Summary of Non Destructive Testing

Nondestructive testing (NDT) is a collection of examination techniques used to evaluate the integrity of materials, welds, and components without causing damage or requiring removal from service. In engineering investigations — including failure analysis, fitness-for-service evaluations, and forensic investigations, NDT is often the first tool applied: it locates and characterizes flaws before any destructive sampling or code analysis proceeds.

NDT methods are codified in ASME Section V – Nondestructive Examination, which specifies the procedures, personnel qualifications, and acceptance criteria referenced by all other ASME Boiler and Pressure Vessel Code sections. O’Donnell Consulting applies NDT interpretation routinely in pressure vessel assessments, weld evaluations, and root cause failure investigations.

Applications of Nondestructive Testing

NDT serves a broad range of engineering purposes. The nine principal application areas are:

Flaw detection — locating cracks, voids, inclusions, and other discontinuities
Leak detection — identifying pathways for fluid or gas escape
Metrology — measuring dimensions, wall thickness, and coating thickness
Location determination — mapping subsurface features or embedded components
Microstructure characterization — assessing grain structure, heat-affected zones, and phase distribution
Estimation of mechanical properties — inferring hardness, strength, or modulus from indirect measurements
Stress and dynamic response determination — measuring residual stress or vibration behavior
Signature analysis — comparing response patterns to known baselines for anomaly detection
Chemical composition determination — identifying alloy content or contamination

Flaw Detection and Evaluation

Flaw detection is the most common NDT application in pressure equipment and structural components. Before performing any flaw detection examination, the following must be specified:

Which types of flaws are rejectable for the application
The size and orientation of flaws that constitute a rejection criterion
The locations within the component where flaws are critical to structural integrity

Rejection criteria should not be arbitrary. Wherever possible, rejectable flaw parameters should be derived from stress analysis and fracture mechanics calculations — so that rejection is tied to actual structural risk rather than generic code tables alone. This integration of NDT findings with engineering analysis is central to a rigorous fitness-for-service assessment.

NDT Methods

Ultrasonic Testing (UT)

Ultrasonic testing introduces high-frequency sound waves into a material using a transducer. In the pulse-echo technique, reflections (echoes) from internal discontinuities or geometrical surfaces return to a receiver and are displayed as amplitude vs. time-of-flight traces or, in phased array configurations, as cross-sectional images.

UT is the primary method for flaw sizing in fitness-for-service assessments — particularly for pressure vessels and piping where through-wall crack depth is needed to perform fracture mechanics calculations under API 579-1/ASME FFS-1.

Advantages:

High sensitivity — capable of detecting minute cracks and subsurface discontinuities
High penetrating power — suitable for examining thick-walled pressure vessel sections
Quantitative — provides depth and through-wall sizing, not just surface indication
No radiation hazard; suitable for field use

Limitations:

Requires surface access for transducer coupling
Coarse grain structures, inclusions, or dispersed precipitates can scatter sound and complicate interpretation
Complex geometry and unfavorable discontinuity orientation can produce ambiguous echo patterns
Requires trained personnel for reliable interpretation

Radiographic Testing (RT)

Radiographic testing uses X-ray or gamma-ray radiation to project a shadow image of a component onto film or a digital detector. Variations in material density — caused by voids, cracks, inclusions, or thickness changes — produce contrast differences in the radiograph.

RT is widely specified for weld examination in pressure vessel fabrication and is required by ASME Section VIII for certain joint categories and service conditions. In failure investigations, RT can document the internal condition of a component before destructive sectioning begins.

Advantages:

Provides a permanent, reviewable record of internal conditions
Effective for detecting volumetric flaws: porosity, slag inclusions, incomplete fusion
Applicable to a wide range of materials and thicknesses

Limitations:

Radiation safety requirements constrain field use and require exclusion zones
Planar flaws (tight cracks) oriented parallel to the beam may not be detected
Does not provide through-wall position of a flaw (depth information)
Film processing and digital equipment add cost and setup time

Magnetic Particle Testing (MT)

Magnetic particle testing induces a magnetic field in a ferromagnetic material, then applies iron particles (dry powder or wet suspension) to the surface. Where a surface or near-surface discontinuity interrupts the induced field, a leakage field forms above it and the particles accumulate to reveal the flaw’s location, size, and shape.

MT is the most reliable method available for detecting surface-breaking cracks — including very fine, shallow, or contamination-filled cracks that would be missed by visual inspection.

Advantages:

Best method for detecting surface and near-surface cracks in ferromagnetic materials
Detects fine cracks and cracks partially filled with foreign matter
Minimal size or shape limitation on the part being tested
Rapid and relatively low cost

Limitations:

Applicable only to ferromagnetic materials (steel, iron); not usable on austenitic stainless steel or aluminum
Not reliable for deep subsurface flaws
The applied magnetic field must be oriented to intercept the principal plane of the discontinuity — requiring multiple field directions for thorough coverage
Requires demagnetization after inspection in some applications

Eddy Current Testing (ET)

Eddy current testing generates alternating electrical currents (eddy currents) in a conductive material by means of a changing magnetic field from a probe coil. Discontinuities, thickness changes, or conductivity variations disrupt the eddy current flow, producing a measurable change in the probe’s impedance that is detected and displayed.

