Non-Destructive Testing: Which Method Finds Which Defect

Each NDT method responds to a different physical property and therefore finds a different family of defect. Specifying the wrong one does not simply weaken the inspection - it produces a clean report on material containing exactly the flaw you were worried about.

No single method sees everything

Non-destructive testing is often written into a specification as a single line — "material shall be NDT tested" — as though the methods were interchangeable. They are not. Each technique responds to a different physical property, and therefore finds a different family of discontinuity. Choosing the wrong one does not simply weaken the inspection; it produces a clean report on material that contains exactly the defect you were worried about.

The five methods used routinely on pipes, plates, bars and fittings are ultrasonic testing, radiography, magnetic particle inspection, dye penetrant inspection and eddy current testing. Here is what each one is genuinely good at.

Ultrasonic Testing (UT)

A transducer injects high-frequency sound into the material and listens for reflections. Anything with an acoustic impedance mismatch — a void, a lamination, a crack, a slag inclusion — returns an echo, and the time of flight gives its depth.

• Finds: internal planar defects, laminations, lack of fusion, wall thinning, inclusions
• Best for: thick sections, plate laminations, seam inspection on welded pipe, in-service wall thickness surveys
• Limitations: requires a couplant and good surface access; needs a skilled operator; defects oriented parallel to the beam can be missed; coarse-grained austenitic material scatters the signal

UT is the workhorse for pipe and plate. Automated ultrasonic testing on modern welded pipe mills inspects the full length of every seam as it is produced.

Radiographic Testing (RT)

X-rays or gamma rays pass through the material onto a film or digital detector. Regions of lower density — porosity, slag, cracks, incomplete penetration — absorb less radiation and appear darker on the image.

• Finds: volumetric defects — porosity, slag inclusions, incomplete penetration, undercut
• Best for: butt welds where a permanent visual record is required; code-mandated weld examination
• Limitations: poor at detecting tight planar cracks lying perpendicular to the beam; radiation safety requires exclusion zones; slower and more costly than UT; access to both sides usually needed

RT and UT are complementary rather than alternatives. RT sees volume; UT sees planes. Critical welds are frequently examined by both.

If you specify only radiography on a weld whose likely failure mode is lack of side-wall fusion, you have specified the one method least likely to find it.

Magnetic Particle Inspection (MPI / MT)

The component is magnetised and fine ferrous particles are applied. A surface or near-surface discontinuity distorts the magnetic field and creates a leakage flux that attracts and holds the particles, drawing a visible indication.

• Finds: surface and slightly subsurface cracks, laps, seams
• Best for: forgings, castings, weld caps, bar and billet surface inspection; fast screening of large areas
• Limitations: ferromagnetic materials only — useless on austenitic stainless, aluminium and titanium; may require demagnetisation afterwards; detects only to a few millimetres depth

Dye Penetrant Inspection (DPI / PT)

A low-viscosity coloured or fluorescent penetrant is applied and drawn into any surface-breaking discontinuity by capillary action. Excess is removed, a developer is applied, and the penetrant bleeds back out to form a visible indication.

• Finds: surface-breaking cracks and porosity only
• Best for: non-magnetic materials — austenitic stainless, aluminium, titanium, nickel alloys; weld root and cap inspection; simple, portable, inexpensive
• Limitations: the defect must break the surface; the surface must be clean and non-porous; blind to anything subsurface

DPI is the standard surface examination for stainless and titanium fabrication, where MPI cannot be used.

Eddy Current Testing (ECT)

An alternating current coil induces eddy currents in a conductive material. Defects disturb those currents and change the coil impedance, which the instrument detects.

• Finds: surface and near-surface cracks, pitting, wall loss; also sorts alloys and verifies heat treatment condition
• Best for: heat exchanger tube inspection, thin-wall tubing, high-speed inline inspection during manufacture, non-ferrous material
• Limitations: conductive materials only; limited penetration depth; geometry-sensitive and requires careful calibration against reference standards

Choosing by defect type

Positive Material Identification deserves its own mention

PMI is not strictly a defect-detection method, but it prevents the most damaging error in metal supply: the wrong grade reaching site. A portable XRF or optical emission analyser reads the elemental composition in seconds and confirms that the pipe stencilled P22 really is P22 and not P11.

Grade mix-ups occur in stockyards, during cutting operations and after re-stencilling. A carbon steel elbow installed in an alloy steel line will not fail on commissioning — it will fail two years later at temperature. PMI on every despatch is cheap insurance and is standard practice at Sarovar Tube Industries.

Specifying NDT sensibly

State the method, the acceptance standard and the extent of coverage. "UT per ASTM A578 Level B, 100% of plate area" is a specification. "NDT to be carried out" is not. Confirm who witnesses the examination, whether reports must accompany the certificate, and whether the operator must hold a specific qualification such as ASNT Level II. Getting these details onto the purchase order avoids the far more expensive alternative of arguing about them at goods inward.