s Law
b) Ohm’s Law
c) a Rowland Ring
d) the right hand rule
d) the right hand rule
Moving a direct current carrying conductor up and down near a conductive test piece will result in
a) no current flow in the test piece
b) DC being induced in the test piece
c) AC being induced in the test piece
d) a short circuit
c) AC being induced in the test piece
Inductance increases improve eddy current sensitivity. Why is increasing coil area not a preferred method of increasing sensitivity even though inductance is increased?
a) it makes the coil too bulky
b) resolution of defects is decreased
c) solid cores cannot be used
d) iron cores must be used
b) resolution of defects is decreased
Using a typical impedance type E.C. machine with storage monitor, electrical resistivity determinations are made by
a) observing resonance effects
b) comparison to reference samples
c) taking measurements at two different frequencies
d) none of the above, impedance instruments cannot be used for resistivity measurements
b) comparison to reference samples
Two insulated wires are wound on a plastic rod such that they are positioned very close to each other but not touching. The ends of one wire are connected to a battery, the ends of the other are connected to a galvanometer. If the wire connected to the battery has 1 amp flowing through it, what will the galvanometer read?
a) 0 A
b) 1 A
c) just a little less than 1 amp
d) just a bit more than 1 amp
a) 0 A
Relative permeability is measured in which units?
a) no units (dimensionless ratio)
b) Webers / Ampere-metre
c) Webers / metre²
d) amperes / metre
a) no units (dimensionless ratio)
Measuring the thickness of conductive layer on another conductor (neither being magnetic) requires
a) two frequencies be used
b) a difference in resistivity between the 2 materials
c) the use of a ferrite cup
d) a gap probe be used
b) a difference in resistivity between the 2 materials
Finned copper tubing used in air conditioning units has smooth land areas at regular intervals along the tube. What is the purpose of these land areas?
a) increase tube rigidity
b) the locations at which the tube is roll expanded into tube supports
c) increase heat transfer rate by wall thinning
d) calibration points for differential coil eddy current inspections
b) the locations at which the tube is roll expanded into tube supports
What does an increase in operating frequency do to the probe coil inductance?
a) it increases
b) it decreases
c) it may increase or decrease depending where you are on the locus
d) none of the above
d) none of the above
Frequency for plate thickness determinations of thin sections can be approximated by _______; where p = resistivity (µohm-cm), t = thickness (mm) and ë = standard depth of penetration(mm).
a) f = 1.6 p/t² (kHz)
b) f = t/ë (Hz)
c) f = 3t²/p (kHz)
d) f = pt (kHz)
a) f = 1.6 p/t² (kHz)
An eddy current test system closely approximates a transformer. In this approximation, what would the “secondary coil” be represented by?
a) the induced eddy currents
b) the eddy current probe
c) the test sample
d) a Hall detector used as a receiver
c) the test sample
Given the permeability of free space is 4piX10^-7 Wb/A/m and the permeability of an iron bar is 7X10^-4 Wb/A/m, what is the relative permeability of the iron?
a) 12.56
b) 87.9
c) 280
d) 557
d) 557
In order to facilitate testing of magnetic materials without the interference of permeability changes you would
a) heat and hold the part over the curie temperature for testing
b) use saturating magnets as part of the probe
c) both a and b
d) stress relieve the part prior to testing
c) both a and b
The method whereby desirable frequency signals are separated from undesirable frequency signals from the modulating envelope of the carrier frequency signal is called
a) phase analysis
b) modulation analysis
c) filtering
d) fast fourier transformer
b) modulation analysis
A practical depth limit for flaw detection and location using eddy current test methods is about
a) 1mm
b) 6mm
c) 12mm
d) 18mm
b) 6mm
Why are cadmium plated steel bolts used as fasteners on aircraft?
a) to ensure maximum shear strength
b) to provide corrosion resistance to the steel bolt
c) to provide a galvanically similar surface next to any aluminium to reduce corrosion of aluminium
d) both b and c
d) both b and c
When a simple bridge made up of 4 impedance arms, the voltage in adjacent arms of the bridge must be equal in
a) amplitude
b) phase
c) both a and b
d) no form
c) both a and b
The apparent impedance curve for two different metals of the same thickness will be the same if
a) different probe diameters are used
b) frequencies are adjusted so åf is equal (where å is conductivity and f frequency)
c) lift-off is adjusted to compensate for skin effects
d) all of the above
b) frequencies are adjusted so åf is equal (where å is conductivity and f frequency)
How does hysteresis manifest itself when testing ferromagnetic materials?
