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ASNT NDT Level III MT study materials
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ASNT NDT Level III MT practice questions

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Question 1Fundamentals of magnetism and magnetic particle testing theory

In the SI system, magnetic flux density is expressed in which unit?

  1. henry
  2. tesla Correct answer.
  3. weber
  4. ampere per metre

Explanation

Flux density B is defined as flux per unit area and its SI unit is the tesla, where 1 T equals 1 Wb/m². The weber measures total flux, ampere per metre measures magnetizing force, and the henry measures inductance.

Question 2Fundamentals of magnetism and magnetic particle testing theory

The relative permeability of air is closest to which value?

  1. 0
  2. 1000
  3. 1 Correct answer.
  4. 100

Explanation

Relative permeability compares a material's permeability with that of a vacuum, so air and vacuum both have a value of approximately 1. Ferromagnetic materials such as iron have values in the hundreds or thousands.

Question 3Fundamentals of magnetism and magnetic particle testing theory

When a ferromagnetic material is heated above its Curie temperature it:

  1. reaches peak permeability
  2. becomes non-ferromagnetic Correct answer.
  3. retains maximum residual magnetism
  4. develops permanent poles

Explanation

Above the Curie point thermal agitation destroys the aligned domain structure, so the material becomes paramagnetic and can no longer be magnetized as a ferromagnetic body. Permeability drops sharply at that temperature rather than peaking.

Question 4Fundamentals of magnetism and magnetic particle testing theory

The magnetic field produced in the space around a straight conductor carrying current takes which form?

  1. concentric circles Correct answer.
  2. helical along the conductor
  3. parallel to the conductor axis
  4. radial, diverging outward from it

Explanation

Current in a straight conductor generates circular flux lines centred on the conductor, with direction given by the right-hand rule. Field strength falls off with increasing distance from the conductor.

Question 5Fundamentals of magnetism and magnetic particle testing theory

Which of these materials is ferromagnetic at room temperature?

  1. Copper
  2. Iron Correct answer.
  3. Titanium
  4. Aluminium

Explanation

Iron, nickel and cobalt are the common ferromagnetic metals and are strongly attracted by a magnet. Copper and aluminium are non-magnetic and titanium is only weakly paramagnetic.

Question 6Fundamentals of magnetism and magnetic particle testing theory

A flux density of 1.5 T corresponds to which value in gauss?

  1. 15 000 G Correct answer.
  2. 150 G
  3. 1 500 G
  4. 150 000 G

Explanation

One tesla equals 10 000 gauss, so a flux density of 1.5 T is 1.5 × 10 000 = 15 000 G. The conversion factor is applied by multiplying the tesla value by 10 000.

Question 7Fundamentals of magnetism and magnetic particle testing theory

Residual magnetism in a finished part is usually removed because it may:

  1. reduce background noise
  2. affect subsequent welding Correct answer.
  3. raise the permeability of the part
  4. improve particle mobility

Explanation

Residual magnetism can deflect welding arcs, attract metal chips during machining and interfere with instruments, so parts are demagnetized after testing. It never improves the inspection process itself.

Question 8Fundamentals of magnetism and magnetic particle testing theory

In a bar magnet, the magnetic flux lines:

  1. form closed loops that continue through the magnet Correct answer.
  2. travel only outside the magnet body
  3. terminate permanently in the surrounding air
  4. start and end on isolated magnetic poles

Explanation

Magnetic flux lines are continuous closed loops; outside the magnet they run from north to south and inside they return from south to north. Isolated magnetic poles have never been observed.

Question 9Fundamentals of magnetism and magnetic particle testing theory

With the prod method, the recommended spacing between contact prods is generally:

  1. 450 to 600 mm
  2. 10 to 25 mm
  3. 250 to 400 mm
  4. 75 to 200 mm Correct answer.

Explanation

Prod spacing of roughly 75 to 200 mm (3 to 8 in) keeps the field strong enough between the contacts while covering a practical inspection area. Wider spacing weakens and deepens the field, and narrower spacing overheats the contact points.

