NDT Methods: UT, RT, MT, PT, VT

Quick Reference

NDT (Non-Destructive Testing) evaluates materials without causing damage. The six main methods are: VT (Visual Testing), PT (Penetrant Testing), MT (Magnetic Particle Testing), UT (Ultrasonic Testing), RT (Radiographic Testing), and ET (Eddy Current Testing). Advanced techniques include TOFD, Phased Array UT, and Digital Radiography. Each detects different defect types; choose based on material, defect location (surface vs. subsurface), code requirements, and sensitivity needs.

Non-Destructive Testing (NDT)

NDT techniques detect defects in materials and welds without destroying the test piece. This allows inspection of every item (unlike destructive testing, which sacrifices samples) and continued use after testing. In oil and gas piping, NDT is governed primarily by ASME B31.3 (process piping), ASME Section V (examination methods), and ASME Section VIII (pressure vessels).

The terms NDT and NDE (Non-Destructive Examination) are used interchangeably in industry. ASME codes generally use “examination,” while API and ISO standards tend to use “testing” or “inspection.”

Detectable Flaws

Defect Type	Description
Cracks	Surface or subsurface fractures from manufacturing, service, or fatigue
Inclusions	Foreign materials embedded during production
Porosity	Voids or gas pockets weakening the structure
Laminations	Layer separations in composites or rolled products
Weld defects	Incomplete penetration, lack of fusion, undercut, porosity
Corrosion	Material loss from chemical attack
Erosion	Surface wear from abrasive substances
Dimensional variations	Thickness deviations from spec

Who Performs NDT?

NDT technicians require certification from recognized bodies such as ASNT (American Society for Nondestructive Testing). Certification levels (I, II, III) indicate increasing expertise and authority to interpret results.

Third-party inspection agencies (SGS, Lloyd’s, DNV, Bureau Veritas) provide independent NDT services for critical applications where unbiased results are required (oil & gas, aerospace, nuclear, and construction).

NDT vs. Destructive Testing

Aspect	NDT	Destructive Testing
Effect on sample	No damage; item remains usable	Sample destroyed
Sample rate	Can test 100% of production	Sample-based only
Data type	Defect detection and location	Ultimate strength, failure mode
Application	Ongoing inspection, quality control	Design verification, material qualification

Selecting the Right NDT Method

Need	Recommended Method
Internal flaws in thick materials	UT (Ultrasonic)
Internal weld structure	RT (Radiographic)
Surface cracks on ferromagnetic steel	MT (Magnetic Particle)
Surface cracks on non-magnetic materials	PT (Penetrant Testing)
Thin-wall tubing inspection	ET (Eddy Current)
Quick initial assessment	VT (Visual Testing)
Accurate flaw sizing for fitness-for-service	TOFD
Cross-sectional weld imaging	Phased Array UT

Combination Testing

For critical applications, multiple NDT methods are often combined. Example: Visual testing first, followed by UT for volumetric examination, then MT or PT for surface verification. ASME B31.3 Table 341.3.2 specifies which methods apply to each joint type.

Pro Tip

Always verify NDT operator certifications before they start work. ASNT Level II minimum for interpretation of results. Level I technicians can perform tests but cannot sign off on acceptance, and inspectors will reject the documentation.

NDT allows inspection of every item without destroying the test piece, unlike destructive testing which sacrifices samples. The six main methods each detect different defect types: VT for surface defects, PT for surface-breaking cracks on non-magnetic materials, MT for surface and near-surface flaws on ferromagnetic steel, UT for internal flaws, RT for internal weld structure, and ET for thin-wall tubing inspection. Advanced techniques such as TOFD, phased array UT, and digital radiography offer improved accuracy, speed, and permanent records.

The 6 Key NDT Methods

Method	Abbreviation	Detects	Materials
Visual Testing	VT	Surface defects	All
Liquid Penetrant	PT / LPT / DPI	Surface-breaking cracks	Non-porous only
Magnetic Particle	MT / MPI / MPT	Surface and near-surface	Ferromagnetic only
Ultrasonic	UT	Internal and surface	Most metals
Radiographic	RT / RX	Internal structure	Dense materials
Eddy Current	ET / ECT	Surface and subsurface	Conductive only

Visual Testing (VT)

The simplest and cheapest NDT method, and the first step in any inspection regime. Inspectors examine surfaces with the naked eye or magnifying tools to identify visible defects. All welds must undergo VT as a minimum requirement per ASME B31.3 para. 344.2.

