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

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Question 1General knowledge of nondestructive testing methods and their applications

Which nondestructive test method relies on the principle that a changing magnetic field induces eddy currents in a conductive material?

  1. Radiographic testing
  2. Acoustic emission testing
  3. Magnetic particle testing
  4. Eddy current testing Correct answer.

Explanation

Eddy current testing induces circulating currents in conductive materials through electromagnetic induction; changes in coil impedance reveal discontinuities or property variations. Magnetic particle testing uses magnetic flux leakage with ferromagnetic materials and a visible medium, not induced currents.

Question 2General knowledge of nondestructive testing methods and their applications

Which method is inherently best suited for detecting internal volumetric discontinuities in thick steel weldments?

  1. Penetrant testing
  2. Radiographic testing Correct answer.
  3. Visual testing
  4. Magnetic particle testing

Explanation

Radiography transmits penetrating radiation through the part; density or thickness differences along the beam path produce contrast that reveals internal volumetric discontinuities. Surface methods cannot detect buried defects.

Question 3General knowledge of nondestructive testing methods and their applications

Liquid penetrant testing is fundamentally limited because it can detect only discontinuities that are:

  1. larger than the part thickness
  2. filled with gas
  3. located at mid-wall depth
  4. open to the surface being inspected Correct answer.

Explanation

Penetrant enters defects by capillary action and is drawn back out during development, so the defect must have a surface opening. Subsurface defects cannot be reached by the penetrant.

Question 4General knowledge of nondestructive testing methods and their applications

Ultrasonic testing detects discontinuities primarily by interpreting:

  1. the leakage of magnetic flux at the surface
  2. the travel time and amplitude of reflected sound waves Correct answer.
  3. the amount of radiation absorbed
  4. the color of a fluorescent indication

Explanation

Ultrasonic testing introduces high-frequency sound into the part and analyzes echoes; echo arrival time gives depth and echo amplitude relates to reflector size and orientation. Acoustic impedance mismatch at an interface produces the reflection.

Question 5General knowledge of nondestructive testing methods and their applications

Which method is most appropriate for detecting surface-breaking cracks in a nonmagnetic aluminum forging?

  1. Magnetic flux leakage
  2. Radiographic testing
  3. Magnetic particle testing
  4. Penetrant testing Correct answer.

Explanation

Aluminum is nonferromagnetic, so magnetic methods are ineffective; penetrant testing is well suited to surface-breaking defects in nonporous metals. Radiography is poor at detecting tight planar cracks oriented unfavorably.

Question 6General knowledge of nondestructive testing methods and their applications

Acoustic emission testing differs from most other NDT methods in that it detects:

  1. only defects present before the test begins
  2. the reflection of externally introduced sound pulses
  3. energy released by defects while the part is under stress Correct answer.
  4. the absorption of gamma radiation

Explanation

Acoustic emission is a passive technique: it listens for transient elastic waves generated by active sources such as crack growth or fiber breakage during loading. It responds to defect activity rather than static geometry.

Question 7General knowledge of nondestructive testing methods and their applications

In magnetic particle testing, a discontinuity is revealed because it:

  1. changes the electrical conductivity locally
  2. causes flux leakage that attracts and holds particles Correct answer.
  3. emits light when magnetized
  4. absorbs the applied magnetic field completely

Explanation

A discontinuity that interrupts the magnetic flux path produces a local leakage field at the surface, which attracts ferromagnetic particles and forms a visible indication. The indication depends on flux leakage, not on absorption or emission.

Question 8General knowledge of nondestructive testing methods and their applications

A technician must inspect a large cast iron component for near-surface defects and wants a fast, relatively low-cost method that indicates surface and slightly subsurface flaws. Which method fits best?

  1. Acoustic emission testing
  2. Magnetic particle testing Correct answer.
  3. Radiographic testing
  4. Leak testing

Explanation

Cast iron is ferromagnetic, and magnetic particle testing is a fast, economical method that reveals surface and near-surface discontinuities. It is well matched to ferromagnetic castings.

Question 9General knowledge of nondestructive testing methods and their applications

Which characteristic is a principal advantage of ultrasonic testing over radiography for flaw detection in thick sections?

  1. It works equally well on all material types without couplant
  2. It produces a permanent radiographic record
  3. It requires no access to either surface
  4. It is sensitive to planar defects oriented perpendicular to the beam Correct answer.

