4.4 Radiographic Testing (RT) and Ultrasonic Testing (UT)
Key Takeaways
- RT and UT are volumetric methods — they find internal flaws; RT excels at volumetric flaws (porosity, slag), UT at planar flaws (cracks, LOF)
- RT prints less-dense flaws darker (porosity, slag, cracks) but tungsten inclusions print lighter; an IQI/penetrameter proves sensitivity (commonly 2-2T)
- Gamma sources: Ir-192 (~74-day half-life, moderate thickness) and Co-60 (~5.27-year half-life, thick sections)
- UT uses 1–6 MHz pulse-echo sound with a couplant; angle-beam (45/60/70°) inspects welds via first-leg/second-leg V-path, calibrated on an IIW block with a DAC curve
- RT = permanent record + radiation hazard + both-side access; UT = depth/size info + one-side access + no radiation but very operator-dependent
- ET is an electromagnetic surface/near-surface method for conductive materials (incl. non-ferromagnetic) — no couplant, ideal for tubing
Volumetric NDE: Radiographic Testing (RT)
Unlike VT, PT, and MT — all surface methods — RT and UT are volumetric methods that probe the full thickness of the weld, finding internal (subsurface) discontinuities. They are largely complementary: RT excels at volumetric flaws (porosity, slag), while UT excels at planar flaws (cracks, lack of fusion). A CWI is not required to perform RT/UT but must understand the methods, interpret reports, and know their relative strengths.
Radiographic Testing (RT) passes penetrating radiation through the weld onto a film or digital detector. Where a discontinuity is present, the material is less dense (or thinner), so more radiation passes through and the film darkens at that location. The result is a two-dimensional shadow image of the weld's interior.
Radiation Sources
| Source | Type | Notes |
|---|---|---|
| X-ray tube | Machine-generated | Energy adjustable; powered on/off; shop use |
| Iridium-192 (Ir-192) | Gamma isotope | ~74-day half-life; ~6–76 mm steel range; very portable |
| Cobalt-60 (Co-60) | Gamma isotope | ~5.27-year half-life; ~1.33 MeV; thick sections (~50–200 mm) |
Gamma sources need no electrical power and reach tight field locations, but the source is always emitting and decays over time, so exposures lengthen as the isotope ages.
Image Quality and Key Terms
| Term | Meaning |
|---|---|
| IQI / penetrameter | Image Quality Indicator — a hole-type or wire-type gauge placed on the part to prove the radiograph's sensitivity; wire type is standard for welds |
| Sensitivity | Smallest detectable detail; commonly a 2% (2-2T) requirement — the IQI's visible hole/wire confirms it |
| Film density | Darkness of the film, read with a densitometer; codes require a density window |
| Geometric unsharpness (Ug) | Edge blur from source size, source-to-film and object-to-film distances |
How Indications Appear
Most flaws are less dense than sound metal, so they print darker: rounded dark spots = porosity; dark irregular lines following the bevel = slag; sharp dark line = a crack (only if reasonably aligned with the beam). Tungsten inclusions are the exception — denser than steel, they print as bright (light) spots. Excess penetration or reinforcement prints lighter.
Ultrasonic Testing (UT)
Ultrasonic Testing (UT) sends high-frequency sound (typically 1–6 MHz, beyond human hearing) into the material through a transducer coupled to the surface with a couplant (gel, oil, or water) that excludes air. Sound travels until it hits a reflector — a discontinuity or the back wall — and echoes back. The time of flight locates the reflector's depth; the echo amplitude indicates its size. Most flaw detection uses the pulse-echo technique (one transducer transmits and receives).
Angle-Beam Weld Inspection
Weld bodies are inspected with an angle-beam transducer mounted on a plastic wedge, which refracts the sound into the steel at a standard angle (45°, 60°, or 70°). The beam travels down to the far surface (the first leg), reflects, and rises back up through the weld (the second leg), letting the inspector scan the whole cross-section from the plate surface beside the weld. The horizontal distance for one full V (down and back to the top surface) is the skip distance; the angled path length is the V-path.
, the IIW block) sets the refraction angle, beam index point, and sweep, and a DAC (Distance-Amplitude Correction) curve compensates for sound attenuation with distance so equal flaws read equally regardless of depth.
| Technique | Description | Use |
|---|---|---|
| Straight beam (0°) | Sound enters perpendicular | Thickness, lamination, plate checks |
| Angle beam (45/60/70°) | Refracted shear wave from the side | Standard weld body inspection |
| Phased array (PAUT) | Many elements steered electronically | Encoded scans + electronic record |
| TOFD | Diffraction from crack tips | Accurate height sizing of planar flaws |
Eddy Current Testing (ET) — Briefly
Eddy Current Testing (ET) is an electromagnetic surface/near-surface method for electrically conductive materials. An AC coil induces swirling eddy currents in the part; a surface or near-surface flaw disturbs those currents, changing the coil's impedance, which the instrument displays. ET needs no couplant and no contact, is very fast, and — unlike MT — works on non-ferromagnetic conductors (austenitic stainless, aluminum, titanium). Its classic applications are tubing/heat-exchanger inspection and rapid surface crack detection.
Limits: shallow depth, conductivity/geometry sensitivity, and skilled signal interpretation.
Cross-Method Comparison
| Method | Surface vs Volumetric | Best detects | Key limitation |
|---|---|---|---|
| VT | Surface | Visible surface flaws; fit-up | Surface only; inspector-dependent |
| PT | Surface | Surface-breaking cracks (any metal) | Surface-breaking only; no depth |
| MT | Surface + near-surface | Surface/near-surface flaws in steel | Ferromagnetic only |
| ET | Surface + near-surface | Surface flaws in conductors; tubing | Shallow; conductive only |
| RT | Volumetric | Volumetric (porosity, slag) | Radiation hazard; both-side access; misses tight planar flaws aligned across the beam |
| UT | Volumetric | Planar (cracks, LOF) | One-side access but very operator-dependent; weak on scattered porosity |
Exam trap: RT = volumetric flaws + permanent record + radiation hazard + both-side access; UT = planar flaws + depth/size info + one-side access + no radiation. UT can miss small scattered porosity that RT shows clearly, while RT can miss a tight crack lying across the beam that UT catches — which is why critical work sometimes uses both.
On a radiograph, a cluster of rounded dark spots in the weld most likely indicates:
In angle-beam UT of a weld, what is the 'first leg'?
Which NDE method is best suited to detect a non-surface-breaking planar crack and also report its depth, using access to only one side of the weld?
Which gamma source has a half-life of roughly 74 days and is commonly used for portable field radiography of moderate-thickness steel?