a) high resistivity indications
b) high penetration of eddy currents
c) cyclic variations in magnetic permeability
d) electrical noise
c) cyclic variations in magnetic permeability
Tesla or Webers per square metre (Wb/m²) are units of
a) eddy current
b) impedance
c) reluctance
d) magnetic flux density
d) magnetic flux density
Defect effects from tests in the mercury cylinder can be applied to ferromagnetic materials for practical applications provided
a) phase is rotated 90ø
b) mercury resistance is subtracted from the results
c) voltages from the mercury tests are multiplied by the relative magnetic permeability of the ferromagnetic material
d) all of the above
c) voltages from the mercury tests are multiplied by the relative magnetic permeability of the ferromagnetic material
Which of the following can cause variability in resistivity readings taken for the purpose of sorting?
a) sample curvature
b) ambient temperature variation
c) coatings
d) all of the above
d) all of the above
Sensitivity of conductivity measurement with the probe coil is
a) proportional to the coil’s geometric field gradient
b) a function of the specimen thickness
c) a function of the effective coil distance
d) all of the above
d) all of the above
The number of lines of magnetic flux divided by a unit area is the
a) permeability
b) flux density
c) pole strength
d) field intensity
b) flux density
P-type semiconductors use ________________ as charge carriers.
a) electrons
b) holes
c) protons
d) positrons
b) holes
The f90 for tubing and plate are found using similar but different equations. These equations were determined
a) empirically
b) by computer simulations
c) from characteristic frequency (fg)
d) from the phase lag equation
a) empirically
Truly effective sorting of aluminium alloys by eddy current determination of resistivity is not possible because
a) variations due to heat treatment overlap ranges of conductivity
b) grain realignments result when eddy currents flow
c) defect free areas can never be found in aluminium
d) differences in meters used are never standardized
a) variations due to heat treatment overlap ranges of conductivity
In remote field eddy current testing, how far does the direct coupling zone extend from the exciter coil?
a) 0.1 coil diameter
b) 1/2 the inside pipe diameter
c) 2 inside pipe diameters
d) there is no direct coupling zone in remote field eddy current testing
c) 2 inside pipe diameters
What is the biggest advantage eddy current test methods have that make them the most frequently used NDT method in the automotive industry?
a) both surface and subsurface defects are found
b) even automotive engineers can perform the tests
c) the speed at which tests can be performed
d) the application to determine heat treatment quality
c) the speed at which tests can be performed
In an AC circuit the total voltage across a resistor and an inductor in series is found by
a) simple addition
b) simple subtraction
c) vector addition
d) a weighted average
c) vector addition
Metal hardness can be indicated by eddy current testing. This is accomplished by
a) indirect measurement of effects on resistivity
b) amplitude measurement
c) multifrequency technique
d) both b and c
a) indirect measurement of effects on resistivity
Which of the following is an advantage of the differential probe compared to the absolute?
a) sensitive to gradual dimensional changes
b) low sensitivity to probe wobble
c) easily interpreted signals
d) all of the above
b) low sensitivity to probe wobble
At 2 standard depths of penetration, eddy current density is about ____ that at the surface.
a) 66%
b) 37%
c) 14%
d) 9%
c) 14%
In the send-receive probe arrangement where the driver and receiver coil are on opposite sides of a plate, signal variation will result from
a) material variations (eg. voids) in the test material
b) coil-to-coil spacing
c) proximity variations of test piece to coils (lift-off)
d) both a and b
d) both a and b
Phase lag in degrees would be represented by (where x = depth, ë = standard depth of penetration).
a) xë
b) x/ë
c) ë/x
d) 57 x/ë
d) 57 x/ë
The purpose of the ferromagnetic core used in a gap probe is to
a) shape the magnetic field
b) saturate the test piece with magnetism
c) compensate for lift-off
d) reduce heating effects caused by eddy current
a) shape the magnetic field
Instrumentation for systems using Hall detectors instead of pickup coils are different in what respect?
