NDT non-destructive testing methods
Non-destructive testing (NDT) constitutes a set of inspection methods that allow evaluating the integrity of a material, component or system without compromising its future usefulness. Applied during manufacturing, assembly and service, they guarantee the quality of the final product by detecting surface and internal discontinuities, property variations or geometric defects. This guide gathers the fundamentals, operating parameters, applicable materials and selection criteria of the main NDT methods used in the manufacturing industry.
Definition and scope
Section titled “Definition and scope”NDT are analysis techniques that examine an object without causing permanent alterations. They are used to verify quality in welds, castings, forgings, composite materials, civil structures and mechanical components. The six most common methods are: visual inspection (VT), liquid penetrant testing (PT), magnetic particle testing (MT), industrial radiography (RT), ultrasonic testing (UT) and eddy current testing (ET). All share the objective of locating discontinuities, measuring thicknesses or characterizing materials with the part intact.
Principle of operation
Section titled “Principle of operation”Each NDT method exploits a different physical phenomenon:
- Visual inspection: Reflection of visible light, magnified with magnifying glasses, borescopes or cameras.
- Liquid penetrant testing: Capillarity to bring discontinuities open to the surface to the surface.
- Magnetic particle testing: Distortion of the magnetic field in ferromagnetic materials, revealed with magnetic powder or ink.
- Radiography: Differential attenuation of ionizing radiation (X-rays or gamma rays) when passing through the material.
- Ultrasonic testing: Reflection and diffraction of high-frequency mechanical waves generated by a piezoelectric transducer.
- Eddy current testing: Variation of the impedance of a coil when inducing eddy currents in conductive materials.
Complementary methods such as infrared thermography (IRT), acoustic emission (AE), acoustic resonance testing (ART) or leak testing (LT) complete the range of options.
Main methods
Section titled “Main methods”| Abbreviation (EN/ES) | Method | Physical principle | Type of detectable defect |
|---|---|---|---|
| VT / IV | Visual inspection | Visible light, optics | Surface, dimensional, surface condition |
| PT / LP | Liquid penetrant testing | Capillarity | Cracks, open porosity, surface folds |
| MT / PM | Magnetic particle testing | Magnetism | Surface and subsurface discontinuities (up to ~3 mm) in ferromagnetic materials |
| RT / RI | Industrial radiography | Ionizing radiation | Internal volumetric defects: pores, inclusions, lack of fusion |
| UT / US | Ultrasonic testing | Mechanical waves | Internal defects, thickness measurement, material characterization |
| ET / CI | Eddy current testing | Electromagnetic induction | Surface and near-surface cracks in conductive materials, conductivity variations |
| IRT / TIR | Infrared thermography | Infrared radiation | Delaminations, lack of insulation, thin walls |
| LT / LF | Leak testing | Fluid or gas penetration | Leaks in pressure vessels, piping |
| AET / EA | Acoustic emission | Elastic waves released by the part | Active defects, crack propagation under load |
| ART | Acoustic resonance analysis | Natural vibration modes | Cracks, cavities, hardness deviations, detached layers |
Applicable materials
Section titled “Applicable materials”| Method | Compatible materials | Limitations |
|---|---|---|
| VT / IV | All | Surface roughness, lighting, accessibility |
| PT / LP | Non-porous metals, glazed ceramics, plastics, composites with sealed surface | Porous or very rough surfaces absorb the penetrant and generate false indications |
| MT / PM | Ferromagnetic materials (carbon steels, ferritic/martensitic stainless steels, cast iron) | Not applicable to non-magnetic materials (aluminum, copper, austenitic steels, titanium, plastics) |
| RT / RI | Steel, aluminum, titanium, nickel alloys, composites (with energy adjustment) | Very dense materials (lead, tungsten) require high-energy sources; practical maximum thickness limitation |
| UT / US | Most metals, ceramic, glass, some plastics and composites | Coarse-grained materials (cast iron, certain stainless steels) attenuate and scatter waves; very complex geometries |
| ET / CI | Electrically conductive materials (metals, graphite) | Penetration depth limited by frequency and conductivity; not applicable to non-conductive materials |
Typical operating parameters
Section titled “Typical operating parameters”The following are the usual working ranges in field conditions. Values may be adjusted according to the reference standard and the particular conditions of the part.