Requirement	Details
Standard	ASME Section V, Article 9
Surface preparation	Clean thoroughly before examination
Defect types	Cracks, porosity, undercut, misalignment, corrosion
Limitation	Surface defects only; cannot detect subsurface flaws
Light requirements	Minimum 1000 lux (100 fc) at the examination surface
Follow-up	If defects found, additional NDT methods determine extent

VT Field Tips

Inspect root pass and each weld layer during fabrication, not just the finished weld. Catching defects early avoids costly repairs. Use a calibrated weld gauge to check reinforcement height, undercut depth, and angular misalignment against the WPS and code limits.

Liquid Penetrant Testing (PT)

PT testing detects surface-breaking defects in non-porous materials (metals, plastics, ceramics) using capillary action to draw colored dye into cracks. It is the standard surface examination method for austenitic stainless steel, duplex, nickel alloys, and aluminum where MT cannot be used.

Step	Action
1. Clean	Remove all contaminants from surface
2. Apply penetrant	Spray or brush colored dye onto surface
3. Dwell	Allow time for penetrant to seep into defects (typically 10-30 minutes)
4. Remove excess	Wipe off surface penetrant with solvent/emulsifier
5. Apply developer	White powder draws penetrant out of defects
6. Inspect	Colored indications reveal crack locations

Penetrant types: Type I (fluorescent, viewed under UV-A light, higher sensitivity) and Type II (visible dye, red contrast, used in field conditions). Most piping fabrication shops use Type II visible dye for convenience; Type I fluorescent is specified for critical components such as high-pressure valves and nuclear piping.

Pros	Cons
Simple, low-cost	Surface defects only
Works on non-magnetic materials	Not for porous materials
Detects cracks invisible to naked eye	Requires clean, smooth surface
Portable, no power needed (visible dye)	Temperature sensitive (typically 10-52 deg C)

Field Note

PT test failures often happen not because of defects, but because of poor surface preparation. Grinding marks, oil residue, or paint remnants trap penetrant and create false indications. Clean properly the first time; it saves re-testing.

Magnetic Particle Testing (MT)

MT testing detects surface and near-surface flaws in ferromagnetic materials only (carbon steel, low-alloy steel, ferritic/martensitic stainless steel). The component is magnetized, and magnetic particles accumulate at flux leakage points caused by defects.

Step	Action
1. Magnetize	Apply magnetic field using yoke, prods, or coil
2. Apply particles	Spray dry powder or wet suspension onto surface
3. Inspect	Particles cluster at defect locations, forming visible indications
4. Demagnetize	Remove residual magnetism (required by most specifications)

Magnetization techniques: AC yoke (most common in field), DC prods (deeper penetration but risk arc strikes), and multi-directional units (detect flaws in all orientations in one shot). For weld inspection, two magnetization passes at 90 degrees are typically required to detect defects in both longitudinal and transverse orientations.

Pros	Cons
Detects surface and near-surface defects	Ferromagnetic materials only
Fast, cost-effective	Cannot detect deep internal flaws
Portable equipment available	Component must be demagnetized after test
Works through thin coatings (up to ~50 microns)	Requires two passes for full coverage

MT vs. PT Selection

For carbon steel and low-alloy steel welds, always specify MT over PT. MT is faster, detects near-subsurface flaws that PT misses, and works through light mill scale. Reserve PT for non-magnetic materials (austenitic stainless, duplex, nickel alloys, aluminum) where MT does not work.

Ultrasonic Testing (UT)

UT uses high-frequency sound waves (typically 1-10 MHz) to detect internal flaws and measure wall thickness. A transducer probe sends ultrasonic pulses into the material; reflections from defects or back walls are analyzed to determine flaw location, size, and characteristics.

Step	Action
1. Setup	Apply couplant (gel/oil) and position probe on surface
2. Calibrate	Set range and sensitivity using reference blocks with known reflectors
3. Transmit	Probe sends ultrasonic waves into material
4. Receive	Probe detects reflected waves from defects or back wall
5. Analyze	Software calculates defect location, size, and depth

Conventional UT techniques: Straight beam (0-degree, for thickness measurement and lamination detection) and angle beam (45, 60, or 70 degrees, for weld examination). Angle beam UT is the standard volumetric examination method for butt welds in process piping per ASME B31.3.

