Find the flaw.
Keep the part flying.
Non-Destructive Testing lets you inspect an aircraft component for hidden defects without ever taking it apart. Walk five inspection stations, run live scan simulators and calculators, clear 50 quiz questions, and earn certification stamps on your way to Certified Inspector.
Five inspection stations
Overview of NDT
NDT vs destructive testing, the method family, merits & limits, visual inspection.
Surface NDE
Liquid penetrant & magnetic particle testing for surface-breaking flaws.
Thermography & Eddy Current
Infrared imaging and induced-current testing of conductive parts.
Ultrasonic & Acoustic Emission
Pulse-echo, A/B/C-scan, phased array, TOFD and passive AE monitoring.
Radiography
X-ray imaging, film characteristics, exposure, safety and fluoroscopy.
Method comparison matrix
| Method | Surface flaws | Sub-surface | Any material | Cost | Portability |
|---|---|---|---|---|---|
| VT · Visual | some | no | yes | low | high |
| PT · Penetrant | excellent | no | non-porous | low | high |
| MT · Magnetic | excellent | near-surface | ferrous | low | med |
| ET · Eddy current | good | near-surface | conductive | med | high |
| UT · Ultrasonic | good | excellent | most | med | high |
| RT · Radiography | fair | excellent | yes | high | low |
Field notes — NDT on the flight line
A Engine borescope
Turbine blades are inspected in place through access ports with a borescope — aided visual testing that catches cracks and burns without pulling the engine.
B Airframe fastener holes
Eddy current probes sweep rivet holes for fatigue cracks hidden under the skin — no disassembly, no paint stripping, quick pass/fail on the impedance plane.
C Composite panels
Ultrasonic C-scans and thermography map disbonds and impact damage in carbon-fibre control surfaces that give no visible sign on the outside.
D Weld & casting integrity
Radiography certifies critical welds and cast fittings, giving a permanent volumetric image that stays on file for the airworthiness record.
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Overview of NDT
Non-Destructive Testing (NDT) finds flaws and characterises materials without impairing usefulness — so the part stays in service. Destructive (mechanical) tests measure properties directly but destroy the specimen.
Side by side
| Aspect | NDT | Destructive |
|---|---|---|
| Part after test | usable | destroyed |
| Coverage | 100% of parts | samples only |
| Output | flaw detection | direct properties |
| Use | screening, in-service | design data |
Every NDT method passes some energy — light, dye, magnetic flux, eddy currents, heat, sound or radiation — into the part and reads how a defect disturbs it.
Methods split into surface techniques (flaws at/near the surface) and volumetric techniques (right through the part).
Surface
VT, PT, MT, ET
Volumetric
UT, RT
Thermal
IRT
Acoustic
AE
Each method exploits a physical property: light reflection (VT), capillarity (PT), ferromagnetism (MT), induction & conductivity (ET), elasticity & sound velocity (UT), radiation attenuation (RT), heat flow (IRT).
Surface methods (PT, MT) are cheap and sensitive to surface cracks but blind to depth; volumetric methods (UT, RT) see inside but cost more and need skill or safety control. MT needs ferrous metal; ET needs a conductor; PT needs a non-porous surface.
Visual Testing (VT) is the oldest, cheapest and most-used method, and the basis of all the others.
- Unaided: the naked eye with good lighting and viewing angle.
- Aided: magnifiers, mirrors, borescopes/endoscopes, CCTV and measuring tools reach inside engines & structures.
- Needs adequate lighting (≥ 500 lux), a clean surface and a trained, rested inspector.
Every method reads a physical property of the material. Choosing a method means matching the property the part actually has.
| Property | Method that uses it | Needs |
|---|---|---|
| Light reflectance | Visual (VT) | accessible, lit surface |
| Capillarity / wetting | Penetrant (PT) | non-porous surface |
| Ferromagnetism | Magnetic particle (MT) | ferrous material |
| Electrical conductivity | Eddy current (ET) | conductor |
| Elasticity / sound velocity | Ultrasonic (UT) | couplant, access |
| Radiation attenuation (density & Z) | Radiography (RT) | two-sided access, safety |
| Thermal conductivity / emissivity | Thermography (IRT) | a heat difference |
Ask three questions first: what material? (ferrous, conductive, porous), where is the flaw? (surface vs internal), and what access do you have? Those answers usually pick the method for you.
