What is Non-Destructive Testing (NDT) and Why is it Used?

Non-destructive testing (NDT) is a set of methods used to evaluate internal and surface discontinuities, geometric deviations, and material properties of a material or component without compromising its integrity and usability. Because the part remains usable after inspection, NDT is a fundamental tool in quality assurance processes and periodic audits.

NDT (Neural Tracking) methods are used to verify the acceptability of weld seams, castings and forgings, pressure vessels, and pipelines during the manufacturing phase, and to monitor service-related defects such as fatigue cracking, corrosion, and wear during the operation phase. This ensures both production quality and supports structural safety and lifespan assessment.

This information focuses on the methods themselves: what each method does, what type of discontinuity it detects, and under what standard it is applied. The qualification and certification of personnel applying the methods (ISO 9712) is a separate topic and is covered on our own page.

• The part remains usable after inspection — there is no material loss.
• Suitable for manufacturing quality control and periodic inspections during operation.
• It includes different methods for detecting surface and internal (volumetric) discontinuities.
• It is commonly used in welding, casting, forging, and pressure vessel inspections.

VT — Visual Examination (Visual Testing)

Visual inspection (VT) is the most basic and often the first method applied in NDT. The surface being inspected is examined directly or with the help of optical instruments (mirror, magnifying glass, endoscope/boroscope) to evaluate surface defects, geometric irregularities, and visible discontinuities. In weld seams, aspects such as profile, seam geometry, surface cracking, porosity, burn-through, and penetration are checked using this method.

VT provides a rapid, economical, and complementary initial assessment when adequate lighting, suitable viewing angles, and observation distances are ensured. The effectiveness of the method depends on the inspection conditions and equipment being defined in accordance with standards.

• Detection area: visible and apparent defects on the surface, geometric and dimensional deviations.
• Relevant standards: ISO 17637 (visual inspection of welded joints), EN 13018 (general principles of visual inspection)
• It can be applied directly (visually) or indirectly (with a borescope, camera).
• It is used as a complementary first step before other NDT methods.

PT — Liquid Penetrant Testing

Liquid penetrant testing (PT) relies on the principle of capillarity to detect surface-open discontinuities. A low surface tension penetrant liquid is applied to a cleaned surface and penetrates open cracks and voids; after a waiting period, excess penetrant is removed, and the applied developer draws the penetrant from within the discontinuity to the surface, making the indication visible.

PT (Percutaneous Transmission) is applicable to metallic and non-metallic (non-porous) materials, regardless of whether the material is magnetic or not. It only reveals surface-open discontinuities; it cannot detect subsurface defects. The success of the method depends on surface preparation, cleaning, and waiting times.

• Detection area: only surface-exposed discontinuities (cracks, pores, folds)
• Relevant standard: ISO 3452 (penetrant testing — general principles and relevant sections)
• Applicable to a wide range of materials, including non-magnetic materials.
• Not suitable for subsurface defects; surface cleanliness is critical.

MT — Magnetic Particle Testing

Magnetic particle inspection (MT) is a surface and near-surface subsurface inspection method applied exclusively to ferromagnetic materials. When a part is magnetized, discontinuities on or near the surface create a leakage field (leakage flux) in the magnetic flux lines. Magnetic particles (dry powder or liquid suspension) applied to the surface collect in this leakage field, indicating the location and orientation of the discontinuity.

MT offers high sensitivity, especially in the detection of surface cracks, and can also reveal near-surface (subsurface) defects at limited depths depending on the penetrant. Sensitivity decreases rapidly with depth. For discontinuities to be detected, the magnetic field direction must be as perpendicular as possible to the discontinuity; therefore, magnetization is usually applied in different directions.

• Detection area: surface and near-surface discontinuities in ferromagnetic materials.
• Relevant standard: ISO 9934 (magnetic particle testing — general principles and relevant sections)
• This is only suitable for ferromagnetic materials (not suitable for stainless austenitic steels, aluminum, etc.).
• It requires omnidirectional magnetization depending on the direction of the discontinuity.