Liquid penetrant testing (PT / LP)
Section titled “Liquid penetrant testing (PT / LP)”| Parameter | Typical value |
|---|---|
| Application temperature | 10 – 50 °C / 50 – 122 °F |
| Penetration time (visible penetrant) | 5 – 30 min |
| Penetration time (fluorescent penetrant) | 10 – 60 min |
| Developing time | 5 – 60 min (visible), 5 – 30 min (fluorescent) |
| Developer layer thickness | ≤ 0.1 mm / 0.004 in (wet) |
| UV-A light intensity (fluorescent) | ≥ 1000 µW/cm² at 38 cm / 15 in |
Magnetic particle testing (MT / PM)
Section titled “Magnetic particle testing (MT / PM)”| Parameter | Typical value |
|---|---|
| Tangential field strength | 2 – 6 kA/m (approximately 25 – 75 Oe) |
| Distance between yoke legs | 150 – 200 mm / 6 – 8 in |
| Magnetizing current (AC yoke) | 500 – 1000 A |
| Electrode spacing (prod technique) | 150 – 250 mm / 6 – 10 in |
| Powder application time | ≥ 2 s of continuous current |
| Background contrast (colored particles) | White or flat black paint; thickness 20 – 50 µm / 0.8 – 2.0 mils |
Industrial radiography (RT / RI)
Section titled “Industrial radiography (RT / RI)”| Parameter | Typical value (X-rays) | Unit |
|---|---|---|
| Tube voltage (steel, 12 mm) | 150 – 200 | kV |
| Tube voltage (steel, 25 mm) | 220 – 300 | kV |
| Tube current | 3 – 10 | mA |
| Source-to-film distance (SFD) | 700 mm / 27.6 in (minimum) | mm / in |
| Exposure time (steel 25 mm, SFD 700 mm, 5 mA) | 3 – 10 | min |
| Film optical density | 1.8 – 4.0 (according to test class) | – |
Ultrasonic testing (UT / US)
Section titled “Ultrasonic testing (UT / US)”| Parameter | Typical value |
|---|---|
| Frequency (fine-grained steel) | 2 – 5 MHz |
| Frequency (cast iron, coarse grain) | 0.5 – 2.25 MHz |
| Frequency (aluminum forging) | 5 – 10 MHz |
| Minimum measurable thickness (steel) | 0.5 mm / 0.02 in (with high-frequency probe) |
| Maximum practical thickness (steel) | Up to 5000 mm / 197 in |
| Probe diameter | 6 – 24 mm / 0.24 – 0.94 in |
| Typical couplant | Gel, water, oil, glycerin |
Visual inspection (VT / IV)
Section titled “Visual inspection (VT / IV)”| Parameter | Typical value |
|---|---|
| Minimum illumination (general inspection) | 500 – 1000 lux |
| Directed illumination (fine crack detection) | ≥ 2000 lux |
| Magnification with magnifying glass | 2× – 10× |
| Borescope camera resolution | ≥ 640 × 480 pixels |
| Minimum viewing angle | 30° (direct), 90° (lateral) |
Tolerances and detection capabilities
Section titled “Tolerances and detection capabilities”| Method | Minimum detectable size (approx.) | Evaluable depth |
|---|---|---|
| VT / IV (without magnification) | ≥ 0.5 mm / 0.02 in (length), ≥ 0.05 mm / 0.002 in (crack width) with good lighting | Surface only |
| VT / IV (with 10×) | Cracks 0.01 mm / 0.0004 in wide | Surface |
| PT / LP (visible) | Cracks width ≥ 0.5 µm / 0.02 mils; length ≥ 0.5 mm / 0.02 in | Only open to surface |
| PT / LP (fluorescent) | Width ≥ 0.1 µm / 0.004 mils | Surface only |
| MT / PM (optimum conditions) | Cracks 0.1 µm wide and 0.5 mm / 0.02 in long | Up to 3 mm / 0.12 in below surface |
| RT / RI (film) | Pores of diameter equivalent to 1–2 % of radiographed thickness | Total (volumetric) |
| UT / US (contact) | Planar discontinuities ≥ 0.5 mm / 0.02 in equivalent diameter | Total, depending on thickness and attenuation |