ET is particularly useful for tubing inspection in heat exchangers and condensers, where it can rapidly screen hundreds of tubes for corrosion, pitting, and cracking from a single end-access probe.

Advantages:

Sensitive to both surface and near-surface discontinuities
No liquid couplant required — unlike UT, ET does not require a coupling medium between the probe and surface.
High inspection speed — well suited to tubing and bar stock screening
Can simultaneously measure coating or cladding thickness

Limitations:

Limited to electrically conductive materials
Depth of penetration is restricted by the skin effect — generally suitable only for near-surface examination
Signal interpretation requires significant operator expertise; permeability variations in ferromagnetic materials complicate response
Not effective on very rough or irregular surfaces

Liquid Penetrant Testing (PT)

Liquid penetrant testing involves applying a visible or fluorescent dye to the test surface and allowing it to dwell, drawing into any surface-open discontinuities by capillary action. Excess penetrant is removed, and a developer is applied to draw the trapped dye back to the surface, producing an enhanced indication of the defect.

PT is applicable to any non-porous material — making it the preferred surface examination method for austenitic stainless steels, aluminum, titanium, and other non-ferromagnetic alloys where MT cannot be used.

Advantages:

Applicable to any non-porous material regardless of magnetic properties
Simple, portable, and low-cost relative to other methods
High sensitivity to tight, fine surface cracks
Results are easy to interpret with minimal training

Limitations:

Detects only surface-breaking discontinuities — no subsurface capability
Surface coatings, scale, or contamination must be removed before examination
Porous materials (castings, sintered parts) produce false indications from background bleed-out
Chemical handling and disposal requirements for penetrant and developer materials

Visual Testing (VT)

Visual testing is the most fundamental NDT method and is required by most codes as a baseline examination before any other technique is applied. Direct VT involves examination of accessible surfaces with the unaided eye or optical aids (magnifiers, borescopes, remote cameras). Indirect VT uses video-assisted equipment to access confined spaces such as vessel interiors, heat exchanger shells, and piping.

In failure analysis investigations, thorough visual examination of fracture surfaces, corrosion patterns, and deformation characteristics often provides more diagnostic information than any other single method.

Advantages:

Applicable to virtually any material and component geometry
Identifies surface corrosion, mechanical damage, deformation, and weld defects directly
Low cost and immediately deployable
Remote VT technology enables internal inspection of vessels and piping without entry

Limitations:

Limited to surface-accessible areas — no subsurface capability
Resolution limited by lighting conditions, surface finish, and inspector acuity
Tight cracks or early-stage corrosion may not be visible without magnification

Selecting the Right NDT Method

No single NDT method is universally superior. Method selection depends on:

Material: Ferromagnetic materials open the option of MT; non-conductive materials limit ET; porous materials preclude PT
Flaw type and orientation: Surface-breaking cracks favor MT or PT; through-wall sizing favors UT; volumetric internal flaws favor RT or UT
Component geometry: Tubing suits ET; complex shapes may require PT or VT; thick sections require UT over RT
Code requirements: ASME, API, and AWS standards specify required methods for particular weld categories, material groups, and service conditions
Investigation objective: Screening for fabrication defects differs from characterizing a service-induced crack for fracture mechanics analysis

In practice, failure investigations and fitness-for-service assessments typically combine methods — for example, UT for flaw sizing and PT or MT for surface crack confirmation — to build a complete picture of component condition before any engineering judgment is rendered.

References

ASM Metals Handbook, Eighth Edition, Volume 10 — Failure Analysis and Prevention. American Society for Metals, 1975.
ASM Metals Handbook, Ninth Edition, Volume 17 — Nondestructive Evaluation and Quality Control. American Society for Metals, 1989.
Glass, Samuel W. III. “A Comprehensive Guide to Nondestructive Evaluation.” Advanced Materials & Processes, ASM International, September 2018, Vol. 176, No. 6.
ASME Boiler and Pressure Vessel Code, Section V — Nondestructive Examination. (2025)

O’Donnell Consulting performs failure analysis services to API, AWS, and ASME code.

Call Tom O’Donnell, PE to Discuss Your Engineering Challenges — (412) 835-5007

Introduction to Non Destructive Testing

Summary of Non Destructive Testing

Applications of Nondestructive Testing

Flaw Detection and Evaluation

NDT Methods

Ultrasonic Testing (UT)

Radiographic Testing (RT)

Magnetic Particle Testing (MT)

Eddy Current Testing (ET)

Liquid Penetrant Testing (PT)

Visual Testing (VT)

Selecting the Right NDT Method

References

Start typing and press enter to search