Explanation

Ultrasonic testing is particularly sensitive to planar discontinuities such as cracks and lack of fusion when the beam is oriented to reflect from them. Radiography is generally less sensitive to tight planar defects aligned with the beam.

Question 10General knowledge of nondestructive testing methods and their applications

The primary purpose of a couplant in contact ultrasonic testing is to:

  1. cool the transducer
  2. increase the electrical conductivity of the part
  3. clean the test surface
  4. transmit sound between the transducer and the part Correct answer.

Explanation

A couplant displaces air at the transducer-part interface, reducing the large acoustic impedance mismatch and allowing sound energy to pass into the part. Without adequate coupling, most energy reflects at the interface.

Question 11General knowledge of nondestructive testing methods and their applications

Which NDT method is best suited to detecting leaks in a sealed pressure vessel?

  1. Eddy current testing
  2. Visual testing
  3. Leak testing Correct answer.
  4. Magnetic particle testing

Explanation

Leak testing is specifically designed to detect and locate the escape of fluid through a barrier, using techniques such as pressure change, tracer gas, or bubble emission. Other methods detect discontinuities but do not directly confirm through-leakage.

Question 12General knowledge of nondestructive testing methods and their applications

A fluorescent penetrant indication appears brighter under ultraviolet light than a visible dye indication under white light primarily because the fluorescent material:

  1. chemically reacts with the defect
  2. has a higher viscosity
  3. re-emits absorbed UV energy as visible light Correct answer.
  4. cools the surface rapidly

Explanation

Fluorescent dyes absorb ultraviolet energy and re-emit it at longer visible wavelengths, producing high contrast against a dark background. This enhances detection of small indications.

Question 13General knowledge of nondestructive testing methods and their applications

Eddy current testing is generally most effective for inspecting:

  1. thin-walled conductive tubing Correct answer.
  2. thick sections of cast steel
  3. ferromagnetic materials with high permeability
  4. nonconductive polymer sheets

Explanation

Eddy currents are confined near the surface by skin effect, making the method well suited to thin conductive products such as heat exchanger tubing. Penetration depth decreases with frequency and conductivity.

Question 14General knowledge of nondestructive testing methods and their applications

In radiographic testing, image contrast between a defect and surrounding material depends primarily on:

  1. differences in radiation absorption along the beam path Correct answer.
  2. the viscosity of the couplant
  3. the color of the film base
  4. the speed of the scanning transducer

Explanation

Radiographic contrast arises because different path lengths or densities absorb differing amounts of radiation, producing intensity differences at the detector. A void of lower density attenuates less than the surrounding metal.

Question 15General knowledge of nondestructive testing methods and their applications

Which method would be least effective for detecting a tight, planar lack-of-fusion defect oriented parallel to the radiation beam in a thick weld?

  1. Ultrasonic testing
  2. Acoustic emission testing under load
  3. Phased array ultrasonic testing
  4. Radiographic testing Correct answer.

Explanation

A tight planar defect aligned with the radiation beam presents little change in path length or absorption, so radiographic contrast is minimal. Ultrasonic methods are more sensitive to such planar reflectors when properly oriented.

Question 16General knowledge of nondestructive testing methods and their applications

The ability of a test method to detect a small discontinuity is generally referred to as its:

  1. sensitivity Correct answer.
  2. reproducibility
  3. reliability
  4. calibration interval

Explanation

Sensitivity describes how small a discontinuity a method or setup can detect under stated conditions. It is distinct from reliability, which concerns consistency of results over repeated tests.

Question 17General knowledge of nondestructive testing methods and their applications

Visual testing is often performed first in an inspection sequence because it:

  1. can reveal gross surface conditions that guide later testing Correct answer.
  2. replaces all other methods
  3. detects internal porosity
  4. requires no lighting

Explanation

Visual testing is inexpensive and can identify surface conditions, geometry, and obvious defects that inform the selection and setup of subsequent methods. It does not replace volumetric or surface-sensitive techniques.

Question 18General knowledge of nondestructive testing methods and their applications

Which of the following best describes the role of a standard reference block in eddy current testing?

