Choosing the wrong NDE method doesn't just waste money — it creates false confidence. A radiograph can miss a tight planar crack oriented parallel to the beam. A yoke MT examination won't detect a subsurface porosity cluster at 1.5 in depth. Each method has a physics-defined sensitivity envelope, and that envelope must match the flaw types most likely to occur given your weld process, joint geometry, and base metal.
AWS D1.1:2025 Clause 8 governs testing and inspection of structural steel welds. It specifies visual testing (VT) as the baseline requirement for all welds and allows or requires radiographic (RT) or ultrasonic (UT) testing for complete joint penetration (CJP) groove welds in the categories specified by contract or by code.
The four methods in structural welding practice
Radiographic testing (RT) passes X-ray or gamma-ray radiation through the weld and records the transmitted pattern on film or a digital detector panel. It excels at volumetric discontinuities: porosity clusters, slag inclusions, incomplete fusion with a measurable gap. Its weak point is orientation: a tight planar crack parallel to the beam produces minimal contrast on the image and can be missed. RT also creates a permanent image record, which is valuable for documentation-heavy projects and owner archives.
Ultrasonic testing (UT) sends high-frequency sound pulses into the weld and measures reflections from internal interfaces. A trained UT technician can locate, size, and characterize flaws in three dimensions. UT is the standard method for thick plate (above approximately 3/4 in) where radiographic sensitivity degrades and for CJP welds in primary structural members where planar-flaw sensitivity matters. Phased-array UT (PAUT) and time-of-flight diffraction (TOFD) offer higher resolution and faster scanning speeds; both are accepted under D1.1:2025 when the procedure is qualified per Clause 8.
Magnetic particle testing (MT) magnetizes the part and applies ferromagnetic particles — dry powder or wet fluorescent suspension — that bridge surface and near-surface discontinuities to form visible indications. MT is fast, inexpensive, and highly sensitive to fatigue cracks, surface-breaking lack of fusion, and linear surface indications on carbon steel and low-alloy steel. It does not work on austenitic stainless or aluminum — the material must be ferromagnetic. MT comes in two field configurations: dry-powder with prods or yokes (common for field work and large structural members), and wet-fluorescent (more sensitive, better for complex geometries in shop inspection).
Liquid penetrant testing (PT) uses capillary action to draw a dye — red visible or fluorescent — into surface-breaking discontinuities, then develops the indication with a white developer. PT works on any material, including non-ferromagnetic alloys. Its limitation is that it only detects surface-open defects. On carbon steel, MT is almost always preferred over PT because MT adds near-surface volumetric coverage that PT cannot provide.
Method selection by joint type and application
CJP groove welds in primary structural members — RT or UT is the default. The choice comes down to plate thickness, access, and project preference. Above approximately 2.5 in thickness, UT is more practical and more sensitive than film RT. For thin plate under about 3/8 in, RT film interpretation is easier and cheaper. Digital radiography (computed radiography and direct digital radiography) is closing that gap on setup time, but the orientation-sensitivity limitation remains.
CJP groove welds in secondary members or minor attachments — VT is the code minimum. MT or PT may be added at the engineer's discretion or in response to elevated repair rates on the joint type.
Fillet welds — VT baseline. MT is added for fatigue-critical welds, cyclic-load applications, or when the quality history for that joint configuration shows surface-cracking issues. Fillet welds are rarely radiographed — the geometry creates excessive scatter that makes film interpretation unreliable.
Repair welds — RT or UT on the repair volume, plus MT at the repair toes to check for surface cracking introduced by grinding. Hold the repair weld to at least the same NDE standard as the original.
T-K-Y tubular connections — UT is typically preferred over RT because the geometry makes radiographic setup difficult and beam orientation hard to optimize. MT at the weld toes is standard for fatigue-rated tube connections.