NDT Techniques

Visual Inspection

Visual inspection is the one NDT method used extensively to evaluate the condition or quality of an item. It is easily carried out, inexpensive and usually doesn't require special equipment.

The method requires good vision, good lighting and the knowledge of what to look for. Visual inspection can be enhanced by various methods ranging from low power magnifying glasses through to boroscopes. These devices can also be used with television camera systems. Surface preparation can range from wiping with a cloth to blast cleaning and treatment with chemicals to show the surface details.

Visual inspection can sometimes identify where a failure is most likely to occur and identify when a failure has commenced. Visual inspection is often enhanced by other surface methods of inspection, which can identify flaws that are not easily seen by the eye.

Dye Penetrant Inspection

Dye penetrant inspection is an NDT method that is used to reveal surface breaking flaws by bleedout of a coloured or fluorescent dye from the flaw.

The technique is based on the ability of a liquid to be drawn into a "clean" surface-breaking flaw by capillary action. After a period of time called the "dwell", excess surface penetrant is removed and a developer applied. The developer acts as a "blotter". It draws out the penetrant from the flaw to reveal its presence on the surface. Colour contrast penetrants require good white light while fluorescent penetrants need to be used in controlled darkened conditions with an ultraviolet "black light".

Penetrant inspection can be used on any solid material. It is essential that the material is carefully cleaned first, otherwise the penetrant will not be able to get into the flaw. If surface penetrant is not fully removed, misleading indications will result.

Magnetic Particle Inspection

Magnetic particle inspection is an NDT method that can be used to find surface flaws in ferromagnetic materials such as steel and iron.

The technique uses the principle that magnetic lines of force (flux) will be distorted by the presence of a flaw in a manner that will reveal its presence. The flaw (for example, a crack) is located from the "flux leakage" following the application of fine iron particles to the area under examination. There are variations in the way the magnetic field is applied, but they are all dependent on the above principle.

The iron particles can be applied dry or wet, suspended in a liquid, coloured or fluorescent. While magnetic particle inspection is primarily used to find surface breaking flaws, it can also be used to locate sub-surface flaws. The method's effectiveness quickly diminishes depending on the flaw depth and type.

Surface irregularities and scratches can give misleading indications. Therefore it is necessary to ensure careful preparation of the surface before magnetic particle testing is undertaken.

Radiographic Inspection

Radiographic inspection is primarily used to find sub-surface flaws in materials.

High voltage x-ray machines produce X-rays whereas gamma rays are produced from radioactive isotopes such as iridium 192. The chosen radiation source is placed close to the material to be inspected and the radiation passes through the material and is then captured either on film or digitally.

The choice of which type of radiation is used (x-ray or gamma) largely depends on the thickness of the material to be tested and the ease of access to area of inspection. Gamma sources have the advantage of portability, which makes them ideal for use in construction site working. High energy portable x-ray machines are available for special applications such as concrete structures.

X-rays and gamma rays are very hazardous. Special precautions must be taken when performing radiography. Therefore the method is undertaken under controlled conditions, inside a protective enclosure or after assessment with appropriate barriers and warning systems to ensure that there are no hazards to personnel.

Eddy Current Inspection

Eddy current testing is an electromagnetic technique and can only be used on conductive materials. It's applications range from crack detection, to the rapid sorting of small components for flaws, size variations or material variation. The method is most commonly used in the aerospace industry, but also in automotive, marine and manufacturing.

When an energised coil is brought near to the surface of a metal component, eddy currents are induced into the specimen. These currents set up a magnetic field that tends to oppose the original magnetic field. The impedance of the coil in close proximity to the specimen is effected by the presence of the induced eddy currents in the specimen.

When the eddy currents in the specimen are distorted by the presence of the flaws or material variations, the impedance in the coil is altered. This change is measured and displayed in a manner that indicates the type of flaw or material condition.

Ultrasonic Inspection

Ultrasonic inspection uses sound waves of short wavelength and high frequency to detect flaws or measure material thickness. It is used as an alternative inspection method to radiography to locate sub-surface flaws in all industry sectors.

Usually, pulsed beams of high frequency ultrasound are used via a hand held transducer (probe) which is placed on the specimen. Any sound from the pulse that is reflected and returns to the transducer (like an echo) is shown on a screen, which gives the amplitude of the pulse and the time taken to return to the transducer. Flaws anywhere through the specimen thickness reflect the sound back to the transducer. Flaw size, distance and reflectivity can be interpreted.

Automated systems are used for testing in a production environment and for some special applications.

Acoustic Emission

Acoustic emission monitoring (AE) involves listening to the sounds (which are usually inaudible to the human ear) made by a material, structure or machine in use or under load. Conclusions are drawn about its "state of health from what is heard, just as a doctor would listen to your heart and lungs.

The technique involves attaching one or more ultrasonic microphones to the object and analysing the sounds using computer-based instruments. The noises may arise from friction (including bearing wear), crack growth, turbulence (including leakage) and material changes such as corrosion.

The advantages of Acoustic Emission are that a whole structure can be monitored from a few locations. The structure can be tested in service and continuous monitoring with alarms is possible. Microscopic changes can be detected if sufficient energy is released and source location is also possible using multiple sensors.