Pros	Cons
Detects internal and surface flaws	Requires skilled operator
Accurate depth/size measurement	Surface must be accessible and smooth
Portable, no radiation hazard	Coarse-grained materials challenging (cast SS, Inconel)
Real-time results	Calibration required for each setup change
Single-sided access sufficient	No permanent image record (conventional UT)

Radiographic Testing (RT)

RT uses X-rays or gamma rays to create images of a component’s internal structure. Radiation passes through the material and exposes film or a digital detector on the opposite side. Defects appear as density variations in the image.

Step	Action
1. Setup	Position radiation source on one side, film/detector on opposite
2. Expose	Activate source; radiation penetrates component
3. Process	Develop film or process digital image
4. Interpret	Trained interpreter examines image for defect indications per acceptance criteria

Radiation sources: X-ray tubes (adjustable energy, better image quality, needs power supply) and gamma ray isotopes: Ir-192 (for steel up to ~75 mm) and Co-60 (for thicker sections). Gamma sources are portable and need no electricity, making them standard for field work on pipelines.

Pros	Cons
Permanent record of inspection	Radiation safety requirements (exclusion zones)
Detects internal voids, inclusions, porosity	Access needed to both sides of the component
Good for weld inspection	Slower than UT (film processing time)
Works on most materials	Equipment cost, licensing, and storage
Image is intuitive to interpret	Poor at detecting planar defects oriented parallel to beam

Radiation Safety

RT requires licensed personnel, radiation monitoring (dosimeters), and controlled access zones. Gamma sources remain radioactive permanently and require secure storage, transport licensing, and emergency response procedures. Follow all applicable safety regulations (10 CFR 34 in the US, local equivalents elsewhere).

Common Mistake

Scheduling RT during day shifts when other trades are working nearby. Radiation exclusion zones disrupt everyone’s work. Plan RT for night shifts or weekends for better productivity and fewer safety incidents.

(Source: Peter Smith, Piping Materials Selection and Applications, 2004)

Eddy Current Testing (ET)

Eddy current testing probe

Eddy current testing finds surface and subsurface defects in conductive materials only through electromagnetic induction. A probe coil generates alternating current that induces eddy currents in the test material; defects cause measurable impedance changes.

Step	Action
1. Setup	Position probe coil near material surface
2. Generate	Alternating field induces eddy currents in material
3. Detect	Defects/thickness changes alter impedance
4. Analyze	Signal variations indicate defect location and size

Pros	Cons
No couplant required	Conductive materials only
Fast inspection speed	Limited penetration depth (typically < 5 mm)
Detects small surface cracks	Sensitive to lift-off variations
Excellent for tube/heat exchanger inspection	Interpretation requires experience

ET testing is particularly effective for inspecting heat exchanger tubes, aircraft structures, and thin-wall tubing where rapid, automated inspection is needed. Internal Rotary Inspection Systems (IRIS) combine UT with a rotating mirror for tube inspection where ET sensitivity is insufficient.

Advanced NDT Methods

Advanced techniques offer improved accuracy, permanent digital records, and faster inspection speeds compared to conventional methods.

Time-of-Flight Diffraction (TOFD)

TOFD uses two angled ultrasonic probes (one transmitter, one receiver) positioned on opposite sides of a weld. Instead of measuring reflected pulse amplitude (like conventional UT), TOFD measures the arrival time of diffracted signals from flaw tips. This provides accurate through-wall flaw sizing independent of flaw orientation.

Feature	Details
Standard	ASME Section V, Article 4; EN ISO 10863
Accuracy	Flaw height sizing to +/- 1 mm
Coverage	Full weld volume in a single scan pass
Record	Permanent encoded digital image (D-scan)
Limitation	Dead zones at OD and ID surfaces (supplemented with PAUT or pulse-echo)

Phased Array UT (PAUT)

Phased array probes contain multiple piezoelectric elements (typically 16-64) pulsed with controlled time delays. This allows electronic beam steering, focusing, and sweeping without moving the probe. PAUT produces cross-sectional images (S-scans) that are similar to RT images but without radiation.