Visual testing is extended by optical aids that reach, magnify or record what the naked eye cannot.
VT still needs the basics right: a clean surface, adequate lighting (≥ 500 lux, up to ~1000 lux for critical work), a sensible viewing angle and distance, and a trained, rested inspector. It is the cheapest method — and the foundation the others build on.
NDT grew from simple tap testing and visual checks into a family of precise, physics-based methods. Today it underpins safety, quality and cost control in aerospace, power, oil & gas, rail, automotive and civil structures.
It is used at three stages:
Manufacturing
screen raw material & welds
Assembly
verify joints & fits
In-service
find fatigue & corrosion
A single undetected fatigue crack in an airframe or turbine disc can be catastrophic. NDT lets us keep parts in service safely and retire them only when a real, measured flaw demands it.
A discontinuity is any break in the normal structure of a material. It becomes a defect only when it exceeds the acceptance criteria for that part. NDT detects discontinuities; the code decides which are defects.
| Origin | Typical discontinuities |
|---|---|
| Inherent (casting) | porosity, shrinkage, inclusions, cold shuts |
| Processing (weld/forming) | cracks, lack of fusion, laps, seams, laminations |
| Service | fatigue cracks, corrosion, creep, wear, hydrogen damage |
Where a flaw sits decides the method: surface-breaking → VT/PT/MT/ET; sub-surface/internal → UT/RT. Orientation also matters — UT and MT are far more sensitive to flaws that face the beam or lie across the field.
Reliable results need more than an instrument. Every inspection follows a controlled procedure and is carried out by qualified personnel to a recognised scheme (e.g. ASNT SNT-TC-1A, ISO 9712).
| Step | Purpose |
|---|---|
| Pre-inspection | method choice, procedure, calibration, surface prep |
| Inspection | apply energy, acquire indications |
| Interpretation | real flaw vs non-relevant / false indication |
| Evaluation | size & compare to acceptance criteria |
| Reporting | document, mark, disposition (accept / reject / repair) |
No method is perfect. The Probability of Detection (POD) depends on flaw size, method, procedure and inspector skill — which is why calibration, coverage and training are mandatory.
🧪 Unit I Quiz
Surface NDE Methods
PT uses capillary action: a dye is drawn into a surface-breaking flaw, excess is removed, and a developer draws the trapped dye back out as a visible indication far wider than the crack. Try it live:
Clean
dry surface
Penetrant
dwell
Remove
wipe excess
Develop
draw out
Inspect
bleed-out
| Class | Options |
|---|---|
| By visibility | Visible (colour-contrast, red, white light) · Fluorescent (UV-A, more sensitive) |
| By removal | Water-washable · post-emulsifiable · solvent-removable |
| Developer | Dry powder · aqueous · non-aqueous (solvent) — a contrasting blotter |
Good penetrants need low surface tension and viscosity, strong capillarity and wetting, good visibility and clean removability.
Simple, low-cost, portable, works on almost any non-porous material and complex shapes. But only surface-breaking flaws are found, the surface must be clean and non-porous, and the chemicals are messy.
For ferromagnetic parts: magnetise the part, and a flaw distorts the field, pushing flux out of the surface (a flux-leakage field). Fine iron particles gather at the leakage and outline the flaw.
A flaw shows best when it lies across the field. So the part is magnetised in two directions (circular, then longitudinal) to catch flaws of all orientations.
After testing, parts retain residual magnetism that can attract debris or disturb instruments. Demagnetisation applies an alternating field of steadily decreasing amplitude, randomising the magnetic domains back toward zero.