a) stabilized DC supplies are needed
b) excitation AC current must be constant for all frequencies
c) both a and b
d) none of the above, Hall detectors can be used on any E.C. instrument
c) both a and b
The analytical method that consists of correlating changes in amplitude, phase and/or quadrature components of a complex test signal voltage to electromagnetic conditions in the test piece is
a) phase analysis
b) impedance analysis
c) differential analysis
d) absolute analysis
b) impedance analysis
Multifrequency techniques are performed using
a) absolute coils
b) differential coils
c) both a and b
d) special multifrequency coils only
c) both a and b
What is the f90 for an encircling coil used on aluminium tubing, p = 5.1µohm-cm, wall thickness 5mm, diameter 40mm?
a) 612 Hz
b) 3.1 kHz
c) 14.7 kHz
d) 61.2 kHz
a) 612 Hz
Eddy current instruments do not have a phase reference, this short coming is compensated for by
a) sensitivity
b) resonance adjustment
c) phase shift control
d) none of the above
c) phase shift control
Seamless pipe and tubing are often made from billets made from continuous cast blooms. The rounds, as the billets are called, are test by eddy current to detect what types of defect?
a) cracks
b) ovalities
c) laps
d) all of the above
d) all of the above
The slope of the induction curve at zero magnetizing force as the test piece is being taken from its demagnetized state is the
a) virgin permeability
b) initial permeability
c) maximum permeability
d) effective permeability
b) initial permeability
When inspecting spheres with an encircling coil, what is the equivalent effect of increasing the coil length?
a) a frequency increase
b) a frequency decrease
c) an increase in material conductivity
d) a decrease in fill factor
d) a decrease in fill factor
Alpha-case forms on titanium and its alloys at elevated temperatures. Eddy currents are used to establish the depth of case. What is the cause of the formation of alpha-case?
a) oxygen diffusion from the heated surface
b) carbide migration
c) active cathodic protection by-products
d) passive anodic protection by-products
a) oxygen diffusion from the heated surface
When does material thickness affect the results of a conductivity test?
a) when eddy current effective penetration is greater than material thickness
b) when conductivity is very high
c) when the material is backed by a higher conductivity material
d) when lift-off is a result of a surface roughness
a) when eddy current effective penetration is greater than material thickness
In what way does computer acquisition and analysis of eddy current signals (particularly heat exchanger tubing) out-perform humans?
a) detection
b) reproducibility
c) accuracy
d) all of the above
d) all of the above
In order that useful results be obtained from an eddy current test, what must be true about the test specimen?
a) only one or two parameters are subject to change
b) it must be ferromagnetic
c) composition must be uniform throughout
d) size and shape must always be small with simple geometric symmetry
a) only one or two parameters are subject to change
The reason for normalizing probe impedance is to
a) allow operators to set lift-off horizontal
b) avoid resonance on impedance graph displays
c) allow study of test specimen variations without concern for probe variations
d) establish a common ground for international discussions of eddy current testing
c) allow study of test specimen variations without concern for probe variations
The most common and reliable method of manufacturing artificial cracks for eddy current standard is by
a) electric discharge machining
b) ion milling
c) TEM (tunneling electron microscopy)
d) saw cuts
a) electric discharge machining
When performing a tubing inspection with a general purpose ECT instrument having an instrument response frequency of 300Hz, maximum scanning speed is about ___ to detect abrupt defects without distortion.
a) 0.1 m/s
b) 0.25 m/s
c) 1.0 m/s
d) 2.0 m/s
b) 0.25 m/s
If a probe for internal tube testing has an average coil diameter of 11mm, what size would the tube inside diameter be to give a 0.9 fill-factor?
a) 12.2
b) 11.6
c) 11.1
d) 10.9
b) 11.6
The limit frequency is
a) the optimum test frequency
b) the maximum limit test frequency
c) the minimum limit test frequency
d) none of the above
d) none of the above
In multifrequency instruments 2 or more operating frequencies are input to a probe simultaneously. What output must be adjusted to permit effective vectorial addition?
