NDT methods compared: UT, MT, PT, RT, VT, ET

NDT methods comparison table

Method	Looks at	What it detects	Materials	Typical uses
UT, ultrasonic	Volume	Internal flaws such as lack of fusion, cracks and laminations, plus thickness	Most metals and many other materials that carry sound	Welds, plate, forgings, corrosion and wall thickness surveys
MT, magnetic particle	Surface and near surface	Cracks, laps and seams that interrupt the magnetic field	Ferromagnetic metals only	Welds, castings, forgings, shafts, in-service crack checks
PT, liquid penetrant	Surface only, open to the surface	Cracks, porosity and laps that are open to the surface	Nonporous materials, magnetic or not	Stainless and aluminum welds, machined parts, castings
RT, radiographic	Volume	Internal voids, inclusions and some planar flaws, as an image	Metals and many other materials	Welds, castings, pipelines
VT, visual	Surface	Surface condition, weld profile, misalignment, corrosion, leaks	Any material you can see	Nearly every job, often the first check
ET, eddy current	Surface and shallow subsurface	Cracks, wall loss and material or coating changes	Conductive materials only	Heat exchanger tubes, aircraft skins, conductivity checks

Which Level II exam covers each method on Angle Beam

Method	Free 10-question check	Related guides
UT	UT General check	UT General exam topics, UT practice test
MT	MT General check	MT practice routine, MT field direction
PT	PT General check	PT Level II exam guide, PT practice by topic
RT, VT, ET	Not covered yet	Requirements by method lists the ASNT route for RT and VT

Each check covers the General exam for its method only. The Specific exams are not covered. Every answer in a check is explained.

Ultrasonic testing (UT)

UT sends high-frequency sound into a part and listens for what comes back. A reflection from inside the part means something is there that is not the back wall: a flaw, or a change in geometry. Timing the echo gives the depth, and the echo height gives a clue to the size.

• Detects: internal discontinuities such as lack of fusion, cracks, laminations and inclusions, and it measures thickness. It is the method that tells you where a flaw sits in depth.
• Works on: most metals and many other materials that carry sound well, including welds when you use angle beams.
• Typical uses: weld inspection, plate and forging checks, corrosion mapping and thickness readings on pipe and vessels.
• Pros: one-sided access, no radiation, and depth information that other methods cannot give.
• Limits: it needs couplant and a trained hand, coarse-grained or rough material can scatter the sound, and orientation matters because a flat flaw reflects best when the beam meets it square on.
• Exam on this site: the UT General exam, with a free 10-question check.

The UT exam is heavy on theory and math. Start with how ultrasonic testing works, then work through the UT formulas guide. The calculators let you check your hand work: the skip distance calculator, the Snell's law calculator, the decibel calculator and the UT beam calculator. The UT outline change guide covers the date ASNT's General topics change.

Magnetic particle testing (MT)

MT magnetizes a part and applies fine magnetic particles. A flaw that interrupts the field makes it leak out of the surface, and the particles gather there as a visible line.

• Detects: surface and slightly subsurface cracks, laps and seams. The flaw shows best when it sits across the field, so direction is the whole game.
• Works on: ferromagnetic metals such as carbon and low-alloy steel. It does not work on austenitic stainless steel, aluminum or copper.
• Typical uses: weld toes, castings, forgings, shafts and in-service crack checks on steel structures.
• Pros: fast, sensitive to tight surface cracks, and the result is easy to see and photograph.
• Limits: only ferromagnetic material, the surface needs to be reasonably clean, and you usually have to check in two directions to find flaws at any angle. Coatings can reduce sensitivity.
• Exam on this site: the MT General exam, with a free 10-question check.

Read how magnetic particle testing works first. Then the MT field direction and technique guide, the MT practice routine and the MT amperage calculator. The MT General outline changes for exams after 15 December 2026, covered in the MT outline change guide.

Liquid penetrant testing (PT)

PT puts a dyed or fluorescent liquid on a clean surface. The liquid soaks into any opening, the excess is removed, and a developer draws the trapped liquid back out so the flaw shows as a bleed-out.