  1. It provides a known signal for setting sensitivity and comparing indications Correct answer.
  2. It absorbs excess couplant
  3. It cools the test coil during operation
  4. It magnetizes the part before inspection

Explanation

Reference standards contain known artificial defects or material properties and are used to establish instrument settings and compare signal responses. This supports consistent, repeatable evaluations.

Question 19General knowledge of nondestructive testing methods and their applications

A phased array ultrasonic system offers an advantage over a single-element transducer mainly because it can:

  1. detect only surface defects
  2. operate without any couplant
  3. electronically steer and focus the sound beam Correct answer.
  4. replace radiographic testing entirely

Explanation

Phased array systems excite multiple elements with programmed delays, allowing electronic steering, focusing, and sweeping of the beam. This improves coverage and flexibility compared with a fixed single-element probe.

Question 20General knowledge of nondestructive testing methods and their applications

Which material property most directly determines whether magnetic particle testing can be applied?

  1. Electrical conductivity
  2. Ferromagnetism Correct answer.
  3. Thermal conductivity
  4. Optical transparency

Explanation

Magnetic particle testing requires the part to be ferromagnetic so that it can be magnetized and support flux leakage at discontinuities. Nonferromagnetic materials cannot be inspected this way.

Question 21General knowledge of nondestructive testing methods and their applications

When selecting an NDT method for a production line, which combination of factors most directly governs the choice?

  1. Defect type and location, material properties, and inspection speed requirements Correct answer.
  2. The cost of the test equipment alone
  3. The color of the part and ambient humidity
  4. The operator's seniority and the plant layout

Explanation

Method selection balances the nature and location of expected discontinuities, material properties that enable or preclude a technique, and practical production constraints such as speed and cost. No single factor determines the choice.

Question 22General knowledge of nondestructive testing methods and their applications

Which statement about the skin effect in eddy current testing is correct?

  1. Eddy currents flow only in nonconductive materials
  2. Eddy current density decreases with depth from the surface Correct answer.
  3. Eddy current density is uniform throughout the part
  4. Eddy currents increase indefinitely with depth

Explanation

Eddy current density attenuates exponentially with depth due to the skin effect, with penetration decreasing as frequency, conductivity, or permeability increases. This limits inspection to near-surface regions.

Question 23General knowledge of nondestructive testing methods and their applications

A weld is to be inspected for surface-breaking cracks on a ferromagnetic steel part in a location where the surface is rough and painted. Which method is generally least suitable without surface preparation?

  1. Penetrant testing Correct answer.
  2. Eddy current testing with a surface probe
  3. Magnetic particle testing
  4. Visual testing with magnification

Explanation

Penetrant testing requires a clean, open surface because paint and roughness can block penetrant entry and trap background dye. Magnetic particle testing can tolerate somewhat rougher, painted surfaces better than penetrant methods.

Question 24General knowledge of nondestructive testing methods and their applications

Which method is most appropriate for detecting a through-wall leak path in a small heat exchanger tube that is otherwise inaccessible internally?

  1. Radiographic testing of the tube bundle
  2. Leak testing with a tracer gas Correct answer.
  3. Visual testing of the shell
  4. Magnetic particle testing of the exterior

Explanation

Tracer gas leak testing can detect and locate small through-leaks even in complex or partially inaccessible geometries. It responds directly to leakage rather than to indirect material discontinuities.

Question 25General knowledge of nondestructive testing methods and their applications

Which pair of methods both depend on the presence of a surface-breaking or near-surface discontinuity to produce an indication?

  1. Penetrant testing and magnetic particle testing Correct answer.
  2. Ultrasonic testing and radiography
  3. Radiography and leak testing
  4. Acoustic emission and leak testing

Explanation

Penetrant testing requires a surface opening, and magnetic particle testing responds to surface and near-surface flux leakage. Both are surface-oriented methods, unlike volumetric techniques such as radiography and ultrasound.

Question 26General knowledge of nondestructive testing methods and their applications

Which NDT method relies on the principle that a discontinuity will interrupt the flow of induced electrical currents in a conductive material?

  1. Radiographic testing
  2. Magnetic particle testing
  3. Eddy current testing Correct answer.
  4. Acoustic emission testing

Explanation

Eddy current testing induces currents in a conductive material; a discontinuity alters the impedance of the coil, revealing the defect. Radiography uses radiation attenuation, magnetic particle testing uses magnetic flux leakage, and acoustic emission detects stress waves.