Question 10Fundamentals of magnetism and magnetic particle testing theory

A tubular part is magnetized with a central conductor running along its bore. This technique is best suited to revealing:

  1. cracks running lengthwise Correct answer.
  2. subsurface porosity near the bore
  3. cracks running across the axis
  4. radial cracks through the wall

Explanation

A central conductor sets up a circular field that encircles the bore, so flux lines run parallel to the axis of the part. Discontinuities oriented along the axis, roughly perpendicular to those circular flux lines, produce the strongest leakage.

Question 11Fundamentals of magnetism and magnetic particle testing theory

A ferromagnetic material that retains magnetism strongly and resists demagnetization is described as:

  1. paramagnetic
  2. magnetically soft
  3. diamagnetic
  4. magnetically hard Correct answer.

Explanation

A wide hysteresis loop, meaning high retentivity and high coercive force, defines a magnetically hard material. Soft magnetic materials have narrow loops and are easily magnetized and demagnetized.

Question 12Fundamentals of magnetism and magnetic particle testing theory

The continuous method is generally preferred to the residual method because:

  1. no particle bath is needed
  2. any material can be used
  3. the residual field is stronger
  4. field applied during spraying Correct answer.

Explanation

In the continuous method the magnetizing force is maintained while the particles are applied, so particles migrate to leakage fields under the influence of the field itself. This yields noticeably better sensitivity than relying on the field left in the part after the current stops.

Question 13Fundamentals of magnetism and magnetic particle testing theory

For maximum indication strength, the angle between the magnetic flux lines and the long axis of a surface crack should be about:

  1. 45 degrees
  2. 0 degrees
  3. 30 degrees
  4. 90 degrees Correct answer.

Explanation

Leakage is greatest when the discontinuity lies perpendicular to the flux path, because the flux must then cross the air gap. Flux running parallel to a crack is barely disturbed and produces little or no indication.

Question 14Fundamentals of magnetism and magnetic particle testing theory

When magnetizing current is passed directly through a solid bar, the resulting field is:

  1. uniform across the section
  2. radial at the centre
  3. circular near the surface Correct answer.
  4. longitudinal at the ends

Explanation

Current through the part creates a circular field that is strongest at the surface and falls to zero at the centre line. This is why direct current through the part is used for discontinuities lying along the axis.

Question 15Fundamentals of magnetism and magnetic particle testing theory

An AC electromagnetic yoke used for surface inspection should be capable of lifting approximately which weight at its maximum pole spacing?

  1. 4.5 kg (10 lb) Correct answer.
  2. 50 kg (110 lb)
  3. 20 kg (45 lb)
  4. 0.5 kg (1 lb)

Explanation

The standard check requires an AC yoke to lift about 4.5 kg (10 lb) at the maximum pole spacing to be used. This verifies that the field strength between the poles is adequate for surface detection.

Question 16Fundamentals of magnetism and magnetic particle testing theory

The reluctance of a magnetic circuit increases when:

  1. permeability increases
  2. the flux path is made longer Correct answer.
  3. cross-sectional area increases
  4. flux density decreases

Explanation

Reluctance is proportional to path length and inversely proportional to permeability and cross-sectional area. Lengthening the flux path therefore raises reluctance, in the same way a longer wire raises electrical resistance.

Question 17Fundamentals of magnetism and magnetic particle testing theory

Wet fluorescent magnetic particle inspection is usually selected when:

  1. the surface is heavily painted
  2. the part is too hot to handle
  3. very fine defects must be found Correct answer.
  4. subsurface flaws are the main concern

Explanation

Wet methods use finer particles in suspension and, combined with fluorescence under ultraviolet light, give the highest sensitivity to small surface discontinuities. Dry powders are coarser and are used where larger defects or rougher surfaces are involved.

Question 18Fundamentals of magnetism and magnetic particle testing theory

Magnetizing force (H) is expressed in which unit?

  1. amperes per metre Correct answer.
  2. henrys per metre
  3. webers per square metre
  4. tesla per second

Explanation

Magnetizing force, also called field intensity, is measured in amperes per metre in SI units and depends on current and coil geometry rather than on the material. Flux density B, in tesla, is the material's response to that force.

Question 19Fundamentals of magnetism and magnetic particle testing theory

A surface-breaking crack is detected because magnetic particles are attracted to:

  1. an electrostatic charge at the crack
  2. light reflected from the crack faces
  3. a leakage field escaping at the crack Correct answer.
  4. particles caught mechanically in the crack

Explanation

Flux crossing an air gap at a discontinuity is refracted out of the surface, producing a local leakage field with north and south poles. Ferromagnetic particles are drawn to and held by this field, forming a visible indication.

