INTRODUCTION TO NON DESTRUCTIVE TESTING

The aim of this section is to provide a general overview of the fundamental nature of NDT, including its role in the engineering, construction and maintenance of in-service industrial systems and components. It has been written to acquaint non NDT personnel looking for a new career. It will cover the basic physical concepts of various testing techniques, how they are selected, who uses them, their capabilities and their limitations.

Inspection of materials, components, and assemblies is often required in order to determine if they comply with design specifications or are defective. Of utmost concern for newly manufactured or constructed items is their ability to withstand the mechanical stresses and chemical attack to which they are expected to be exposed. For items that are already in service, the question is whether or not the item will continue to function satisfactorily in its service environment until the next scheduled inspection. NDT by virtue of its ability to detect flaws, measure dimensions, and assess material characteristics, has become a primary tool in addressing the predictions of component performance and remaining service life. The selection of an appropriate NDT method for a specific application is based upon the physical characteristics of the material and the discontinuities of most interest, including the probable location, orientation, size, and morphology. The assumption is that if an item is free from unacceptable flawed conditions, it should meet performance expectations, that is, carry the load, withstand the vibration, or maintain dimensional integrity.

The NDT approach to materials testing is ‘non-destructive’ in that each inspected item is expected to be placed in service. This is in stark contrast to the destructive methods of materials evaluation where sacrificial specimens are required. In destructive testing, specially prepared specimens are stressed in a manner that simulates the stresses to which the item may be subjected and at levels exceeding the design limits.

The destructive approach results in specimens fracture, distortion, of some other form of degradation, and thus is applied on only a sampled basis.

NDT is most commonly used where component failure may have catastrophic consequences, such as in aircraft, electric power plants and petrochemical plants, as well as gas lines and offshore installations. The periodic inspection of components in these fields determines if they are suitable for continued service. The advantages of NDT for in service applications include safety evaluation with a minimum of system down time, early detection of potentially dangerous and costly unscheduled service interruption and the ability to monitor degradation processes from which realistic and cost effective re-inspection schedules can be developed.

The five main methods of NDT described in this brief are:

Magnetic Particle Inspection
Liquid Penetrant Inspection
Ultrasonic Inspection
Eddy Current Inspection
Radiographic Inspection and Interpretation

MAGNETIC PARTICLE INSPECTION

Magnetic Particle Inspection (MT) is one of the best-known and commonly used methods of NDT. Its aim is to detect the presence of surface braking discontinuities (cracks) in the part under inspection. Only Ferromagnetic materials can be inspected by the MT method. This is because Ferromagnetic materials develop strong internal magnetic fields when an electrical current is passed through them. An electric current can be introduced in to the test part in several ways. It can be wrapped in encircling coils and rods or the current can be applied directly with the use of the yoke producing a magnetic field perpendicular to the current flow. When these internal magnetic fields encounter a change in permeability (i.e. an open fissure/crack) the magnetic field is forced outside of the materials surface, and produces flux leakage. This leakage will attract any other Ferromagnetic materials that may be close to the leakage site.

Prior to any MT being carried out the part is cleaned of any loose scale, oil/grease, and then covered with a very thin layer of background contrast paint (this is applied by painting or by aerosol). The aim of the contrast paint is to make any defects or anomalies stand out, and help the Inspector in locating the defect. Once the contrast paint is dry, particles with an affinity for leakage fields are passed over the part, these Ferromagnetic particles are applied by aerosol i.e. wet or dry powder form depending on the temperature or the part. These particles are highly visible against the contrast paint. When the particles are attached to the leakage field around the surface flaw, they take the shape of the anomaly that has broken the magnetic field. The pattern of the particles clearly shows the shape and contours of the anomaly, allowing for easy monitoring and recording by the inspector.

LIQUID PENETRANT INSPECTION

Liquid Penetrant Inspection (PT) is another common method of NDT, and is solely used to detect surface breaking discontinuities, which are free from debris that can limit the entrance of the dye (oil, grease and paint). This method of Inspection can be applied to Ferromagnetic and non-ferromagnetic materials, however it is most commonly used in non-ferromagnetic components, for example when MT is not practical. The Liquid Penetrants used have a low viscosity and a high affinity for metallic surfaces. The dyes are applied to the test part by aerosol spray or by being submerged in a tank were the dye penetrates any surface breaking flaws. After a required soak time the dye can be removed so no excess fluid remains on the surface and a thin coating of a highly absorbent developer is sprayed over the test area. The developer draws any entrapped dye out of any cracks of fissures by capillary action and the dye spreads throughout the developer surface magnifying the size of the indication. The contrast in colour between the red penetrant and the white developer plus the magnifying effect caused by the spreading of the dye leads to a clearly visible indication.

ULTRASONIC INSPECTION

Ultrasonic Inspection (UT) is a method of NDT that is used to detect internal anomalies in a part, which may contain welding, or stress defects that can be detrimental to the integrity of the component. It is also a commonly accepted method of checking the wall thickness of pipelines and vessels, which are suspected of being eroded internally, when access is limited to one side of the material. UT is very sensitive to critical defects in a material like cracks, welding defects, porosity, lack of fusion and inclusions, which may weaken the weld. It is also very portable and can be used on a wide range of materials. This method of inspection, though, is very reliant on having an experienced and well-trained Inspector to interpret the indications they come across in field conditions and to determine if the part is satisfactory or if remedial action is required.

UT uses very short duration sound pulses which when included into a material reflects off different media i.e. air interfaces and inclusions. The time for the reflection from these media are monitored and compared against the known travel speeds for the given material. The careful measurement of these pulse times becomes a measure of the distance the pulse has travelled and these are monitored via signals on a visual display screen. These signals may represent cracks, back wall echo and lack of fusion (common air interfaces), slag, tungsten and copper (common welding inclusion). The tracking of these signal levels during the inspection enables the inspector to gather information about the size, type and location of the anomalies detected. During wall thickness monitoring of pipelines and vessels where access is limited to one side only, the sound pulse reflects off the inner wall of the component. These signal times become a measure of the distance travelled and any internal erosion/corrosion can be detected due to a reduction in wall thickness (a faster signal time).

EDDY CURRENT INSPECTION

Eddy Current Inspection (ET) is one of the most widely applicable of the Non-Destructive Inspection methods. It depends on measuring the changes in the impedance of a coil due to change in the flow of eddy current in a conductor. Any material change that affects the flow of the induced eddy current sufficiently can be detected. Because so many things affect the flow, eddy current inspection can be applied to a wide variety of test situations. Principle areas of application are flaw detection, material-thickness measurements, alloy sorting and the monitoring of metallurgical conditions such as hardness and heat treatment. Besides its versatility, the major advantages of eddy current inspection are the speed at which tests can be performed and its ability to inspect through painted coatings. It can be used also to inspect both ferromagnetic and non-magnetic materials. The principle disadvantages of the method is its limited depth of inspection into the material or part being inspected.

RADIOGRAPHIC INSPECTION AND INTERPRETATION

Radiography (RT) and Radiographic Interpretation (RI) uses X-rays or gamma-rays to produce an image of an object on film. The image is usually natural-size. X-rays and gamma-rays are very short wavelength electromagnetic radiation which can pass through solid material, being partly absorbed during transmission. Thus, if an X-ray source is placed on one side of a specimen and a photographic film on the other side, an image is obtained on the film of the thickness variations in the specimen, whether these are surface or internal.

This is a well-established technique which gives a permanent record and is widely used to detect internal flaws in weldments and castings and to check for mis-constructions in assemblies.