MT sensitivity depends on getting flux across the flaw. Different set-ups create the field in different directions.
| Technique | Field | Finds |
|---|---|---|
| Yoke (electromagnet) | longitudinal, portable | transverse surface cracks |
| Prods | local circular | cracks between the prods |
| Head shot (current through part) | circular | longitudinal flaws |
| Central conductor | circular in bore | ID/OD flaws of tubes & rings |
| Coil / solenoid | longitudinal | transverse flaws |
Inspection media: dry powder (hot/rough parts) or a wet bath (finer particles, better sensitivity); visible or fluorescent particles viewed under UV-A. Continuous application (field on while particles flow) is most sensitive.
Equipment: handheld yokes, portable prod kits, and bench wet-horizontal machines (head/tail stock + coil) with UV lamps.
A sharp, tightly-held particle build-up marks a real flaw. Non-relevant indications come from geometry, section changes or magnetic writing. Always assess against the acceptance standard — then demagnetise.
A liquid-penetrant test is a fixed sequence. Skipping or rushing a step (especially cleaning or dwell) directly loses sensitivity.
| Stage | Key control |
|---|---|
| Pre-clean | remove all contaminants; dry fully |
| Penetrant dwell | 5–30 min so dye enters fine flaws |
| Excess removal | remove surface dye only — do not over-wash |
| Developer | thin, even layer; correct development time |
| Inspection | correct light; note size, type & location |
| Post-clean | remove chemicals to prevent corrosion |
Penetrant systems are graded by sensitivity level (from Level ½ low up to Level 4 ultra-high). Higher sensitivity fluorescent systems find tighter cracks but demand cleaner surfaces and darker viewing conditions.
Under-cleaning leaves flaws blocked so dye can’t enter; over-washing strips dye back out of real flaws. Both cause missed indications.
MT sensitivity depends on getting the right field strength in the right direction. Too little field = weak indications; too much = confusing background "furring".
| Current | Behaviour | Best for |
|---|---|---|
| DC / rectified | deep penetration | sub-surface flaws |
| AC | concentrates at surface (skin effect) | surface flaws, fine cracks |
| Half-wave DC | deep + mobile dry powder | rough / large parts |
Continuous method (particles applied while the field is on) is the most sensitive and standard. The residual method relies on retained magnetism and suits only high-retentivity steels.
Not every particle build-up or bleed-out is a defect. Distinguishing relevant, non-relevant and false indications is the core skill of a surface inspector.
| Indication | Meaning |
|---|---|
| Relevant | caused by a real discontinuity — evaluate it |
| Non-relevant | from geometry: threads, keyways, section changes, press fits |
| False | not caused by leakage/flaw: dirt, lint, over-wash, magnetic writing |
A sharp, tightly-held, repeatable indication that reappears after re-testing is treated as real until proven otherwise. Record location, length, orientation & type, then accept / reject per the acceptance code — and remember to demagnetize after MT.
🧪 Unit II Quiz
Thermography & Eddy Current Testing
Thermography maps surface temperature. A sub-surface defect changes heat flow, so it shows as a hot or cold spot when the part is heated and viewed with an IR camera.
| Approach | How it works |
|---|---|
| Contact | Liquid crystals / thermal paints change colour with temperature |
| Non-contact | IR cameras sense emitted radiation; active = apply a heat pulse and watch it dissipate |
Fast, non-contact, full-field — ideal for composites, disbonds and electrical surveys. But mainly near-surface, affected by emissivity/reflections, and needs a temperature difference.
An AC coil induces circulating eddy currents in a conductive part. A flaw, or a change in conductivity/thickness, disturbs them and changes the coil’s impedance, which the instrument reads.
Eddy currents are strongest at the surface and weaken with depth (the skin effect), so ET is mainly a surface/near-surface method. Explore it:
- Higher frequency → shallower penetration, better surface sensitivity.
- Probes: surface (pencil), encircling (bar/tube), internal bobbin (tubes).
- Arrangements: absolute, differential, reflection — read on an impedance plane.
ET also measures conductivity, coating thickness and sorts alloys — not just cracks.
Passive thermography watches a part that is already hot or cold in service (motors, bearings, electrical joints). Active thermography adds a controlled heat stimulus and watches how it flows.
Shiny metal has low emissivity and reflects surroundings, faking hot/cold spots. Dull, high-emissivity surfaces (or a matt coating) read far more reliably. Always account for reflections and ambient temperature.