Feature	Details
Standard	ASME Section V, Article 4; ASME CC 2235
Probe elements	16 to 128 elements typical
Output	S-scan (sectorial), B-scan (side view), C-scan (plan view)
Speed	3-5x faster than conventional angle beam UT
Acceptance	ASME, API, EN codes accept PAUT as RT alternative

TOFD + PAUT Combination

The most reliable modern approach combines TOFD for accurate flaw sizing with PAUT for full volume coverage and surface zone inspection. This TOFD+PAUT combination is increasingly replacing RT for process piping weld inspection, eliminating radiation hazards and reducing inspection time.

Digital Radiography (DR) and Computed Radiography (CR)

Digital radiography replaces conventional film with flat-panel detectors (DR) or phosphor imaging plates (CR). Benefits include instant image availability, adjustable contrast/brightness, reduced radiation dose, and elimination of chemical film processing. Digital images are stored electronically and can be reviewed remotely.

Automated Ultrasonic Testing (AUT)

AUT uses mechanized scanners to drive ultrasonic probes around a weld, producing encoded and repeatable data. AUT typically combines PAUT, TOFD, and conventional UT channels in a single scan pass. It is the standard inspection method for cross-country pipeline girth welds under API 1104 and is increasingly specified for shop-fabricated pressure vessel and piping welds.

NDT Method Comparison Table

Criterion	VT	PT	MT	UT	RT	ET
Defect location	Surface	Surface only	Surface + near-surface	Internal + surface	Internal	Surface + near-surface
Ferromagnetic steel	Yes	Yes	Yes	Yes	Yes	Yes
Austenitic SS / Nickel	Yes	Yes	No	Difficult	Yes	Yes
Aluminum / Non-ferrous	Yes	Yes	No	Yes	Yes	Yes
Minimum detectable size	~1 mm visible	~1 micron width	~0.5 mm	~1 mm	~2% of wall thickness	~0.5 mm
Subsurface capability	No	No	Shallow (~3 mm)	Full thickness	Full thickness	Shallow (~5 mm)
Permanent record	Photos/reports	Photos	Photos	Digital (PAUT/TOFD)	Film or digital image	Strip chart/digital
Radiation hazard	No	No	No	No	Yes	No
Relative cost	Low	Low	Low-Medium	Medium	Medium-High	Medium
Speed	Fast	Slow (dwell time)	Fast	Medium	Slow	Fast
Personnel level (min.)	VT Level II	PT Level II	MT Level II	UT Level II	RT Level II	ET Level II

NDT Selection Guide by Defect Type

Selecting the right NDT method depends on the defect being sought, the material, and access conditions.

Defect Type	Primary Method	Secondary Method	Notes
Surface cracks (CS/alloy)	MT	VT	MT detects tighter cracks than VT
Surface cracks (SS/nickel)	PT	VT	MT not applicable to non-magnetic alloys
Subsurface cracks	UT (angle beam)	RT	UT better for planar flaws
Porosity	RT	UT	RT more sensitive to scattered porosity
Slag inclusions	RT	UT	Volumetric defects show clearly on RT
Lack of fusion	UT	TOFD/PAUT	Planar defect; RT often misses if parallel to beam
Incomplete penetration	RT	UT	Consistent orientation makes RT effective
Wall thinning / corrosion	UT (straight beam)	ET	UT measures remaining thickness precisely
Laminations in plate	UT (straight beam)	—	RT cannot detect laminations (parallel to beam)
Heat exchanger tube defects	ET	IRIS (UT)	ET is fast and automated for large tube bundles

Personnel Qualification Levels

NDT personnel must be certified per ASNT SNT-TC-1A (employer-based, North America), ASNT CP-189 (central certification), or ISO 9712 (international). Certification is method-specific, so an inspector qualified in UT is not automatically qualified in RT.

Level	Authority	Typical Requirements
Level I	Perform tests under direct supervision of Level II or III; cannot interpret or accept/reject	Classroom training + OJT hours (method-specific); pass general, specific, and practical exams
Level II	Set up equipment, perform tests, interpret results, accept/reject per procedures, train Level I	Additional training + OJT hours beyond Level I; pass all exams; most project specifications require Level II minimum
Level III	Develop procedures, establish techniques, interpret codes, manage NDT programs, train all levels	Extensive experience; pass ASNT Level III Basic and Method exams; typically requires engineering background

Certification Verification

Always verify that NDT personnel hold current, valid certifications for the specific method being performed. Certifications expire (typically every 5 years for ISO 9712, employer-determined for SNT-TC-1A). Expired or wrong-method certifications invalidate all examination results and require re-inspection.