Question 20Fundamentals of magnetism and magnetic particle testing theory

Longitudinal magnetization of a part is produced by:

  1. using prods on the surface
  2. a central conductor in a bore
  3. passing current through the part
  4. a coil or electromagnet Correct answer.

Explanation

Placing the part inside a solenoid coil or between the poles of an electromagnet drives flux along the length of the part, giving a longitudinal field. Current through the part or through prods and central conductors produces a circular field instead.

Question 21Fundamentals of magnetism and magnetic particle testing theory

Compared with direct current of the same value, alternating current produces a magnetic field that is:

  1. deeper within the part
  2. independent of frequency
  3. identical throughout the section
  4. crowded toward the surface Correct answer.

Explanation

Alternating current concentrates near the surface because of the skin effect, so the resulting field is strongest at the surface and weak at depth. This makes AC well suited to surface discontinuities and to demagnetization.

Question 22Fundamentals of magnetism and magnetic particle testing theory

Once a ferromagnetic part reaches magnetic saturation, further increases in magnetizing force produce:

  1. a rapid rise in permeability
  2. a proportional increase in flux density
  3. immediate reversal of the field
  4. almost no further rise in flux Correct answer.

Explanation

At saturation the magnetic domains are already aligned, so additional magnetizing force cannot increase flux density appreciably. The curve of flux density against magnetizing force flattens out beyond that point.

Question 23Fundamentals of magnetism and magnetic particle testing theory

The most effective way to demagnetize a ferromagnetic part is to apply:

  1. a reversing, decaying field Correct answer.
  2. a steady field of high strength
  3. heating above the Curie temperature
  4. a single pulse of direct current

Explanation

Demagnetization requires driving the material around successively smaller hysteresis loops by reversing polarity while steadily reducing amplitude to zero. A steady field or a single pulse leaves the material on a minor loop with residual magnetism still present.

Question 24Fundamentals of magnetism and magnetic particle testing theory

A yoke is placed on a part with its poles 150 mm apart and the field between them is weak. If the pole spacing is increased further, the field between the poles will:

  1. weaken Correct answer.
  2. become more uniform
  3. double in strength
  4. stay the same

Explanation

Flux spreads over an increasing volume as the poles are separated, so the field strength in the gap and the useful flux density in the part both fall. Pole spacing must therefore be kept within the range the yoke was designed to handle.

Question 25Fundamentals of magnetism and magnetic particle testing theory

Which material property most strongly determines whether a usable residual field will remain for the residual magnetization technique?

  1. permeability
  2. retentivity Correct answer.
  3. reluctance
  4. conductivity

Explanation

The residual technique relies on the field left in the part after the magnetizing current is removed, which is a direct expression of the material's retentivity. Low-retentivity materials lose nearly all their flux and are unsuited to this technique.

Question 26Fundamentals of magnetism and magnetic particle testing theory

The temperature above which a ferromagnetic material loses its ferromagnetic behaviour and becomes paramagnetic is called the:

  1. Curie temperature Correct answer.
  2. Saturation temperature
  3. Austenitizing temperature
  4. Néel temperature

Explanation

Each ferromagnetic material has a characteristic temperature at which thermal agitation destroys the alignment of magnetic domains, and the material becomes paramagnetic. For iron this transition occurs at about 770 °C. The temperature is named after Pierre Curie, who described the phenomenon.

Question 27Fundamentals of magnetism and magnetic particle testing theory

Magnetic particles used for inspection should have which combination of properties?

  1. Low permeability and high retentivity
  2. High permeability and low retentivity Correct answer.
  3. Low permeability and low retentivity
  4. High permeability and high retentivity

Explanation

Particles must be easily magnetized by the leakage field, which requires high permeability, so they are strongly attracted to indications. They must also give up their magnetism readily, which requires low retentivity, so they do not clump together or cling to the part and form confusing background.

