Nondestructive Testing (NDT)
Also: NDT, NDI, NDE, non-destructive testing, nondestructive inspection, dye penetrant inspection, liquid penetrant testing, FPI, magnetic particle inspection, MPI, ultrasonic testing, UT, radiographic testing, X-ray inspection, eddy current testing, ASTM E1417, ASTM E1444
NDT finds cracks, porosity and inclusions without damaging the part, by penetrant, magnetic particle, ultrasonic, X-ray or eddy current inspection.
186 shops tagged NDT among 344 inspection and testing companies in the Noramark directory
Nondestructive testing looks for flaws in a part without cutting it up. Two methods find surface cracks. Liquid penetrant (PT) draws a fluorescent or red dye into surface-breaking cracks, laps and porosity; after the excess is removed, a developer pulls it back out as a visible indication. ASTM E1417 is the governing practice, with penetrant sensitivity levels from 1/2 to 4, and it works on any non-porous material. Magnetic particle inspection (MT or MPI) magnetizes a ferromagnetic part and flows fine iron particles over it, usually fluorescent in a wet bath; they gather where a crack at or just below the surface breaks the field. ASTM E1444 governs it. On steel it is faster and more sensitive to tight cracks than penetrant, but it only works on magnetic material, and the part is demagnetized afterwards.
Three methods look inside. Ultrasonic testing (UT) sends high-frequency sound into the part and reads echoes from flaws and the back wall; it finds laminations, inclusions and cracks in forgings, bar, plate and welds, and it measures wall thickness. Phased-array (PAUT) and time-of-flight diffraction (TOFD) techniques image welds. Radiography (RT) passes X-rays or gamma rays through the part onto film or a digital detector and shows porosity, shrinkage, inclusions and lack of fusion in castings and welds; computed tomography (CT) does the same in 3D. Eddy current testing (ET) induces currents in a conductive part with a coil and detects the change a crack or a conductivity shift causes; it checks bolt holes, tubing and heat-treat condition quickly, with no couplant or chemicals.
An NDT result is only as good as the inspector and the procedure. Inspectors are qualified to a written practice under ASNT SNT-TC-1A or ANSI/ASNT CP-189, or NAS 410 in aerospace, at three levels. A Level I performs tests under supervision. A Level II sets up and calibrates equipment, performs the test, and interprets and reports results against the procedure. A Level III writes and approves procedures, selects techniques and qualifies the others. Aerospace primes require NDT sources to hold Nadcap accreditation for each method they perform.
NDT reports indications; the drawing, the spec or the code decides whether they are acceptable. The acceptance criteria (no linear indications, for example, or porosity up to a stated severity level against ASTM E446 reference radiographs for steel castings or E155 for aluminum castings) belong on the drawing or PO, with the method, the sensitivity and the areas to inspect.
At a glance
| Typical tolerances | For NDT this is detection capability, set by reference standards rather than a dimension. Fluorescent penetrant at sensitivity level 3 or 4 on a clean, etched surface reliably finds tight surface cracks on the order of 1-2 mm (0.04-0.08") long, and often smaller. Magnetic particle finds similar cracks at the surface or up to about 1-2 mm below it. Ultrasonic sensitivity is calibrated on flat-bottom holes or notches in a reference block of the same material. Radiographic sensitivity is proved with image quality indicators, commonly to 2% of section thickness. Eddy current is calibrated on EDM notches of a stated depth. The spec and class called out set each of these. |
|---|---|
| Size limits | Penetrant and magnetic particle lines at contract labs take parts up to about 1-3 m (3-10 ft); portable kits, spray cans and electromagnetic yokes handle larger parts, welds and field work. Ultrasonic immersion tanks take bar, forgings and plate a few metres long, and contact UT goes anywhere. Industrial X-ray tubes up to about 450 kV penetrate steel to roughly 75-100 mm (3-4"); iridium-192 gamma sources reach about 75 mm (3") and cobalt-60 about 200 mm (8"); thicker sections need linear accelerators. Radiography needs a shielded bay, or a cleared exclusion zone on site. |
| Surface finish | NDT leaves the part as it found it, apart from penetrant residue, which is cleaned off, and residual magnetism after MT, which is removed to the level the spec sets. Surface condition limits sensitivity. Penetrant needs a clean, dry, open surface: machining, grinding and blasting smear soft metals such as aluminum and titanium over cracks, so aerospace procedures etch the surface before penetrant. Magnetic particle tolerates thin coatings. Contact ultrasonic needs a smooth, flat surface for the probe, roughly 6.3 µm Ra (250 µin) or better, so weld ripple and scale are ground off. Radiography is indifferent to finish, though surface irregularities show on the image. |
| Lead time | Penetrant and magnetic particle at a contract lab: same day to 3 business days. Ultrasonic and radiography: 2-5 business days, longer for new parts that need technique sheets, reference standards or Level III approval of a procedure. Field inspections are scheduled around the work and, for radiography, around site access. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Penetrant works on any non-porous material: aluminum, titanium, stainless, nickel alloys, steel, copper alloys, ceramics and many plastics; unsealed powder metal and porous castings bleed and mask indications. Magnetic particle works only on ferromagnetic materials: carbon, alloy and tool steels, martensitic and precipitation-hardening stainless (410, 420, 440C, 17-4 PH) and cast iron, and not on austenitic stainless (304, 316), aluminum, titanium, copper or most nickel alloys. Ultrasonic works on most metals and many composites, but coarse-grained castings and austenitic welds scatter the sound. Radiography works on everything, limited by thickness and density. Eddy current needs an electrically conductive material.