Question 27General knowledge of nondestructive testing methods and their applications

Which of the following is a primary advantage of ultrasonic testing over radiographic testing for detecting planar discontinuities?

  1. Ultrasonic testing requires no access to the test surface.
  2. Ultrasonic testing provides a permanent visual record of the internal structure.
  3. Ultrasonic testing is better at detecting planar discontinuities oriented perpendicular to the beam. Correct answer.
  4. Ultrasonic testing is more effective for detecting volumetric porosity.

Explanation

Ultrasonic testing is highly sensitive to planar defects that reflect sound waves, especially when the beam is perpendicular to the defect. Radiography is generally better for volumetric defects but less effective for tight planar defects unless oriented favorably.

Question 28General knowledge of nondestructive testing methods and their applications

Which NDT method is most suitable for detecting surface-breaking defects in a nonmagnetic material such as aluminum?

  1. Magnetic particle testing
  2. Eddy current testing
  3. Liquid penetrant testing Correct answer.
  4. Radiographic testing

Explanation

Liquid penetrant testing can detect surface-breaking defects in any nonporous material, magnetic or not. Magnetic particle testing is limited to ferromagnetic materials, eddy current is mainly for conductive materials but less effective for surface-breaking defects, and radiography is for internal defects.

Question 29General knowledge of nondestructive testing methods and their applications

What is the main reason a couplant is used in contact ultrasonic testing?

  1. To align the transducer perpendicular to the surface.
  2. To protect the transducer from high temperatures.
  3. To increase the frequency of the ultrasonic wave.
  4. To reduce the acoustic impedance mismatch between the transducer and the test piece. Correct answer.

Explanation

A couplant removes air gaps and provides a medium that better matches the acoustic impedance of the transducer and the test material, allowing sound to enter the part. It does not change frequency, protect against heat, or align the transducer.

Question 30General knowledge of nondestructive testing methods and their applications

In magnetic particle testing, which material property is essential for the test to be effective?

  1. Ferromagnetism Correct answer.
  2. High electrical conductivity
  3. High thermal conductivity
  4. Low density

Explanation

Magnetic particle testing relies on the material being ferromagnetic so that magnetic flux can be induced and will leak at discontinuities. Conductivity, density, and thermal conductivity do not determine suitability for magnetic particle testing.

Question 31General knowledge of nondestructive testing methods and their applications

Which NDT method would be most appropriate for detecting a tight, planar crack oriented parallel to the surface in a thick steel weld?

  1. Visual testing
  2. Ultrasonic testing Correct answer.
  3. Radiographic testing
  4. Liquid penetrant testing

Explanation

Ultrasonic testing with an angle beam can direct sound waves perpendicular to a planar crack, making it highly detectable. Radiography may miss tight planar cracks if they are not oriented favorably, and penetrant and visual testing only detect surface-breaking defects.

Question 32General knowledge of nondestructive testing methods and their applications

In radiographic testing, which factor primarily determines the ability to detect a small discontinuity?

  1. The distance from the source to the film.
  2. The difference in radiation absorption between the defect and the surrounding material. Correct answer.
  3. The type of film developer used.
  4. The speed of the film.

Explanation

Image contrast in radiography depends on the difference in radiation attenuation between the defect and the surrounding material. Developer type, film speed, and source-to-film distance affect image quality but not the fundamental contrast mechanism.

Question 33General knowledge of nondestructive testing methods and their applications

Which NDT method is most effective for detecting leaks in a sealed, evacuated tube?

  1. Magnetic particle testing
  2. Helium leak testing Correct answer.
  3. Radiographic testing
  4. Ultrasonic testing

Explanation

Helium leak testing is highly sensitive for detecting leaks in sealed systems by tracing helium ingress or egress. Ultrasonic testing, radiography, and magnetic particle testing are not primarily leak-detection methods.

Question 34General knowledge of nondestructive testing methods and their applications

What is the primary purpose of a standard reference block in eddy current testing?

  1. To calibrate the equipment and establish a baseline for comparison. Correct answer.
  2. To physically support the test piece during inspection.
  3. To measure the thickness of the test piece.
  4. To provide a source of magnetization.

Explanation

A standard reference block contains known defects or features and is used to calibrate the eddy current system and set acceptance levels. It does not support the part, magnetize it, or directly measure thickness.