Question 28Fundamentals of magnetism and magnetic particle testing theory

Direct current flows in a straight conductor toward the reader, out of the page. Viewed from the reader's position, the magnetic field around the conductor runs:

  1. Clockwise
  2. Radially outward from the conductor
  3. Counterclockwise Correct answer.
  4. Radially inward toward the conductor

Explanation

The field around a straight conductor forms closed circular lines in a plane perpendicular to the current, as given by the right-hand rule. With the right thumb pointing out of the page toward the reader, the fingers curl counterclockwise as seen by that reader.

Question 29Fundamentals of magnetism and magnetic particle testing theory

Which type of magnetizing current is most effective for revealing discontinuities located below the surface of a ferromagnetic part?

  1. Alternating current
  2. High-frequency alternating current
  3. Direct current Correct answer.
  4. Pulsed alternating current

Explanation

Direct current produces a steady field that penetrates the full cross-section of the part, so leakage fields can form at subsurface discontinuities. Alternating current is concentrated near the surface by the skin effect and is therefore limited mainly to surface and near-surface detection.

Question 30Fundamentals of magnetism and magnetic particle testing theory

A solenoid 200 mm long has 400 turns and carries a current of 3 A. Neglecting the core, the approximate magnetizing force inside the coil is:

  1. 600 A/m
  2. 6000 A/m Correct answer.
  3. 1200 A/m
  4. 24000 A/m

Explanation

For a long solenoid the magnetizing force is H = NI/L, where N is the number of turns, I the current and L the coil length in metres. Substituting gives H = (400 × 3)/0.20 = 6000 A/m. The result depends on ampere-turns per unit length, not on the core material.

Question 31Fundamentals of magnetism and magnetic particle testing theory

A coil produces a magnetizing force H in a region of space. A soft iron core of relative permeability 500 is then inserted into that region. The flux density B in the region:

  1. Increases by a factor of approximately 500 Correct answer.
  2. Increases by a factor of approximately 500 only if the coil current is also increased
  3. Decreases by a factor of approximately 500
  4. Remains the same because H is unchanged

Explanation

The magnetizing force H is fixed by the coil current and geometry, but flux density is related to it by B = μ0μrH. Replacing air (μr near 1) with iron of relative permeability 500 therefore raises B by roughly the same factor, until the iron approaches saturation.

Question 32Fundamentals of magnetism and magnetic particle testing theory

The area enclosed by the hysteresis loop of a ferromagnetic material represents:

  1. The energy dissipated as heat per cycle of magnetization Correct answer.
  2. The maximum flux density the material can support
  3. The residual magnetism left after the magnetizing force is removed
  4. The permeability of the material at low field strength

Explanation

Tracing the loop once means magnetizing and demagnetizing the material in both directions, and the work not returned to the circuit appears as heat. The enclosed area is therefore the energy lost per unit volume per cycle, which is why materials for transformers are chosen to have narrow loops.

Question 33Fundamentals of magnetism and magnetic particle testing theory

When a part is magnetized with alternating current, the field is concentrated in a thin layer adjacent to the surface. This behaviour is known as:

  1. Flux refraction
  2. The skin effect Correct answer.
  3. Magnetic saturation
  4. The demagnetizing effect

Explanation

Alternating current and its associated alternating field are crowded toward the surface of a conductor, and the crowding becomes more pronounced as frequency rises. This so-called skin effect is the reason AC magnetization is used mainly for surface and near-surface discontinuities.

Question 34Fundamentals of magnetism and magnetic particle testing theory

A current of 600 A passes through a long straight conductor. In air, at a distance of 30 mm from the conductor axis, the magnetizing force is approximately:

  1. 64 A/m
  2. 20000 A/m
  3. 3200 A/m Correct answer.
  4. 1000 A/m

Explanation

The field around a long straight conductor is H = I/(2πr). Substituting I = 600 A and r = 0.030 m gives H = 600/(2π × 0.030) ≈ 3180 A/m, or about 3200 A/m. The field falls off inversely with distance from the axis.

Question 35Fundamentals of magnetism and magnetic particle testing theory

Dry magnetic powder is generally preferred over a wet suspension when:

  1. The part has a smooth machined finish and very fine discontinuities
  2. The part is hot, roughly cast, or must be tested in the field Correct answer.
  3. Fluorescent indications are to be viewed under ultraviolet light
  4. The greatest possible sensitivity to tight surface cracks is required

Explanation

Dry powder consists of coarser particles and carries no liquid vehicle, so it performs well on rough surfaces, at elevated temperatures and in portable field work where wet handling is impractical. Wet suspensions use finer particles and give higher sensitivity on smooth, critical parts.