ASME B31.3 Examination Requirements

ASME B31.3 specifies minimum examination requirements based on fluid service category. Project piping specifications and pipe class sheets may increase these minimums but never reduce them.

Examination Extent by Fluid Service

Fluid Service	Butt Welds (RT or UT)	Branch Welds	Socket/Fillet Welds	Hardness Testing
Normal (Category D)	5% random	VT 100%	VT 100%	Per WPS
Normal	5% random	VT 100%	VT 100%	Per WPS
Category M (lethal)	100% RT or UT	100% MT or PT	100% MT or PT	100%
High-Pressure	100% RT or UT	100% MT or PT	100% MT or PT	100%
Severe Cyclic	100% RT or UT	100% MT or PT	100% MT or PT	100%

Typical Project Examination by Pipe Class

In practice, EPC companies define examination percentages in project piping specifications that exceed B31.3 minimums based on pipe class criticality.

Pipe Class Criticality	Butt Weld RT/UT	Socket/Fillet MT or PT	Visual
Utility (water, air)	5-10%	0-10%	100%
Standard process	10-20%	10-20%	100%
High-temperature / HP	100%	100%	100%
Lethal / toxic service	100%	100%	100%
Sour service (NACE)	100%	100%	100% + hardness

Spot vs. Random Examination

“5% random” means the inspector selects 5% of welds from any welder; if a defect is found, the same welder’s additional welds are examined. ASME B31.3 para. 341.3.4 provides the progressive examination rules: a rejected weld triggers examination of two additional welds by the same welder. If either fails, 100% of that welder’s work is examined.

Acceptance Criteria Overview

NDT results are evaluated against acceptance criteria defined in the applicable code. The two most commonly referenced codes for piping and pressure vessels are ASME B31.3 and ASME Section VIII Division 1.

ASME B31.3 (Process Piping)

Examination Method	Acceptance Standard	Key Limits
RT	ASME B31.3 Table 341.3.2; acceptance per ASME Section VIII UW-51	No cracks; porosity limited by wall thickness charts; slag length limited
UT	ASME B31.3 para. 344.6.2; ASME Section V Article 4	Reflectors exceeding reference level evaluated for acceptance
MT	ASME B31.3 para. 344.3.2	Relevant linear indications > 1.5 mm rejected; no cracks
PT	ASME B31.3 para. 344.4.2	Relevant linear indications > 1.5 mm rejected; no cracks
VT	ASME B31.3 Table 341.3.2	Per workmanship standards: undercut depth, reinforcement height, misalignment

Critical Rule

Any crack or crack-like indication, regardless of length, is always rejected under ASME B31.3 for all examination methods. There is no minimum acceptable crack size. Even a 1 mm crack indication requires repair.

Applicable Standards Reference

Standard	Title / Scope
ASME Section V	Nondestructive Examination: methods, techniques, calibration
ASME B31.3	Process Piping: examination requirements by fluid service
ASME Section VIII Div. 1	Pressure Vessels: UW-51 (RT acceptance), UW-52 (spot RT)
ASNT SNT-TC-1A	Personnel Qualification and Certification (employer-based)
ASNT CP-189	Standard for Qualification and Certification (central)
ISO 9712	NDT Personnel Qualification and Certification (international)
ASTM E94	Standard Guide for Radiographic Examination
ASTM E164	Standard Practice for Contact Ultrasonic Testing of Weldments
ASTM E165	Standard Practice for Liquid Penetrant Testing
ASTM E709	Standard Guide for Magnetic Particle Testing
ASTM E1416	Standard Practice for Radioscopic Examination of Weldments
ASTM E2698	Standard Practice for Radiographic Examination Using Digital Detector Arrays
ASTM E2373	Standard Practice for Use of the UT Time-of-Flight Diffraction Technique
EN ISO 10863	Welding: Use of TOFD Technique
EN ISO 13588	Welding: Use of Phased Array Technique
API 1104	Welding of Pipelines: includes AUT acceptance criteria

Practical Tips for NDT in Piping Projects

Plan NDT early in the project schedule. Build NDT hold points into the Inspection and Test Plan (ITP) before fabrication starts. Late NDT planning causes rework, schedule delays, and inflated costs.