| ET / CI (surface) | Cracks 0.1 mm / 0.004 in deep and 1 mm / 0.04 in long | Up to ~ 5 mm / 0.2 in in aluminum (low frequencies) |
Advantages and limitations
Section titled “Advantages and limitations”| Method | Advantages | Limitations |
|---|---|---|
| VT / IV | Fast, low cost, no special equipment required | Only visible surface defects; depends on lighting and inspector |
| PT / LP | High sensitivity for open discontinuities; applicable to almost all non-porous materials | Surface must be clean and dry; defects must be open; post-cleaning required; penetrant can be difficult to remove from threads and rough surfaces |
| MT / PM | Immediate detection of fine cracks; indicates shallow subsurface defects; real-time results | Only ferromagnetic materials; needs post-demagnetization; field orientation influences detectability |
| RT / RI | Permanent record; detects internal defects and thickness variations; inspects hard-to-reach areas | Ionizing radiation (health risk); high equipment and consumable cost; low ability to detect cracks perpendicular to the beam |
| UT / US | Very deep; high accuracy in thickness measurement; portable; immediate results; safe | Requires couplant; very rough or cylindrical surfaces make coupling difficult; expert interpretation; dead zones under the surface |
| ET / CI | High speed; no direct contact (in many configurations); sensitive to small changes in conductivity and permeability | Only conductive materials; limited depth; very sensitive to geometry and temperature |
Method selection guide
Section titled “Method selection guide”The choice of the appropriate non-destructive test is based on the nature of the material, the type of defect expected, its location and the inspection conditions. The following matrix guides the initial decision.
| Situation | Recommended method(s) |
|---|---|
| Non-magnetic materials (aluminum, austenitic stainless steel, titanium) with surface defects | PT / LP, ET / CI |
| Ferromagnetic materials with surface or shallow defects | MT / PM, PT / LP |
| Internal volumetric defects in welds, castings or forged parts | RT / RI, UT / US |
| Precise thickness measurement in corroded piping | UT / US |
| Rapid inspection of extensive surfaces (coatings, delaminations) | VT / IV, IRT / TIR |
| Leak detection in pressurized systems | LT / LF |
| Detection of active cracks in service (online monitoring) | AET / EA |
| 100 % verification of small parts suspected of hardness variations or microcracks | ART (acoustic resonance) |
| Requirement for permanent documentary record | RT / RI, advanced UT with digital storage (TOFD, phased array) |
| Limited access to one side of the part | UT / US, RT / RI |
| Tight budget and basic personnel training | VT / IV, PT / LP, MT / PM |
Industrial applications
Section titled “Industrial applications”NDT are implemented in virtually all sectors:
- Aerospace: Inspection of turbine blades, fuselages, composites (UT, RT, ET).
- Automotive: Chassis welds, castings, engine components (MT, PT).
- Petrochemical and energy: Piping, pressure vessels, storage tanks (UT, RT, MT, LT).
- Steel construction and shipbuilding: Welded joints in load-bearing structures (UT, RT, MT).
- Capital goods manufacturing: Shafts, gears, bearings (ET, UT, ART).
- Railway: Wheel axles, rails (UT, MT).