Eddy-current results are read on the impedance plane — a plot of coil resistance vs inductive reactance. Different influences move the signal in different, recognisable directions.
| Effect | Signal on impedance plane |
|---|---|
| A crack | a repeatable loop at a characteristic angle |
| Lift-off (probe gap) | large swing — nulled out by phase rotation |
| Conductivity change / alloy | moves along the conductivity curve |
| Coating / paint thickness | measured from the lift-off direction |
Applications: surface-crack detection, tube inspection (heat-exchangers, boilers) with bobbin probes, conductivity & heat-treat sorting, coating-thickness gauging, and rivet-hole/fastener inspection in aircraft skins.
Fast, no couplant, no consumables, easily automated and great on tubing. But it only works on conductors, is mostly near-surface (skin effect), and the impedance signals need a skilled reader and reference standards.
Every object above absolute zero emits infrared radiation; the amount rises steeply with temperature (Stefan–Boltzmann) and shifts with wavelength (Planck / Wien). Thermography turns this radiation into a temperature map.
| Element | Role |
|---|---|
| IR detector | photon (cooled, sensitive) or microbolometer (uncooled, rugged) |
| Optics & filters | focus IR; select 3–5 µm (MWIR) or 8–14 µm (LWIR) band |
| Processor / display | build the false-colour thermogram, log sequences |
| Emissivity setting | corrects raw radiance to true temperature |
Shiny metals have low emissivity and mirror their surroundings, faking hot/cold spots. Apply a matt coating or set emissivity correctly, and account for ambient reflection & the atmosphere between camera and part.
An alternating current in the probe coil creates a changing magnetic field. By Faraday’s & Lenz’s laws, this induces circulating eddy currents in any nearby conductor, which in turn oppose the coil field and change its impedance.
The standard depth of penetration δ = 1/√(π·f·µ·σ). Currents fall to ~37% of surface value at one δ, so ET is inherently a surface / near-surface method — exactly why frequency is the main tuning knob (see the skin-depth tool in 3.3).
A crack, lift-off and a conductivity change each move the signal in a different direction on the impedance plane. Rotating phase lets the inspector separate the flaw from the lift-off nuisance.
Choosing the probe and connection arrangement tailors ET to the job — from a single surface crack to a full heat-exchanger tube bundle.
| Probe / mode | Where used |
|---|---|
| Surface / pencil | local crack detection on skins & fastener holes |
| Encircling coil | bars, wires & tube OD scanned at speed |
| Internal bobbin | in-service heat-exchanger & boiler tubing |
| Array (ECA) | fast wide-area coverage with C-scan imaging |
ET also sorts alloys & heat-treat by conductivity, gauges non-conductive coating thickness (from lift-off) and inspects around rivets/fasteners in aircraft skins without removing paint.
🧪 Unit III Quiz
Ultrasonic Testing & Acoustic Emission
A transducer (piezoelectric crystal) sends ultrasound (0.5–25 MHz) into the part. Echoes from a flaw or the back wall return to the transducer; flaw depth comes from the time of flight and the known sound velocity.
| Term | Meaning |
|---|---|
| Transmission | Separate Tx/Rx; a flaw reduces the through-signal |
| Pulse-echo | One probe sends & receives — the most common |
| Straight beam | Normal to surface — laminations, thickness |
| Angle beam | Injected at an angle — welds |
Acoustic Emission (AE) — a passive method
As a material is stressed, growing cracks release tiny stress waves. Sensors detect and locate these emissions in real time, monitoring active defects rather than static ones.
Hits/counts, amplitude, energy, rise time, duration and source location. Used for proof-testing pressure vessels, monitoring bridges, pipelines and aircraft, and leak detection.