UT — Ultrasonic Testing

Ultrasonic testing (UT) is based on the principle of high-frequency sound waves propagating through a material and reflecting off discontinuities or the back surface. When an ultrasonic wave is sent through a probe and strikes internal discontinuities or surfaces, it reflects back; by evaluating the arrival time and amplitude of the reflected signal, information about the location, size, and type of discontinuity is obtained. UT is also commonly used for thickness measurement.

Ultrasound (UT) is a powerful method capable of detecting volumetric internal discontinuities (internal cracks, slag, pores, lack of adhesion/fusion) and operating with single-sided access. Phased array ultrasonic (PAUT) technique allows for electronically directed beam scanning with a multi-element probe and wide-sectional scanning in a single pass; TOFD (Time-of-Flight Diffraction) is a complementary technique, particularly for height (sizing) measurement, using waves scattered from the ends of discontinuities.

The accuracy of ultrasonic testing depends on the calibration of the probe and device, the reference blocks, and the sensitivity/range settings. These settings are made according to relevant standards.

• Detection area: internal (volumetric) discontinuities and thickness measurement.
• Relevant standards: ISO 16810 (ultrasonic testing — general principles), ISO 16811 (sensitivity and range adjustment); ISO 13588 for PAUT.
• PAUT: Electronic beam steering with multi-element probe and wide-section scanning in a single pass.
• TOFD: Complementary technique in measuring discontinuity height (sizing).

RT — Radiographic Testing

Radiographic examination (RT) is based on the principle that ionizing radiation, such as X-rays or gamma rays, is absorbed at different rates depending on the thickness and density of the material as it passes through the sample. In the image formed on the film or digital detector placed behind the sample, discontinuities (pores, slag, cracks, lack of penetration) appear as indications with different intensities compared to the surrounding area.

Radiography (RT) stands out for providing a permanent and visual record of internal volumetric defects. In addition to traditional film radiography, digital radiography and computed tomography techniques are also used. Due to radiation safety requirements, the procedure necessitates qualified personnel, appropriate equipment, and regulatory compliance with radiation safety regulations.

• Detection area: visualization of internal volumetric discontinuities (pores, slag, lack of fusion)
• Relevant standards: ISO 17636-1 (X-ray and gamma radiography with film), ISO 17636-2 (digital radiography techniques)
• It provides permanent image recording; film or digital detectors can be used.
• Radiation safety requirements and compliance with regulations are mandatory.

ET — Eddy Current Testing

Eddy current testing (EC) is based on the principle of electromagnetic induction and is only applicable to electrically conductive materials. An alternating current passing through a coil creates eddy currents on the surface of the conductive material; surface and near-surface discontinuities disrupt the flow of these currents, leading to a measurable change in the coil's impedance. This change is evaluated to identify the discontinuity.

Eddy current testing (ET) is used in applications such as detecting surface and near-surface cracks, inspecting heat exchanger and condenser tubes, measuring coating thickness, and conductivity/material differentiation. Its contactless and rapid operation provides an advantage, especially for tubes and thin-section conductive parts. The specifications of the devices and probes used in eddy current testing are defined by relevant standards.

• Detection area: surface and near-surface discontinuities in conductive materials; tube inspection.
• Relevant standard: ISO 15548 (edge current testing equipment — device, probe and system specifications)
• This applies only to electrically conductive materials.
• Heat exchanger/condenser tubes are commonly used in coating thickness and conductivity measurement.

Method Selection: How to Determine the Right NDT Method?

The choice of NDT method cannot be reduced to a single rule; the type and location of the defect being sought, material properties, geometry, access conditions, and acceptance criteria are all considered together. As a general approach, the first distinction is whether the defect is being sought on the surface or internally (volumetrically).

For surface discontinuities, VT, PT, and MT (in ferromagnetic materials), and ET (in conductive materials) are prominent. For internal/volumetric discontinuities, UT and RT are the primary methods; UT offers advantages in one-sided access and sizing, while RT provides permanent image recording and visibility of specific defect types. Whether the material is magnetic or conductive (determining for MT and ET), surface condition, part thickness, and the possibility of two-sided access influence the choice.

In practice, multiple methods are often used together in a complementary manner. The choice of method should be determined within the framework of a technical assessment by a qualified engineer, along with the applicable standard and acceptance level.