Equipment and consumables
Section titled “Equipment and consumables”| Method | Typical equipment | Consumables |
|---|---|---|
| VT / IV | Magnifying glasses, flexible/rigid borescopes, digital camera, lux meter, mirrors, flashlights | Batteries, isopropyl alcohol for cleaning |
| PT / LP | Penetrant kit (cleaner, penetrant, developer), UV-A lamp (fluorescent), lux meter | Penetrant (visible red or fluorescent), developer (wet/dry), non-woven wipes |
| MT / PM | Electromagnetic yoke, bench magnetizer, UV-A lamp, magnetic field meter, white light meter | Dry magnetic powder or suspension (black, fluorescent), contrast paint, demagnetizer |
| RT / RI | X-ray tube, isotopic source (Ir-192, Co-60), radiographic film, intensifying screen, densitometer, personal dosimeter | Film, developing solutions, fixer, image quality indicators (IQI), lead shielding |
| UT / US | Ultrasonic flaw detector, probes (straight, angle, phased array), coaxial cable, calibration block | Couplant (gel, water, oil) |
| ET / CI | Eddy current generator, coils (absolute, differential), calibration standard parts | Calibrated tape, conductivity standards |
Reference standards
Section titled “Reference standards”Correct application of NDT requires following standardized procedures. The main international references that govern these methods include:
- ISO 9712: Qualification and certification of NDT personnel.
- ASME BPVC Section V: Articles for NDT examination in pressure vessels and boilers.
- ASTM E1444: Practice for magnetic particle testing.
- ASTM E1417: Practice for liquid penetrant testing.
- ASTM E1742: Practice for industrial radiography with film.
- ISO 17635: General rules for non-destructive examination of welded joints.
- ISO 5817: Quality levels for imperfections in welded joints (applicable to NDT). These standards establish acceptance criteria, test parameters and minimum equipment requirements.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the minimum working temperature allowed for liquid penetrant testing according to most standards?
Section titled “What is the minimum working temperature allowed for liquid penetrant testing according to most standards?”The temperature of the part and the penetrant must be maintained between 10 °C / 50 °F and 50 °C / 122 °F, although special penetrants exist for extended ranges.
What maximum thickness of steel can a conventional ultrasonic equipment inspect in a single scan?
Section titled “What maximum thickness of steel can a conventional ultrasonic equipment inspect in a single scan?”With low-frequency probes (2 MHz) and good attenuation, it is possible to inspect up to 5000 mm / 197 in of rolled steel.
What radiation dose does an operator receive during a typical radiograph of a 25 mm thick weld?
Section titled “What radiation dose does an operator receive during a typical radiograph of a 25 mm thick weld?”The annual occupational dose is limited by law (generally 20 mSv/year). In a well-planned individual exposure, the dose received by the operator, positioned behind barriers, can be less than 0.01 mSv.
What minimum crack size can a visual inspection with a 10× magnifying glass detect under high illumination conditions?
Section titled “What minimum crack size can a visual inspection with a 10× magnifying glass detect under high illumination conditions?”With 2000 lux illumination and 10×, cracks of approximately 0.01 mm / 0.0004 in width and ≥ 0.2 mm / 0.008 in length can be discerned.
How long must the penetrant remain on the surface to ensure defect detection in stainless steel?
Section titled “How long must the penetrant remain on the surface to ensure defect detection in stainless steel?”The standard penetration time for visible penetrants in stainless steel is 15 min; for fluorescent penetrants, 30 min to 60 min is recommended, depending on temperature.
What frequency is typically used in ultrasonic testing to inspect aluminum forgings?
Section titled “What frequency is typically used in ultrasonic testing to inspect aluminum forgings?”For fine-grained aluminum forgings, frequencies from 5 MHz to 10 MHz are used, which allows detecting discontinuities of 0.5 mm / 0.02 in equivalent diameter.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/ndt-non-destructive-testing-d_314.html
- efunda.com: https://www.efunda.com/designstandards/gdt/introduction.cfm