Question 35General knowledge of nondestructive testing methods and their applications

Which characteristic of acoustic emission testing makes it particularly suited for continuous monitoring of structures under load?

  1. It can only be used on ferromagnetic materials.
  2. It detects only defects that are actively growing. Correct answer.
  3. It provides a direct image of the defect.
  4. It requires no sensors on the structure.

Explanation

Acoustic emission testing detects transient elastic waves released by active processes such as crack growth, making it ideal for monitoring during loading. It requires sensors, is not limited to ferromagnetic materials, and does not produce a direct image.

Question 36General knowledge of nondestructive testing methods and their applications

Which NDT method is best suited for detecting surface discontinuities in a nonporous, nonconductive material?

  1. Liquid penetrant testing Correct answer.
  2. Magnetic particle testing
  3. Eddy current testing
  4. Ultrasonic testing

Explanation

Liquid penetrant testing can be used on any nonporous material, regardless of conductivity or magnetism, to find surface-breaking defects. Eddy current requires conductivity, magnetic particle requires ferromagnetism, and ultrasonic testing is mainly for internal defects.

Question 37General knowledge of nondestructive testing methods and their applications

What is the main advantage of phased array ultrasonic testing over conventional single-element ultrasonic testing?

  1. It can electronically steer and focus the beam. Correct answer.
  2. It can inspect non-conductive materials.
  3. It does not require a couplant.
  4. It provides a permanent record without additional equipment.

Explanation

Phased array systems use multiple elements that can be pulsed with time delays to steer and focus the beam electronically, allowing rapid scanning of complex geometries. They still require couplant, work on similar materials, and do not inherently provide permanent records.

Question 38General knowledge of nondestructive testing methods and their applications

Which of the following is a primary limitation of visual testing?

  1. It is limited to ferromagnetic materials.
  2. It cannot be used in the field.
  3. It can only detect surface discontinuities. Correct answer.
  4. It requires expensive equipment.

Explanation

Visual testing is limited to surface conditions and cannot detect internal or subsurface defects. It is generally inexpensive, not limited to ferromagnetic materials, and is commonly used in the field.

Question 39General knowledge of nondestructive testing methods and their applications

In eddy current testing, the skin effect causes the eddy currents to concentrate near the surface of the material. What is the practical consequence of this?

  1. The test is only sensitive to surface and near-surface defects. Correct answer.
  2. The test can detect deep internal defects easily.
  3. The test is limited to nonmagnetic materials.
  4. The test requires a coupling medium.

Explanation

The skin effect limits eddy current penetration, so the method is most sensitive to surface and near-surface discontinuities. It does not enhance deep defect detection, require couplant, or restrict to nonmagnetic materials.

Question 40General knowledge of nondestructive testing methods and their applications

Which NDT method would be most effective for detecting a small fatigue crack on the surface of a steel component?

  1. Ultrasonic testing
  2. Acoustic emission testing
  3. Magnetic particle testing Correct answer.
  4. Radiographic testing

Explanation

Magnetic particle testing is highly sensitive to surface and near-surface discontinuities in ferromagnetic materials like steel. Radiography and ultrasonic testing are better for internal defects, and acoustic emission detects active defects but is not a routine surface inspection method.

Question 41General knowledge of nondestructive testing methods and their applications

Which of the following statements about the use of a penetrant in liquid penetrant testing is correct?

  1. The penetrant must be removed from the surface before inspection.
  2. The penetrant should be applied before any pre-cleaning.
  3. The penetrant must be applied to a clean, dry surface. Correct answer.
  4. The penetrant must be electrically conductive.

Explanation

Liquid penetrant testing requires a clean, dry surface so the penetrant can enter surface-breaking defects without contamination. Pre-cleaning is essential, excess penetrant must be removed before development, and conductivity is not required.

Question 42General knowledge of nondestructive testing methods and their applications

What is the primary reason for using a high-energy X-ray source in industrial radiography?

  1. To improve the resolution of the image.
  2. To penetrate thick or dense materials. Correct answer.
  3. To eliminate the need for film.
  4. To reduce the exposure time.

Explanation

High-energy X-rays are used to penetrate thick or dense materials that would absorb lower-energy radiation. While exposure time may be reduced, the primary reason is penetration capability, and film may still be used.