a) gain
b) phase
c) frequency difference
d) both a and b
d) both a and b
The main reason an eddy current coil can detect support plates in heat exchangers when testing tubes from the inside diameter is
a) support plates are always ferro-magnetic
b) support plates are always the same material as the tube
c) magnetic flux is not restricted by the tube wall
d) support plates act as resonance amplifiers in the circuit
c) magnetic flux is not restricted by the tube wall
Linear multichannel Hall detector arrays are ideal for
a) single pass inspections of large surfaces
b) improving depth resolution
c) increased depth of penetration
d) increasing frequency response
a) single pass inspections of large surfaces
The method of eddy current testing that uses a dedicated coil to induce eddy currents in a test piece and another coil to detect eddy current variations in the test piece is the _______method.
a) pulse-echo
b) impedance
c) send-receive
d) sing-a-round
c) send-receive
The total impedance in a tuned circuit appears as
a) a phase shift
b) pure resistance
c) pure inductance
d) pure capacitance
b) pure resistance
What is the disadvantage of the multi-pancake probe used for internal tube inspections as compared to the axial bobbin type probe?
a) difficulty in interpreting signals
b) over sensitivity to wobble
c) reduced sensitivity to outside wall defects
d) insensitivity to circumferential cracks
c) reduced sensitivity to outside wall defects
What is the most effective way of assessing heat or fire damage to heat-treatable aluminium on aircraft?
a) eddy current conductivity tests
b) ultrasonic velocity tests
c) Brinnel hardness tests
d) thermography
a) eddy current conductivity tests
When using an eddy current surface probe for flaw detection in plate, a frequency f = 1.6p/t² (kHz) will provide good discrimination between (p is resistivity)
a) all possible indications
b) lift-off and defects but not resistivity and ferromagnetic signals
c) lift-off and defects and ferromagnetic signals
d) lift-off and defects and resistivity changes
b) lift-off and defects but not resistivity and ferromagnetic signals
When an eddy current probe is brought near a conductive sample the net magnetic flux in the system
a) increases
b) decreases
c) remains unchanged
d) drops to zero when the part is contacted
b) decreases
In the send-receive method of eddy current testing the variations in eddy current flow due to flaws in the test piece are monitored by
a) the primary coil
b) the secondary coil
c) the receive coil
d) both b and c
d) both b and c
An electromagnetic sorting based on a signal response from the material under test above or below a level established by two or more calibration standards is
a) a two way sort
b) a three way sort
c) threshold sorting
d) standard deviation testing
a) a two way sort
Applying a DC electric field to a ferromagnetic coil is done for what purpose?
a) reduce background noise
b) improve signal to noise ratio
c) eliminate permeability variations that might affect eddy current coil response
d) all of the above
d) all of the above
A voltage in induced in a region of space when there exists a changing magnetic field. This is a statement of
a) Faraday’s Law
b) Oersted’s Law
c) Helmholtz’s Theorem
d) Ohm’s Law
a) Faraday’s Law
What is the purpose of pulsed saturation eddy current testing?
a) to allow sequenced multifrequency application
b) to achieve greater penetration in ferromagnetic materials
c) synchronization of gates
d) it allows time for ring to die down and so improves far wall resolution
b) to achieve greater penetration in ferromagnetic materials
What are slip rings used for in eddy current inspection systems?
a) electrical contacts in rotating heads
b) clutch mechanisms in probe pushers
c) to allow ease of motion by encircling probes
d) to allow ease of motion by bobbin probes
a) electrical contacts in rotating heads
A circuit block that accepts a binary number and translates it to an analogue voltage or current proportional to the binary number is a(n)
a) CPU
b) analogue-to-digital converter
c) digital-to-analogue converter
d) retro-virus
c) digital-to-analogue converter
Which of the following are Hall effect detectors not sensitive to?
a) magnitude of magnetic field
b) direction of magnetic field
c) rate of change of total flux linkage
d) both a and b
c) rate of change of total flux linkage
Two or more coils in electrical series opposition arranged so EM conditions not common to the areas of the specimen being tested produce a bridge imbalance is a(n)
a) absolute probe
b) axial probe
c) differential probe
d) multipancake probe
c) differential probe
The purpose of spring loading an eddy current probe against the test material is
a) greater wear protection
b) to maintain constant capacitance
c) to minimize lift-off
d) to prevent Barkhausen noise
c) to minimize lift-off
Given a coil with 50 ohm resistance and 50 microhenries inductance and operated at 50 kHz; what is the coil inductive reactance?
