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NDT Method Summary

No single NDT method will work for all flaw detection or measurement applications. Each of the methods has advantages and disadvantages when compared to other methods. The table below summarizes the scientific principles, common uses and the advantages and disadvantages for some of the most often used NDT methods.

Magnetic Particle Testing
Eddy Current
Scientific Principles        
Penetrant solution is applied to the surface of a precleaned component. The liquid is pulled into surface-breaking defects by capillary action. Excess penetrant material is carefully cleaned from the surface. A developer is applied to pull the trapped penetrant back to the surface where it is spread out and forms an indication. The indication is much easier to see than the actual defect. A magnetic field is established in a component made from ferromagnetic material. The magnetic lines of force travel through the material, and exit and reenter the material at the poles. Defects such as crack or voids cannot support as much flux, and force some of the flux outside of the part. Magnetic particles distributed over the component will be attracted to areas of flux leakage and produce a visible indication. High frequency sound waves are sent into a material by use of a transducer. The sound waves travel through the material and are received by the same transducer or a second transducer. The amount of energy transmitted or received and the time the energy is received are analyzed to determine the presence of flaws. Changes in material thickness, and changes in material properties can also be measured. Alternating electrical current is passed through a coil producing a magnetic field. When the coil is placed near a conductive material, the changing magnetic field induces current flow in the material. These currents travel in closed loops and are called eddy currents. Eddy currents produce their own magnetic field that can be measured and used to find flaws and characterize conductivity, permeability, and dimensional features. X-rays are used to produce images of objects using film or other detector that is sensitive to radiation. The test object is placed between the radiation source and detector. The thickness and the density of the material that X-rays must penetrate affects the amount of radiation reaching the detector. This variation in radiation produces an image on the detector that often shows internal features of the test object.
Main Uses        
Used to locate cracks, porosity, and other defects that break the surface of a material and have enough volume to trap and hold the penetrant material. Liquid penetrant testing is used to inspect large areas very efficiently and will work on most nonporous materials. Used to inspect ferromagnetic materials (those that can be magnetized) for defects that result in a transition in the magnetic permeability of a material. Magnetic particle inspection can detect surface and near surface defects. Used to locate surface and subsurface defects in many materials including metals, plastics, and wood. Ultrasonic inspection is also used to measure the thickness of materials and otherwise characterize properties of material based on sound velocity and attenuation measurements. Used to detect surface and near-surface flaws in conductive materials, such as the metals. Eddy current inspection is also used to sort materials based on electrical conductivity and magnetic permeability, and measures the thickness of thin sheets of metal and nonconductive coatings such as paint. Used to inspect almost any material for surface and subsurface defects. X-rays can also be used to locates and measures internal features, confirm the location of hidden parts in an assembly, and to measure thickness of materials.
Main Advantages        

Large surface areas or large volumes of parts/materials can be inspected rapidly and at low cost.

Parts with complex geometry are routinely inspected.

Indications are produced directly on surface of the part providing a visual image of the discontinuity.

Equipment investment is minimal.

Large surface areas of complex parts can be inspected rapidly.

Can detect surface and subsurface flaws.

Surface preparation is less critical than it is in penetrant inspection.

Magnetic particle indications are produced directly on the surface of the part and form an image of the discontinuity.

Equipment costs are relatively low.

Depth of penetration for flaw detection or measurement is superior to other methods.

Only single sided access is required.

Provides distance information.

Minimum part preparation is required.

Method can be used for much more than just flaw detection.

Detects surface and near surface defects.

Test probe does not need to contact the part.

Method can be used for more than flaw detection.

Minimum part preparation is required.

Can be used to inspect virtually all materials.

Detects surface and subsurface defects.

Ability to inspect complex shapes and multi-layered structures without disassembly.

Minimum part preparation is required.


Detects only surface breaking defects.

Surface preparation is critical as contaminants can mask defects.

Requires a relatively smooth and nonporous surface.

Post cleaning is necessary to remove chemicals.

Requires multiple operations under controlled conditions.

Chemical handling precautions are necessary (toxicity, fire, waste).

Only ferromagnetic materials can be inspected.

Proper alignment of magnetic field and defect is critical.

Large currents are needed for very large parts.

Requires relatively smooth surface.

Paint or other nonmagnetic coverings adversely affect sensitivity.

Demagnetization and post cleaning is usually necessary.

Surface must be accessible to probe and couplant.

Skill and training required is more extensive than other technique.

Surface finish and roughness can interfere with inspection.

Thin parts may be difficult to inspect.

Linear defects oriented parallel to the sound beam can go undetected.

Reference standards are often needed.

Only conductive materials can be inspected.

Ferromagnetic materials require special treatment to address magnetic permeability.

Depth of penetration is limited.

Flaws that lie parallel to the inspection probe coil winding direction can go undetected.

Skill and training required is more extensive than other techniques.

Surface finish and roughness may interfere.

Reference standards are needed for setup.

Extensive operator training and skill required.

Access to both sides of the structure is usually required.

Orientation of the radiation beam to non-volumetric defects is critical.

Field inspection of thick section can be time consuming.

Relatively expensive equipment investment is required.

Possible radiation hazard for personnel.

Magnetic Particle Testing
Eddy Current