Question 43General knowledge of nondestructive testing methods and their applications

Which NDT method is most appropriate for detecting a through-wall leak path in a small heat exchanger tube that is otherwise inaccessible?

  1. Visual testing
  2. Liquid penetrant testing
  3. Helium leak testing Correct answer.
  4. Magnetic particle testing

Explanation

Helium leak testing is highly sensitive and can detect small leak paths in inaccessible areas by detecting helium that passes through the leak. Penetrant and magnetic particle testing require surface access, and visual testing cannot access the interior.

Question 44General knowledge of nondestructive testing methods and their applications

Which of the following pairs of NDT methods both rely on the detection of discontinuities that are open to the surface?

  1. Liquid penetrant testing and magnetic particle testing Correct answer.
  2. Radiographic testing and ultrasonic testing
  3. Acoustic emission testing and eddy current testing
  4. Visual testing and radiographic testing

Explanation

Liquid penetrant testing and magnetic particle testing are both surface and near-surface inspection methods; penetrant requires surface-breaking defects, and magnetic particle testing can detect surface and slightly subsurface defects in ferromagnetic materials. Radiography and ultrasonic testing are primarily for internal defects.

Question 45General knowledge of nondestructive testing methods and their applications

Which of the following is a key advantage of eddy current testing for tube inspection?

  1. It provides a direct image of the tube wall.
  2. It can detect defects in nonconductive tubes.
  3. It requires no reference standards.
  4. It can inspect tubes from the inside without contact. Correct answer.

Explanation

Eddy current testing can inspect conductive tubes from the inside using a probe that does not require contact, making it fast and effective for heat exchanger tubes. It is limited to conductive materials, does not produce direct images, and requires reference standards.

Question 46General knowledge of nondestructive testing methods and their applications

What is the main purpose of a penetrant remover in liquid penetrant testing?

  1. To increase the sensitivity of the penetrant.
  2. To develop the indication.
  3. To remove excess penetrant from the surface before development. Correct answer.
  4. To clean the defect before penetrant application.

Explanation

After penetrant dwell, excess penetrant must be removed from the surface so that indications from defects are not masked. Pre-cleaning is done before penetrant application, development is a separate step, and remover does not increase sensitivity.

Question 47General knowledge of nondestructive testing methods and their applications

Which NDT method is most suitable for detecting delaminations in a composite laminate?

  1. Eddy current testing
  2. Magnetic particle testing
  3. Ultrasonic testing Correct answer.
  4. Liquid penetrant testing

Explanation

Ultrasonic testing is effective for detecting delaminations and other internal defects in composites because the defects affect sound transmission and reflection. Magnetic particle and eddy current testing require conductive or ferromagnetic materials, and penetrant testing only finds surface-breaking defects.

Question 48General knowledge of nondestructive testing methods and their applications

In magnetic particle testing, what does the term 'flux leakage' refer to?

  1. The electrical current used to magnetize the part.
  2. The particles that remain on the surface after testing.
  3. The magnetic field that escapes from the material at a discontinuity. Correct answer.
  4. The residual magnetism left after magnetization.

Explanation

Flux leakage occurs when magnetic flux exits the surface at a discontinuity, attracting magnetic particles and forming an indication. It is not the magnetizing current, the particles themselves, or residual magnetism.

Question 49General knowledge of nondestructive testing methods and their applications

Which of the following is a primary advantage of radiographic testing over ultrasonic testing?

  1. Requires less safety precautions.
  2. Provides a permanent visual record of the internal structure. Correct answer.
  3. Better detection of planar defects.
  4. More portable equipment.

Explanation

Radiography produces a permanent image (film or digital) that shows internal structure and defects, which is valuable for documentation. Ultrasonic testing is generally better for planar defects, requires safety precautions for radiation, and is often more portable.

Question 50General knowledge of nondestructive testing methods and their applications

Which NDT method would be most effective for detecting a crack on the inside surface of a small-diameter pipe that is accessible only from the outside?

  1. Visual testing with a borescope
  2. Radiographic testing Correct answer.
  3. Ultrasonic testing with a dual-element transducer
  4. Magnetic particle testing

Explanation

Radiographic testing can detect internal defects, including inside-surface cracks, without direct access to the inside surface. Visual testing requires access, magnetic particle is limited to surface/near-surface, and ultrasonic may have difficulty with small-diameter geometry.