Composite service & NDT
Scientific Principles
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 so it is possible to measure and find flaws and characterize conductivity, permeability, and dimensional features.
Main Uses
Used to detect surface and near-surface flaws in conductive materials, such as metals. Eddy current inspection is also used to sort materials based on electrical conductivity and magnetic permeability.
Main Advantages
- Detects surface and near-surface defects.
- The method can be used for more than flaw detection.
- Minimum part preparation is required.
Disadvantages
- Only conductive materials can be inspected.
- Ferromagnetic materials require special treatment to address magnetic permeability.
- The depth of penetration is limited.
- The skill and training required are more extensive than other techniques.
- Surface finish and roughness may interfere.
- Reference standards are needed for setup.
Penetrant Inspection
Scientific Principles
The 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.
Main Uses
Used to locate cracks, porosity, and other defects that break the surface of a material. Liquid penetrant testing is used on most nonporous materials.
Main Advantages
- Parts with complex geometry are routinely inspected.
- Indications are produced directly on the surface of the part providing a visual image of the discontinuity.
- Equipment investment is minimal.
Disadvantages
- 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 paints, sealing, chemicals, and more.
- Requires multiple operations under controlled conditions.
- Chemical handling precautions are necessary (toxicity, fire, waste).
Magnetic Particle Testing
Scientific Principles
A magnetic field is established in a component made from ferromagnetic material. The magnetic lines of force travel through the material, and exit and re-enter the material at the poles. Defects such as cracks 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
Main Uses
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.
Main Advantages
- Can detect surface and subsurface flaws.
- Surface preparation is less critical than it is in the penetrant inspection.
- Magnetic particle indications are produced directly on the surface of the part and form an image of the discontinuity.