Question 36Fundamentals of magnetism and magnetic particle testing theory

A part has been magnetized in a single direction. To ensure that discontinuities of all orientations are detected, the inspector should:

  1. Magnetize in at least two approximately perpendicular directions Correct answer.
  2. Increase the current amplitude only
  3. Demagnetize the part between successive passes
  4. Use the residual method only

Explanation

A discontinuity produces its strongest leakage field when it lies roughly perpendicular to the flux lines, and a discontinuity parallel to the flux produces almost none. Magnetizing in two approximately perpendicular directions ensures that every orientation meets a transverse field in at least one pass.

Question 37Fundamentals of magnetism and magnetic particle testing theory

Excessive magnetizing current applied to a part with a rough surface is most likely to produce:

  1. False indications caused by the skin effect
  2. A background of particles that masks relevant indications Correct answer.
  3. An increase in the permeability of the part
  4. A permanent loss of residual magnetism

Explanation

Very high current creates strong leakage fields at surface irregularities, edges and joints, and particles collect at all of them. The resulting background obscures the indications that matter. Magnitude should be just sufficient to reveal discontinuities without flooding the surface.

Question 38Fundamentals of magnetism and magnetic particle testing theory

A flux density of 0.8 T crosses a steel-to-air interface at right angles to the surface. The flux density in the air immediately outside the steel is:

  1. About zero
  2. About 0.8 T Correct answer.
  3. About 0.8 multiplied by the relative permeability of the steel
  4. About 0.8 divided by the relative permeability of the steel

Explanation

At a boundary, the component of flux density normal to the surface is continuous, so the flux passes into the air unchanged in magnitude. It is the magnetizing force H that changes across the boundary, by the ratio of the permeabilities. The field outside therefore remains about 0.8 T.

Question 39Fundamentals of magnetism and magnetic particle testing theory

Two bars of the same steel and the same cross-section are magnetized in the same coil, one being 50 mm long and the other 500 mm long. Compared with the long bar, the short bar will:

  1. Be more strongly opposed by its own free poles and retain less flux Correct answer.
  2. Reach a higher flux density for the same applied field
  3. Retain more flux because its poles are closer together
  4. Behave identically because the material is the same

Explanation

Free poles at the ends of a magnetized bar set up a demagnetizing field that opposes the applied field inside the bar. As the length-to-diameter ratio falls, this demagnetizing field grows, so the short bar reaches a lower internal flux density and retains less when the field is removed.

Question 40Fundamentals of magnetism and magnetic particle testing theory

Which condition in a steel part would most reduce its response to a given magnetizing force?

  1. A coarse grain size
  2. A slight increase in carbon content
  3. A small amount of cold work
  4. A large fraction of retained austenite Correct answer.

Explanation

Austenite is paramagnetic, so retained austenite contributes essentially nothing to the ferromagnetic response of the part. A large fraction of it dilutes the effective ferromagnetic volume and lowers the flux density obtained for a given magnetizing force, which can cause discontinuities to be missed.

Question 41Equipment, materials, and magnetization/particle application methods

For hand-held prods used on steel plate, the recommended spacing between the contact tips is generally:

  1. 25 to 50 mm (1 to 2 in)
  2. 75 to 200 mm (3 to 8 in) Correct answer.
  3. 250 to 350 mm (10 to 14 in)
  4. 400 to 500 mm (16 to 20 in)

Explanation

Hand-held prods are normally spaced about 75 to 200 mm (3 to 8 in) apart, with current set in proportion to that spacing. Spacing outside this range either concentrates the field too narrowly near the tips or spreads it so thinly that small discontinuities are missed.

Question 42Equipment, materials, and magnetization/particle application methods

Which magnetic particle method generally provides the highest sensitivity for fine, surface-breaking discontinuities?

  1. Dry powder used with an AC yoke
  2. Nonfluorescent dry powder in a cloud
  3. Fluorescent wet method Correct answer.
  4. Visible-dye wet method

Explanation

Wet fluorescent particles are the finest and most mobile particles available, and the fluorescent response under ultraviolet light gives high contrast against a dark background. The combination of small particle size and high contrast makes the fluorescent wet method the most sensitive for fine surface indications.