A UT transducer uses a piezoelectric crystal that turns electrical pulses into sound and echoes back into voltage. The beam has a near field (noisy interference zone) and a spreading far field; frequency and crystal size set the resolution and penetration trade-off.
| Set-up | How it works |
|---|---|
| Contact | probe on the part with a thin couplant film |
| Immersion | part & probe in water — water is the couplant, fast scanning |
| Through-transmission | separate Tx/Rx on opposite faces; a flaw drops the signal |
Couplant (gel, oil, water) bridges the air gap because sound barely crosses air. Instruments are calibrated on reference blocks with known holes/notches, building a DAC (distance-amplitude correction) curve so equal flaws read equally at any depth.
Higher frequency → shorter wavelength → finds smaller flaws and better resolution, but attenuates faster and penetrates less. Lower frequency penetrates thick/attenuating material but blurs small defects.
Ultrasound travels as different wave modes, and the way it bends at an interface is what lets us make angle-beam probes for welds.
| Mode | Motion | Note |
|---|---|---|
| Longitudinal | particles vibrate along travel | fastest; used straight-beam |
| Shear (transverse) | particles vibrate across travel | ~half the velocity; angle beams |
| Surface (Rayleigh) | travels along the surface | near-surface flaws |
| Lamb / plate | whole thin plate oscillates | thin sheet & tube inspection |
At a boundary, sound reflects and refracts; the refracted angle follows Snell’s law (sinθ₁/v₁ = sinθ₂/v₂). By choosing a wedge angle we mode-convert into a pure shear wave at 45°, 60° or 70° for weld scanning.
Reflection strength depends on the impedance mismatch (Z = ρ·v) across the interface. A crack backed by air reflects almost 100% of the sound — which is why even tight flaws show up.
The same echo data can be presented three ways, each answering a different question about the flaw.
| Display | Axes | Answers |
|---|---|---|
| A-scan | amplitude vs time | how big & how deep is this echo? |
| B-scan | depth vs scan position | a side cross-section — flaw profile |
| C-scan | plan view, colour = depth/amp | a top-down flaw map over an area |
Depth & sizing are only as good as the calibration. Set range & velocity on a reference block and build a DAC/TCG curve so equal reflectors read equally at any depth before trusting any scan.
Acoustic emission is passive — it listens for stress waves that the growing flaw itself releases under load. The chain is: sensor → pre-amp → filter → threshold → feature extraction → source location.
| AE parameter | What it tells you |
|---|---|
| Hits / counts | how much activity is occurring |
| Amplitude | severity / energy of an event |
| Rise time & duration | source type & wave shape |
| Arrival-time difference | position of the source (triangulation) |
The Kaiser effect: no significant new AE until the previous maximum load is exceeded. When emission does restart below that load (the Felicity effect), it warns of a structurally significant, growing defect — key in pressure-vessel proof testing.
🧪 Unit IV Quiz
Radiography (RT)
Radiation passes through the part onto film/detector. A flaw (less material) lets more radiation through, recording as a darker image — a permanent volumetric picture.
X-rays interact mainly by the photoelectric effect (low energy) and Compton scattering (higher energy), which govern attenuation and contrast.
Sharpness improves with a small source, a large source-to-film distance and the part close to the film (less geometric unsharpness). Intensity falls with the square of distance — the basis of radiation safety:
Filters (thin metal at the source) absorb soft scatter to improve contrast; screens (lead/fluorescent) next to the film intensify the image and cut scatter, shortening exposure.
| Property | Meaning |
|---|---|
| Density | Degree of film blackening (set by exposure) |
| Contrast | Density difference — high contrast makes flaws stand out |
| Speed | How quickly the film responds (fast = grainier) |
| Graininess | Grain texture — fine grain = better definition |
- Penetrameters (IQIs): step/wire/hole gauges that prove the image quality achieved.
- Exposure charts: relate thickness to the kV / mA-min needed.
- Radiographic equivalence: factors to adapt exposure between materials.
- Film-less: computed radiography (phosphor plates) and digital detector arrays — instant, storable images.
Fluoroscopy gives a real-time moving X-ray image; xeroradiography is an older electrostatic method with edge enhancement.
X-rays and gamma rays are ionising. RT demands controlled areas, shielding, dosimetry, and distance/time controls with trained personnel — the method’s principal limitation.
