Non-Destructive Testing

Non destructive testing is a descriptive term used for the examination of materials and components in such a way that allows materials to be examined without changing or destroying their usefulness.

NDT plays a crucial role in everyday life and is necessary to assure safety and reliability. Typical examples are found in aircraft, motor vehicles, pipelines, bridges, trains, power stations, refineries and oil platforms, to name a few, which are all inspected using NDT.

NDT is a Quality Assurance management tool which can give impressive results when used correctly. It requires an understanding of the various methods available, their capabilities and limitations, knowledge of the relevant standards and specifications for performing the tests.

Materials, products and equipment which fail to achieve their design requirements or projected life due to undetected defects may require expensive repair or early replacement. Such defects may also be the cause of unsafe conditions or catastrophic failure, as well as loss of revenue due to unplanned plant shutdown.

Non destructive testing can be applied to each stage of an item's construction. The materials and welds can be examined using NDT and either accepted, rejected or repaired. NDT techniques can then be used to monitor the integrity of the item or structure throughout it's design life.

TESTING METHODS

The method that can be used for non-destructive testing depends on the physical properties of the material. A thorough knowledge of each NDT method is required to ensure the correct selection of the appropriate method for each application.

The aptGroup can assist in the provision of the following most commonly used NDT methods:

Visual Inspection

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

It is used most effectively for the inspection of welds where quick detection and the correction of flaws or process related problems can result in significant cost savings. It is the primary evaluation method of many quality control programs.

It 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 treating with chemicals to show the surface details.

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

Liquid Penetration Inspection

Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleed out 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. This acts as a "blotter". It draws the penetrant from the flaw to reveal its presence. Coloured (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".

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

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 and drawing conclusions about it's "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 AE are that a whole structure can be monitored from a few locations, the structure can be tested in use (without taking it out of 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.

Applications include testing pipelines and storage tanks (above and below the ground), fibreglass structures, rotating machinery, weld monitoring and biological and chemical changes.

Magnetic Particle Inspection

Magnetic particle inspection is a method that can be used to find surface and near 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 it's 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 dependant 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. But its 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.

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. Commonly it is used in the aerospace, automotive, marine and manufacturing industries.

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 magnetic field that tend to oppose the original magnetic field. The impedance of coil in close proximity to the specimen is affected 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 on aircraft, the power stations generating plant, or welds in pressure vessels at an oil refinery or paper mill.

Usually pulsed beams of high frequency ultrasound are used via a hand-held transducer, which is placed on the specimen. Any sound from that pulse that 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. Defects anywhere through the specimen thickness reflect the sound, back to the transducer. Flaw size, distance and reflectivity can be interpreted.

Because of it's complexity considerable technician training and skill is required.

Radiography

X-rays are produced by high voltage x ray machines whereas gamma rays are produced from radioactive isotopes such as Iridium 192 The x-ray or gamma rays are placed close to the material to be inspected and they pass through the material and are then captured on film This film is then processed and the image is obtained as a series of grey shades between black and white.

The choice of which type of radiation is used (x ray or gamma depends on the thickness of the material to be tested. Gamma sources have the advantage of portability, which makes them ideal for use in construction site working.

X-rays and gamma rays are very hazardous. Special precautions must be taken when performing radiography therefore the operator will use these inside a protective enclosure or with appropriate barriers and warning signals to ensure there are no hazards to personnel.