Question 43Equipment, materials, and magnetization/particle application methods

An AC electromagnetic yoke is used primarily to detect:

  1. Subsurface inclusions
  2. Deep internal porosity
  3. Surface-breaking discontinuities Correct answer.
  4. Mid-wall laminations

Explanation

Alternating current concentrates the magnetic flux in a thin layer at the surface through the skin effect, so the strong field exists only near the surface. Subsurface and internal conditions require direct or rectified current for adequate flux penetration.

Question 44Equipment, materials, and magnetization/particle application methods

Under typical specifications, the minimum ultraviolet (black light) intensity at the inspection surface for fluorescent magnetic particle testing is:

  1. 500 µW/cm²
  2. 100 µW/cm²
  3. 1000 µW/cm² Correct answer.
  4. 300 µW/cm²

Explanation

Standard practice requires at least 1000 µW/cm² (about 10 W/m²) of black light intensity at the inspection surface. The measurement is made at the working distance with the filter in place and the lamp fully warmed up.

Question 45Equipment, materials, and magnetization/particle application methods

Parts are usually demagnetized by:

  1. Heating the part above its Curie temperature
  2. Applying a reversing field of steadily decreasing amplitude Correct answer.
  3. Quenching the part in oil from the austenitizing temperature
  4. Applying steady DC of the same polarity used in testing

Explanation

A reversing field whose amplitude is steadily reduced leaves the magnetic domains randomized, so the residual field approaches zero. This is the basis of AC and reversing-DC demagnetization.

Question 46Equipment, materials, and magnetization/particle application methods

An encircling coil produces which type of magnetic field in a cylindrical part?

  1. A radial field through the wall thickness
  2. A longitudinal field along the part axis Correct answer.
  3. A random field with no preferred direction
  4. A circular field around the part axis

Explanation

Current flowing in the coil turns produces flux lines parallel to the coil axis, giving longitudinal magnetization of the part inside it. This field is best for detecting discontinuities lying across the axis.

Question 47Equipment, materials, and magnetization/particle application methods

When inspecting with a yoke, a crack is most readily detected when it lies:

  1. Parallel to the line joining the poles
  2. Perpendicular to the line joining the poles Correct answer.
  3. Directly beneath one of the poles
  4. At 30 degrees to the line joining the poles

Explanation

Flux travels between the yoke poles, so a discontinuity that crosses that path at a large angle interrupts the flux and creates a strong leakage field. Indications are strongest when the discontinuity is roughly perpendicular to the flux direction.

Question 48Equipment, materials, and magnetization/particle application methods

Compared with wet-method particles, dry magnetic particles are typically:

  1. Smaller and suspended in a liquid vehicle
  2. Larger and applied by dusting or in a cloud Correct answer.
  3. Fluorescent and used only under black light
  4. Coated with a surfactant to improve mobility

Explanation

Dry particles are milled to a larger average size so they flow freely and can be blown into a cloud or dusted onto the surface. Wet particles are much finer so that they remain in suspension and give higher sensitivity.

Question 49Equipment, materials, and magnetization/particle application methods

For an encircling coil, as the length-to-diameter (L/D) ratio of the part increases, the amperage needed to produce an adequate longitudinal field:

  1. Remains constant
  2. First decreases and then rises sharply
  3. Increases in direct proportion
  4. Decreases Correct answer.

Explanation

The coil rule of thumb is NI = 45,000/(L/D), so the ampere-turns required fall as L/D rises because a long part is more easily magnetized along its length. For L/D below about 2, the value for L/D = 2 is used to avoid over-magnetization.

Question 50Equipment, materials, and magnetization/particle application methods

If the prod spacing on a steel plate is increased from 4 in to 8 in, the magnetizing current should be:

  1. Reduced to one quarter
  2. Approximately halved
  3. Left unchanged
  4. Approximately doubled Correct answer.

Explanation

Current for prod magnetization is proportioned to the spacing between the contact tips, commonly 100 to 125 A per inch. Doubling the spacing therefore roughly doubles the current needed to maintain field density.