• Surface or volume: VT/PT/MT/ET for surface; UT/RT for internal defects.
• Material type: MT requires ferromagnetic, ET requires conductive material.
• Access and geometry: UT may be advantageous for one-way access, while RT may be advantageous for two-way access.
• The type of defect and acceptance criteria guide the procedure according to the relevant standard.

In which sectors are NDT methods used?

Non-destructive testing (NDT) methods are applied in many sectors where structural integrity and weld quality are critical. In the manufacturing industry, welded steel structures, cast and forged parts; and in the pressure equipment sector, pressure vessels, boilers, and pipelines are regularly evaluated using NDT.

In addition, NDT methods are widely used in fields such as petrochemicals, energy, transportation equipment manufacturing, shipbuilding, and general machinery manufacturing, both in production quality control and in periodic inspections during operation. The selection of the method and standard is determined according to sector-specific acceptance criteria and relevant technical specifications.

• Manufacturing: welded constructions, cast and forged parts.
• Pressure equipment: pressure vessels, boilers, pipelines
• Energy, petrochemicals, shipbuilding, and general machinery manufacturing.
• Both production quality control and periodic inspections during operation.

Information Note

This content is for general informational purposes only. AES is NOT an accredited inspection/certification body in this context. The methods and standards described above may vary depending on the specific conditions, materials, and acceptance criteria where the application will be carried out; the content is for informational purposes only and is subject to the technical assessment of an authorized engineer.

AES is an accredited Type A inspection body under TÜRKAK accreditation, specifically in the 6.1 Electrical Installation field. The NDT methods described on this page are for informational purposes only and should not be interpreted as expanding AES's accreditation scope. NDT personnel qualifications (ISO 9712) are discussed on a separate page.

• This content is for informational purposes only and does not constitute binding technical evaluation.
• AES's TÜRKAK accreditation scope is limited to 6.1 Electrical Installations.
• For your specific inspection needs, an assessment by a qualified engineer is essential.

Related Services

• TS EN ISO 9712 NDT Personnel Competency
• Welding Inspection and WPS Proficiency Assessment

Frequently Asked Questions

What is the main difference between NDT (non-destructive testing) methods?

The fundamental difference lies in the location of the discontinuity being sought. VT, PT, and MT focus on surface and near-surface defects; UT and RT detect volumetric defects within the material; and ET is used for surface and near-surface defects and tube inspection in conductive materials. Furthermore, MT is only applicable to ferromagnetic materials, while ET is only applicable to conductive materials.

Which NDT method finds defects inside a material?

The two main methods for detecting internal (volumetric) discontinuities are ultrasonic examination (UT) and radiographic examination (RT). UT evaluates internal defects and thickness by means of sound wave reflection and can be performed with unilateral access; RT provides permanent image recording of internal defects. PT, MT, and VT are surface-focused methods.

What are PAUT and TOFD; how do they differ from traditional UT?

PAUT (phased array ultrasonic) allows for electronically directing the sound beam with a multi-element probe and scanning a wide cross-section in a single pass. TOFD (Time-of-Flight Diffraction) is a complementary technique, particularly for dimensioning defect heights, using waves scattered from discontinuity ends. Both are advanced techniques in ultrasonic inspection and are often used together.

Is every NDT method applicable to every material?

No. Magnetic particle inspection (MT) is only applicable to ferromagnetic materials, and eddy current inspection (ET) is only applicable to electrically conductive materials. Penetrant inspection (PT) can be used on a wide range of materials, including non-porous and non-magnetic materials. The appropriate method is determined by the material type, defect location, and access conditions.

According to which standards are NDT methods applied?

The main general principle standards for the methods are: ISO 17637 and EN 13018 for visual inspection; ISO 3452 for penetrant inspection; ISO 9934 for magnetic particle inspection; ISO 16810 and ISO 16811 for ultrasonic inspection (ISO 13588 for PAUT); ISO 17636 for radiographic inspection; and ISO 15548 for eddy current equipment. In practice, sector and product-specific acceptance standards may also apply.

Does this page also cover NDT personnel qualifications (ISO 9712)?

No. This page only describes the NDT methods themselves for informational purposes. The qualification and certification of NDT personnel (ISO 9712) is a separate topic and is covered on our separate page.