• Detects: discontinuities that are open to the surface, such as cracks, porosity and laps. A flaw that is closed, smeared over or buried will not show.
• Works on: nonporous materials, magnetic or not. That makes it the usual choice for stainless steel, aluminum and nonferrous parts where MT does not apply.
• Typical uses: welds on stainless and aluminum, machined parts, castings and aerospace components.
• Pros: simple equipment, works on complex shapes, and sensitive to fine surface openings.
• Limits: surface-open flaws only, cleaning is critical because contamination blocks the opening, and rough or porous surfaces give false indications. Temperature and dwell time matter.
• Exam on this site: the PT General exam, with a free 10-question check.

Read how liquid penetrant testing works. Then the PT Level II exam guide, PT practice by topic and PT process control and indications.

Radiographic testing (RT)

RT passes X-rays or gamma rays through a part onto film or a digital detector. Thicker or denser spots absorb more, so internal voids and some inclusions show up as differences in darkness on the image.

• Detects: internal voids, porosity, slag and other volume flaws. Flat flaws such as cracks show only when the beam runs close to their plane.
• Works on: metals and many other materials, within the limits of the source and thickness.
• Typical uses: pipe and vessel welds, castings and pipeline girth welds.
• Pros: a permanent image you can review, and a clear picture of volume flaws.
• Limits: radiation safety rules and exclusion zones, access to both sides of the part, and poor sensitivity to tight planar flaws that sit across the beam.
• Exam on this site: not covered yet. ASNT lists 12 training days and 120 days of RT experience for Level II, the same as UT, in our requirements guide.

Visual testing (VT)

VT is looking, done properly. A qualified inspector checks surface condition, weld profile, alignment, corrosion and leaks, with the right lighting, distance and gauges.

• Detects: surface discontinuities and conditions you can see directly or through an aid such as a mirror, borescope or magnifier.
• Works on: any material you can see.
• Typical uses: almost every inspection starts with it, including weld fit-up, final weld checks and structural walkdowns.
• Pros: the cheapest and quickest method, and it often finds the problem before any other method is needed.
• Limits: it sees only what the light and your eye can reach, and it depends heavily on the inspector's skill and the surface condition.
• Exam on this site: not covered yet. ASNT lists 4 training days and 30 days of VT experience for Level II.

Eddy current testing (ET)

ET passes alternating current through a coil near a conductive part. The coil induces circulating currents in the metal, and a flaw, wall loss or change in material alters those currents, which the instrument reads as a signal change.

• Detects: surface and shallow subsurface cracks, wall loss in tubing, and changes in conductivity or coating thickness.
• Works on: conductive materials, magnetic or not. Depth of reach is shallow, and it falls as frequency rises.
• Typical uses: heat exchanger and condenser tube inspection, aircraft skin and fastener-hole checks, and conductivity sorting.
• Pros: fast, needs no couplant and can run without touching the surface, and works well in automated tube scanning.
• Limits: conductive material only, limited depth, signals take training to read, and the geometry of the part affects the response.
• Exam on this site: not covered yet.

Want the study order for UT, MT and PT in one place? Use the ASNT Level 2 study guide.

How to choose between methods

Ask three questions. First, where could the flaw be: on the surface, just under it, or inside the volume? Second, what is the part made of: if it is steel you can use MT, and if it is stainless or aluminum you use PT or ET instead. Third, what do you need to know: depth and size (UT), a picture of internal flaws (RT), or a surface answer fast (VT). Many jobs use two methods together, such as PT on a stainless weld surface and UT for its volume.

Methods also differ in how long they take to certify. Which NDT method to certify in first compares the routes, and NDT Level II requirements by method lists the training and experience days.

Start with the free 10-question check

Take the free 10-question check for UT General and see which topics cost you points. It needs no signup and every answer is explained. The full course adds timed practice exams, a lesson for each published topic and a study plan built from your exam date. Start the free 10-question check.

Working toward MT or PT instead? Take the free MT check or the PT check.

Sources

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• ASNT NDT Level II Certification, ASNT Certification Services.
• ASNT NDT Level II Examinations, ASNT Certification Services.