Radiographs need a penetrating source. X-ray tubes are electrically switchable (kV sets penetration, mA·min sets exposure). Gamma sources are radioactive isotopes — no power needed, very portable, but always "on".
| Source | Energy / use | Half-life |
|---|---|---|
| X-ray tube | adjustable kV; thin–medium sections | n/a (switchable) |
| Iridium-192 | steel up to ~75 mm; most common isotope | ≈ 74 days |
| Cobalt-60 | thick steel (up to ~200 mm) | ≈ 5.3 years |
| Caesium-137 | medium sections | ≈ 30 years |
Exposure charts relate material thickness and kV to the exposure needed; radiographic equivalence factors adapt an exposure from one material to another.
Protect with the three levers: time (work quickly), distance (inverse-square law — step back), and shielding (lead, concrete). Use controlled/barriered areas, warning signs, personal dosimeters, survey meters and trained radiographers. This safety burden is RT’s biggest limitation.
In an X-ray tube, electrons are boiled off a filament, accelerated through a high voltage (kV) and slammed into a target. Two processes make the X-rays:
| Control | Effect |
|---|---|
| kV (tube voltage) | penetrating power / beam hardness |
| mA (tube current) | beam intensity → exposure rate |
| Time | total exposure (mA × min) |
| Focal-spot size | geometric sharpness (smaller = sharper) |
Gamma sources (Ir-192, Co-60, Cs-137) give fixed-energy radiation from radioactive decay — portable and power-free, but always "on" and decaying with their half-life.
Both X-rays and gamma rays are ionising and hazardous. Controlled areas, shielding, dosimetry and the inverse-square law (see 5.2 / 5.5) are mandatory — ALARA at all times.
Accessories shape the beam and the image to raise quality and cut exposure.
| Accessory | Job |
|---|---|
| Filter (at source) | absorbs soft, scattered rays → cleaner contrast |
| Lead screen (at film) | intensifies image & absorbs scatter → shorter exposure |
| Fluorescent screen | converts X-rays to light → much faster, coarser image |
| Masking / collimation | limits the beam to the area of interest |
The radiographic equivalence factor lets an exposure worked out for one material be adapted to another (e.g. steel vs aluminium vs titanium) by scaling for how strongly each absorbs radiation.
A penetrameter / IQI (wire, hole or step type) is placed in the beam. Being able to resolve its known small features on the radiograph proves the achieved sensitivity meets the code.
Modern radiography increasingly replaces wet film with electronic detectors — faster, storable and easier to enhance.
| Method | How it works |
|---|---|
| Computed Radiography (CR) | reusable phosphor plate scanned by laser into a digital image |
| Digital Radiography (DR) | flat-panel detector gives an instant digital image |
| Fluoroscopy | real-time moving X-ray image on a screen — dynamic inspection |
| Xeroradiography | older electrostatic (selenium) plate with strong edge enhancement |
Digital detectors (DR/DDA) now dominate high-volume work: no darkroom, immediate results, and images that can be shared, measured and stored indefinitely.
🧪 Unit V Quiz
Inspector's toolbox
NDT glossary
Common defect types
| Defect | Where it forms | Best-suited methods |
|---|---|---|
| Surface crack | Fatigue, grinding, machining | PT (any), MT (ferrous), ET (conductive) |
| Porosity | Welds & castings (trapped gas) | RT, UT |
| Lack of fusion | Weld interfaces | UT (angle beam), RT |
| Lamination | Rolled plate & bar | UT (straight beam) |
| Inclusion | Foreign matter in the melt | RT, UT |
| Disbond / delamination | Composites & bonded joints | UT C-scan, thermography, tap test |
| Corrosion / wall loss | In-service surfaces & tubing | UT thickness, ET, RT |
Method selector
ASNT SNT-TC-1A sets personnel qualification levels (I, II, III). ASME Section V and AWS D1.1 govern procedures & acceptance in industry, while aerospace leans on NAS 410 and OEM specs. The exam tests the physics; the workplace tests the paperwork too.
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NDT Inspection Bay · Debrief
24MBAV31 · Non-Destructive Testing · Siddy Learning System
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NDT Process Visualizer
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