NON DESTRUCTIVE TESTING
ULTRASONIC INSPECTION
Type of operation: Manual or mechanised.
Equipment: Main unit comprising pulse generator, display oscilloscope, probe (chosen to suit work).
Mode of operation:
A pulse of electrical energy is fed to the probe in which a piezo-electric crystal converts it to mechanical vibrations at an ultrasonic frequency. The vibrations are transmitted (via a layer of grease to exclude the air) through the work. If they encounter a defect some are reflected back to the probe, where they regenerate an electrical signal. A cathode ray tube trace, started when the original signal is sent, displays the reflected defect signal and from it time - indicating distance from probe, and amplitude - indicating defect size, can be calculated.
Materials: Most metal except those with coarse or varying grain structure.
Typical welding applications.
- Welds in thick wall vessels.
- Welds with access to one side only.
Operating parameters.
| Probe frequency |
1 - 5MHz |
| Portability |
Good |
| Access |
Good (can be battery operated) |
| Thickness range |
5 - 500mm |
| Minimum defect size |
5mm wide |
Overall advantages.
- Immediate presentation of results.
- Not necessary to evacuate personnel.
- Can be battery powered.
- Depth location of defects.
Overall limitations.
- Trained and skilled operator needed.
- No pictorial record.
Safety.
Moderate care needed as for all electrical equipment.
MAGNETIC PARTICLE INSPECTION
Type of operation: Manual or mechanised
Equipment: Power supply. Contacts or coil. Ultra-violet lamp (optional). Portable or fixed installation.
Mode of operation:
The work is magnetised either by passing a current through it, or through a coil surrounding it. Defects on or near the surface disrupt the magnetic field (unless they are parallel to it). A magnetic particle fluid suspension is applied which concentrates around the defects. The work is viewed either directly or by ultra-violet light using a dye which fluoresces - i.e. emits visible light (this must be done where normal lighting is subdued). After testing, work may be demagnetised if required.
Materials: Magnetic materials only - ferritic steels and some nickel alloys.
Operating parameters:
| Current |
500 - 10,000 Amps (AC or DC) |
| Supply load |
1 - 100 kVA |
| Portability |
good |
| Access |
restricted |
| Minimum defect size |
0.025mm wide at surface |
| Testing time |
10 - 80 seconds |
Typical welding applications:
- Rapid inspection of welded structural details.
- Production rate inspection of small components.
Overall advantages:
- Direct indication of defect location.
- Initial inspection by unskilled labour.
- Some indication of sub-surface defects but of low sensitivity.
- Not critically dependent on surface condition.
Overall limitations:
- No use for non-magnetic materials.
- Defect detection critically dependent on alignment across magnetic field.
- Sub-surface flaws require special procedures.
Safety
Moderate care needed in handling electrical equipment and flammable fluids.
RADIOGRAPHY
Gamma Radiography.
Type of operation:
Static - development may be mechanised.
Equipment:
Radioactive isotope in storage container. Remote handling gear. Lightproof cassette. Photographic development facilities. Darkroom and illuminator for assessment.
Mode of operation:
Gamma rays, similar to X-rays but of shorter wavelength, are emitted continuously from the isotope. It cannot be ‘switched off’ so when not in use, it is kept in a heavy storage container that absorbs radiation. They pass through the work to be inspected. Parts of the work presenting less obstruction to gamma rays, such as cavities or inclusions, allow increased exposure of the film. The film is developed to form a radiograph with cavities or inclusions indicated by darker images. Section thickness increases (such as weld) appear as less dense images.
Operating parameters:
| Wavelength of radiation |
0.001 - 0.015 nm
0.01 - 1 nm (1.25MeV - 80KeV) |
| Portability |
good (except for container) |
| Access |
good |
| Exposure time |
1 second - 24 hours |
| Thickness range |
up to 250 mm |
| Minimum defect size |
1% of thickness |
Materials:
Most weldable materials can be inspected.
Typical welding applications:
- Site inspection.
- Panoramic exposure for small work.
Advantages, limitations, consumables and safety as for X-ray radiography.
X-ray Radiography
Type of operation:
Static or transportable.
Equipment:
X-ray tube. Stand and control gear. Lightproof cassette. Photographic development facilities. Dark room and illumination for assessment.
Mode of operation:
X-rays are emitted from the tube and pass through the work to be inspected. Parts of the work presenting less obstruction to X-rays, such as cavities or inclusions, allow increased exposure of the film. The film is developed to form a radiograph with cavities or inclusions indicated by darker images. Section thickness increases (such as weld under-bead) appear as less dense images.
Operating parameters:
| Tube voltage |
10 - 500 kV |
| Tube current |
10 - 250 mA |
| Power consumption |
1 - 10 kW |
| Portability |
fair |
| Access |
fair |
| Exposure time |
1 sec - 10 min |
| Thickness range |
up to 100 mm |
| Minimum defect size |
0.1% of thickness X 0.05 mm |
Materials:
Most weldable materials may be inspected.
Typical welding applications:
- Pipelines
- Pressure vessels.
Overall advantages:
- Accurate pictorial presentation of results.
- Radiographs may be kept as a permanent record.
- Not confined to welds.
Overall limitations:
- Personnel must be clear of area during exposure.
- Cracks parallel to film may not show up.
- Film expensive.
Consumables:
- Film.
- Processing chemicals.
- Water.
- Isotope replacements - for gamma radiography
Safety
Cumulative radiation risk to personnel requires stringent precautions.
DYE PENETRANT INSPECTION
Type of operation: Manual or mechanised.
Equipment:
- Minimum - aerosols containing dye, developer, cleaner.
- Maximum - Tanks, work handling gear, ultra-violet lamp.
Mode of operation:
A special dye is applied to the surface of the article to be tested. A suitable time interval allows it to soak into any surface defects. The surface is then freed from surplus dye and the dye in the crack revealed by either: applying a white powder developer into which the dye is absorbed producing a colour indication,
or, illuminating with ultra-violet light under which the dye fluoresces, that is, emits visible light. This must be done where normal lighting is subdued.
Operating parameters:
| Portability |
excellent (for aerosols) |
| Access |
good |
| Minimum defect size |
0.025 mm wide |
| Time |
30 minutes approx. |
Materials:
Any - non porous.
Typical welding applications:
- Root runs in pipe butt welds.
- Leak paths in containers.
Overall advantages:
- Low cost.
- Direct indication of defect location.
- Initial examination by unskilled labour.
Overall limitations:
- Surface defects only detected.
- Defects cannot readily be rewelded due to trapped dye.
- Rough welds produce spurious indications.
Safety: Dye and propellant gases have low flash points. |