Specify methods in the piping specification. Define RT/UT percentages, MT/PT requirements for branch connections and socket welds, and hardness testing requirements for each pipe class. Do not leave it to the fabricator to decide.

Coordinate RT scheduling with other disciplines. Radiation exclusion zones shut down nearby work. Schedule RT during off-hours or weekends. Some projects have switched entirely to PAUT/TOFD to avoid radiation disruptions.

Get surface preparation right. The number one cause of false PT/MT indications is inadequate surface cleaning. This wastes time on unnecessary repairs and re-examination.

Keep calibration blocks traceable. UT reference blocks must have current calibration certificates traceable to national standards. Expired calibration invalidates all examination results obtained with that block.

Document everything. NDT reports, film/digital images, operator certifications, calibration records, and procedure qualifications must be filed in the project data book. Missing documentation is treated the same as missing examination, and the weld must be re-examined.

Frequently Asked Questions

Which NDT method is best for detecting internal weld defects?

For internal defects (lack of fusion, porosity, slag inclusions), radiographic testing (RT) and ultrasonic testing (UT) are the primary methods. RT provides a permanent film record and detects volumetric defects well, but requires radiation safety precautions. UT is faster, more portable, detects planar defects (cracks) better than RT, and doesn't require evacuating the work area. For critical welds in piping, ASME B31.3 typically specifies RT or UT depending on the service class. Many modern projects use TOFD or phased array UT (PAUT) as RT replacements to eliminate radiation hazards while providing superior flaw detection and permanent digital records.

What is the difference between RT and UT for weld inspection?

RT (Radiographic Testing) passes X-rays or gamma rays through the weld to create an image on film or digital detector, and is best for volumetric defects (porosity, slag inclusions, incomplete penetration). UT (Ultrasonic Testing) sends sound waves through the material and analyzes reflections, and is better for planar defects (cracks, lack of fusion). RT provides a visual record that is intuitive to interpret but is slower and requires radiation safety zones. UT is faster, requires single-sided access, and can measure defect depth and height, but requires more operator skill and (in conventional form) does not produce an image. Many modern projects use TOFD or phased array UT as RT replacements, combining the advantages of both.

When is magnetic particle testing (MT) used instead of liquid penetrant testing (PT)?

MT is preferred for ferromagnetic materials (carbon steel, alloy steel, ferritic stainless); it detects both surface and near-surface defects, is faster than PT, and works through thin paint or coatings up to about 50 microns. PT is used for non-magnetic materials (austenitic stainless steel, duplex, aluminum, nickel alloys) where MT cannot work, and for detecting surface-breaking defects only. PT is also used when surface finish quality must be verified. Both methods are typically applied to welds after visual testing (VT). Rule of thumb: if the material is magnetic, use MT; if it is non-magnetic, use PT.

What NDT qualifications do inspectors need?

NDT personnel must be certified to ISO 9712 (international) or ASNT SNT-TC-1A / CP-189 (North American). Certification levels: Level I can perform tests under direct supervision; Level II can set up, perform, and interpret tests independently and accept/reject per written procedures; Level III can develop procedures, train others, interpret codes, and manage NDT programs. Most project specifications require Level II minimum for performing and interpreting inspections. Certification is method-specific, so an inspector must be qualified separately for each NDT method they use. Certifications have defined validity periods and require renewal through re-examination or continued activity documentation.

What percentage of welds must be examined per ASME B31.3?

ASME B31.3 examination percentages depend on fluid service category. Normal fluid service requires a minimum of 5% random RT or UT of butt welds, with 100% VT. Category M (high toxicity/lethal) and severe cyclic conditions require 100% RT or UT of butt welds plus 100% MT or PT of all other welds. High-pressure piping (Chapter IX) also requires 100% volumetric examination. Project specifications often exceed these minimums; for example, specifying 10-20% RT for standard process piping and 100% for high-temperature, sour service, or high-pressure classes. When a random examination reveals a defect, ASME B31.3 para. 341.3.4 requires progressive examination of additional welds by the same welder.