What drives the cost
- Method: penetrant and magnetic particle are cheapest per part; immersion UT, radiography and CT cost the most
- Part size and coverage: the number of radiographic shots or views, the scan area, and how many zones are inspected
- Technique development: procedures, technique sheets and reference standards for a new part, approved by a Level III
- Sensitivity and spec: aerospace levels and classes take more preparation and time than commercial ones
- Surface preparation: etching before penetrant, grinding welds for UT, cleaning and demagnetizing
- Sampling: 100% inspection versus a sample per lot
- Documentation and accreditation: signed reports, film or digital archives, Nadcap, customer source approval
When to use it
- Castings: radiography for internal porosity and shrinkage, penetrant or magnetic particle for surface cracks
- Forgings, bar and plate: ultrasonic for internal flaws, magnetic particle or penetrant for surface defects
- Welds under a code or customer spec (AWS D1.1, ASME, aerospace): visual plus MT or PT, and UT or RT where required
- Flight-critical and pressure-retaining parts after machining and heat treatment
- Checking for grinding cracks, quench cracks and other heat-treat cracks
- In-service inspection for fatigue cracks, and sorting material by conductivity or heat-treat condition (eddy current)
When not to
- Dimensional verification: that is CMM or gauge inspection
- Confirming alloy or chemistry: use positive material identification (XRF or spark OES) and the mill cert
- Measuring strength or hardness: use hardness testing and mechanical test coupons
- Magnetic particle on aluminum, titanium or austenitic stainless: use penetrant or eddy current
- Radiography for tight planar cracks that are not aligned with the beam: use UT, MT or PT
- As a substitute for process control: NDT finds defects, it does not prevent them
Design tips
- Call out method, spec, sensitivity and acceptance on the drawing: "FPI per ASTM E1417, Type I, Sensitivity Level 3, 100%; no linear indications permitted".
- For magnetic particle, name the spec and technique: "MPI per ASTM E1444, wet fluorescent, continuous method, 100%", with acceptance criteria.
- Define acceptance criteria, including zones where different criteria apply; NDT reports indications, and the drawing decides what is rejectable.
- Sequence it: final NDT after heat treatment and machining, and before shot peening, plating or painting, which can close or hide cracks.
- Require inspectors qualified to NAS 410 for aerospace or SNT-TC-1A for industrial work, and a Nadcap-accredited source when the customer does.
- Design for inspectability: flat, accessible surfaces for UT probes, weld joints that can be radiographed or scanned, and casting zones with stated radiographic grades.
- Order reference standards and technique approval early on a new part; they are often the long-lead item.
Frequently asked questions
- What are the main types of NDT?
- The five common methods are liquid penetrant (PT), magnetic particle (MT), ultrasonic (UT), radiographic (RT) and eddy current (ET), plus visual inspection (VT), which comes first. PT and MT find surface cracks; UT and RT find internal flaws; ET finds surface and near-surface cracks in conductive metals and sorts material.
- What is the difference between dye penetrant and magnetic particle inspection?
- Penetrant works on any non-porous material but only finds flaws open to the surface. Magnetic particle works only on magnetic steels and irons, but it is faster, less sensitive to surface cleanliness, and also finds cracks just below the surface. On steel, MT is usually preferred; on aluminum, titanium and austenitic stainless, penetrant is the choice.
- What does ASNT Level II mean?
- A Level II inspector is qualified to set up and calibrate equipment, perform the test, and interpret and report results against a procedure, and to guide Level I trainees. A Level III writes and approves procedures and qualifies others. The employer certifies inspectors under a written practice to SNT-TC-1A or CP-189, ASNT also certifies Level III inspectors directly, and aerospace uses NAS 410.
- What is Nadcap NDT accreditation?
- Nadcap is an industry audit program for aerospace special processes, run by the Performance Review Institute. A Nadcap-accredited NDT source has passed an audit of its procedures, personnel qualification, equipment control and records for each method. Most aerospace primes require it for NDT on their parts.
- Can you use magnetic particle inspection on stainless steel?
- Only on the magnetic grades: martensitic (410, 420, 440C) and precipitation-hardening (17-4 PH, 15-5 PH) stainless. Austenitic 304 and 316 are not magnetic enough; inspect them with penetrant or eddy current.
