Introduction to the functions and limitations of different nondestructive testing methods
Non-destructive testing (NDT) methods are essential tools in the inspection and quality control of materials and structures without causing any damage. Here, we explore several key NDT techniques: Radiographic Testing (RT), Ultrasonic Testing (UT), Eddy Current Testing (ET), Magnetic Particle Testing (MT), and Penetrant Testing (PT).

Pipe fitting ready for radiographic test
Radiographic Testing (RT)
Capabilities:
Detects weld defects such as incomplete penetration, porosity, and slag inclusions.
Identifies casting defects including shrinkage cavities, slag inclusions, porosity, porosity, and thermal cracks.
Determines the size, type, and planar projection location of detected defects.
Limitations:
Difficult to detect defects in forgings and profiles.
Less effective at identifying fine cracks and lack of fusion in welds.
Notes:
The penetration thickness for RT is mainly determined by the radiation energy. For steel materials:
400 kV X-ray can penetrate up to 85 mm.
Cobalt-60 gamma rays can penetrate up to 200 mm.
9 MeV high-energy X-ray can penetrate up to 400 mm.

Ultrasonic testing of welds
Ultrasonic Testing (UT)
Capabilities:
Detects defects in forgings such as cracks, white spots, delaminations, and extensive or dense slag inclusions.
Identifies weld defects including cracks, incomplete penetration, lack of fusion, slag inclusions, and porosity.
Examines profiles (including plates, pipes, rods, and other profiles) for defects like cracks, folds, delaminations, and laminar slag inclusions.
Detects casting defects such as hot cracks, cold cracks, porosity, slag inclusions, and shrinkage cavities.
Can measure the coordinate position and relative size of detected defects, although it is challenging to determine defect types.
Limitations:
Difficult to detect defects in coarse-grained materials like austenitic steel castings and welds.
Less effective for complex-shaped or rough-surfaced workpieces.
Notes:
Using straight beam techniques, UT can detect internal or surface-parallel defects up to approximately 1 meter deep in steel.
Using angle beam or surface wave techniques, it can detect surface and non-surface-parallel defects.
When testing steel welds with 2.5 MHz ultrasound, the effective detection depth is about 200 mm.

Eddy Current Testing Of Flange
Eddy Current Testing (ET)
Capabilities:
Detects surface and near-surface defects in conductive materials (ferromagnetic and non-ferromagnetic metals, graphite, etc.), such as cracks, folds, pits, inclusions, and porosity.
Measures the coordinate position and relative size of detected defects, but identifying defect types is challenging.
Limitations:
Not suitable for non-conductive materials.
Cannot detect deep internal defects in conductive materials.
Less effective for complex-shaped workpieces.

Magnetic particle testing for large diameter elbow welds
Magnetic Particle Testing (MT)
Capabilities:
Detects surface and near-surface defects in ferromagnetic materials (including forgings, castings, welds, profiles, etc.), such as cracks, folds, delaminations, inclusions, and porosity.
Determines the surface location, size, and shape of detected defects but struggles to ascertain defect depth.
Limitations:
Not applicable for non-ferromagnetic materials such as austenitic steel, copper, and aluminum.
Cannot detect deep internal defects in ferromagnetic materials.

PT inspection (penetration inspection) of pipe fittings
Penetrant Testing (PT)
Capabilities:
Detects surface-opening defects in metallic and dense non-metallic materials, such as cracks, folds, porosity, and pinholes.
Determines the surface location, size, and shape of detected defects but cannot measure defect depth.
Limitations:
Not suitable for porous materials.
Cannot detect internal or near-surface defects that do not open to the surface.
Each NDT method has unique strengths and limitations, making them suitable for different applications and material types. Understanding these techniques’ capabilities and restrictions helps ensure the appropriate method is chosen for specific inspection needs, ultimately enhancing the safety and reliability of materials and structures.