a) 0.4 ohms
b) 1.6 ohms
c) 3.9 ohms
d) 15.7 ohms
d) 15.7 ohms
Magnetic shielding technique provides the magnetic field lines of the eddy current probe with
a) increased penetrating ability
b) decreased coupling ability
c) a path of low magnetic reluctance
d) both a and c
c) a path of low magnetic reluctance
When eddy current probes used for resistivity readings are required to be used on small surfaces (eg. bolt heads), what can be done to overcome edge effects?
a) use field collimators
b) use correction factors from a pre-made edge-distance curve
c) both a and b
d) use higher frequencies
c) both a and b
The ratio of the square of the diameter of a cylindrical test piece to the square of the average diameter of the test coil is the
a) flux ratio
b) fill factor
c) physical impedance
d) test ratio
b) fill factor
Vectorial addition of signals at conductive non-magnetic support plates is not usually viable because
a) signals are too large making small defects hard to see
b) no magnetite occurs to use as a reference
c) elimination frequencies are too high
d) elimination frequencies are too low
a) signals are too large making small defects hard to see
What is the purpose of DC magnetic bias in eddy current testing?
a) increases near surface sensitivity
b) reduce magnetic permeabilities of ferromagnetic test materials
c) increase magnetic permeabilities of ferromagnetic test materials
d) eliminate probe wobble signals
b) reduce magnetic permeabilities of ferromagnetic test materials
The frequency providing the highest signal-to-noise ratio for detection of an individual property of the test piece is the
a) optimum frequency
b) test frequency
c) limit frequency
d) characteristic frequency
a) optimum frequency
To eliminate magnetic deposits as a possible cause of “defect” signals (ie. a non-relevant indication) it is recommended that
a) you reinspect the area at 2f90
b) you reinspect the area at 4f90
c) you reinspect the area at 0.1f90
d) both a and b
c) you reinspect the area at 0.1f90
Which of the following is not an example of electromechanical energy conversion devices?
a) a dry cell battery
b) a generator or alternator
c) a microphone
d) an electric motor
a) a dry cell battery
Phase angle differences of eddy currents greater than about 100° is not recommended for tube testing with encircling coils because
a) eddy current density on the inner wall is too low for crack detection
b) there is no sensitivity to ferromagnetic inclusions
c) no discrimination between inner and outer wall is possible
d) variation in wall thickness and cracks look the same
a) eddy current density on the inner wall is too low for crack detection
The inductance in the excitation coil is proportional to the diameter squared (D²) and the number of turns squared (N²). The voltage induced in the pickup coil is proportional to
a) N and D
b) N² and D
c) N² and D²
d) N and D²
c) N² and D²
The property of a test system that allows separation of signals from defects on close proximity to each other is
a) phase separation
b) amplitude discrimination
c) defect resolution
d) multifrequency demodulation
c) defect resolution
Hall detectors used in ECT send-receive systems have the advantage of
a) being independent of test frequency
b) being useful for low frequency inspection
c) being used in very tight locations as size can be as small as 1mmý
d) all of the above
d) all of the above
Multifrequency eddy current testing utilizes
a) a single probe operating at more than one frequency
b) a single probe operated at one frequency then rescanning the flaw at a different frequency with the same probe
c) two or more probes in tandem each at a different frequency
d) any of the above constitutes multifrequency ECT
a) a single probe operating at more than one frequency
The lift-off locus is
a) a straight line connecting the zero lift-off point to the empty coil value
b) bent slightly left towards increasing f/fg values
c) bent slightly right towards increasing f/fg values
d) bent slightly right towards decreasing f/fg values
b) bent slightly left towards increasing f/fg values
What is done to correct for reduced field coupling when making conductivity measurements on curved surface?
a) a Vee block is used to maintain parallelism
b) curved calibration standards are used
c) lower operating frequencies are used
d) both a and b
d) both a and b
The magnetic line of force is
a) a mythical quantity
b) a imaginary but useful concept
c) equal to 1gm mass when converted by Einstein’s equation
d) 1 micron diameter and 10 microns long
b) a imaginary but useful concept
