Why Method Selection Matters
Selecting the wrong NDT method for a given inspection task can mean missed defects, wasted time, or unnecessary equipment downtime. The four most commonly applied methods — UT, MT, PT, and RT — each have distinct capabilities, limitations, and optimal application domains.
Ultrasonic Testing (UT)
Ultrasonic testing uses high-frequency sound waves to detect internal and surface defects, and to measure wall thickness. Advantages include: detection of subsurface and embedded defects, precise depth and sizing information, applicability to a wide range of materials including composites, and portability for field applications. Phased Array UT (PAUT) extends conventional UT with electronic beam steering, enabling rapid scanning and detailed volumetric imaging.
Best for: Weld inspection, corrosion mapping, wall thickness measurement, forgings and pressure vessels.
Magnetic Particle Testing (MT)
MT detects surface and near-surface discontinuities in ferromagnetic materials by applying a magnetic field and then iron particles. MT is fast, inexpensive, and highly sensitive to surface-breaking cracks, but is limited to ferromagnetic materials and cannot detect subsurface defects beyond approximately 3 mm depth.
Best for: Weld root and cap inspection, casting inspection, in-service cracking detection in carbon and low-alloy steel components.
Liquid Penetrant Testing (PT)
PT uses a penetrating liquid drawn into surface-breaking discontinuities by capillary action. PT is applicable to virtually any non-porous material including austenitic stainless steel, titanium, and ceramics — materials that cannot be inspected by MT.
Best for: Surface crack detection in non-ferromagnetic materials, valve body and seat inspection, weld quality control.
Radiographic Testing (RT)
RT uses X-rays or gamma rays to produce an image of the internal structure of a component. It provides a permanent visual record and can detect volumetric defects such as porosity, inclusions, and incomplete fusion in welds.
Best for: Weld inspection where volumetric sensitivity is required, pipe and vessel inspection, complex geometries.
Selection Matrix
- Surface crack in carbon steel weld: MT (primary), PT (alternative if not ferromagnetic)
- Internal porosity in weld: RT (primary), PAUT (alternative)
- Wall thickness and corrosion mapping: UT
- Surface crack in stainless steel: PT