Non-Destructive Testing (NDT) Instrumentation

The increasing requirement to demonstrate the integrity and reliability of engineering materials, products and plant has contributed immensely to the development of non-destructive testing instrumentation. Efficient materials manufacture, the assurance of product quality and re-assurance of plant at regular intervals during use represent the need for non-destructive testing (NDT).

The failure in engineering components generally results from a combination of conditions, the major ones being inadequate design, incorrect use, or the presence of defects in materials. The use of non-destructive testing seeks to eliminate failures caused mostly by defects. During manufacture, these defects may, for instance, be shrinkage and porosity in castings, laps and folds in forgings, laminations in plate material, lack of penetration and cracks in weldments. Then again, with increasing complexity of materials and conditions of service, less obvious factors may require control through non-destructive testing, for instance, features such as composition, microstructure, and homogeneity.

In addition to its applications in manufacturing, non-destructive testing is also applied to on-site testing of bridges, pipelines in the oil and gas industries, pressure vessels in the power-generation industry and in-service testing of nuclear plant, aircraft and refinery installations. Defects at this stage may be deterioration in plant due fatigue and corrosion. The objective of non-destructive testing during service is to look for deterioration in plant to ensure that adequate warning is given of the need to repair or replace. Periodic checks also give assurance that, ‘everything is okay’.

Broadly speaking, non-destructive testing methods are normally classified in terms of whether their suitability is primarily for the examination of the surface materials or the internal features of materials. In the following sections, we explore the different techniques of non-destructive testing and briefly mention some of the advanced methods employed in NDT.

Visual Examination/Inspection

Integrity is verified principally through visual inspection in most components. As a matter of fact, even for components that need further inspection using ultrasonic or radiography, visual inspection still make ups  a key aspect of practical quality control.

Visual inspection is the most widely utilized method. It is relatively easy to apply and can have the following advantages:

• Can be applied while work is in progress.
• Allows early correction of faults.
• Gives indication of incorrect procedures.
• Gives early warning of faults developing when item in use.
• Low cost.

The equipment used in visual inspection may range from that suitable for determining dimensional non-conformity e.g. visual inspection gauges, to illuminated magnifiers, and the more advanced fiberscope (for example a high-resolution flexible fiberscope for viewing inaccessible areas in boilers, heat exchanges, castings, turbines, interior welds, etc.).

Non-Destructive Testing Techniques for Surface Inspection

The inspection of surfaces for defects at or close to the surface can be done via visual and electromagnetic methods such as magnetic particle, potential drop and eddy current.

Visual Methods

In many cases, defects are visible to the eye on the surface of components. But, for purposes of recording or gaining access to difficult locations, photographic and photomicrographic methods are used. In hazardous environs, such as in the nuclear and off-shore fields, remote television cameras coupled to video recorders allow inspection results to be assessed after the test. When coupled to remote transport systems, these cameras can be used for pipeline inspection, the cameras themselves being miniaturized for very narrow pipe sections. 

In cases where surface-breaking defects are not immediately apparent, their presence may be enhanced by the use of dye penetrants. A penetrating dye-loaded liquid is applied to a material surface where, due to its surface tension and moist properties, a strong capillary effect exists, which causes the liquid to penetrate into fine openings on the surface.  After a short time (about 10 minutes), the surface is cleaned and an absorbing powder applied which blots the dye penetrant liquid, causing a stain around the defects. Since the dye is either a bright red or fluorescent under ultraviolet light, small defects become readily visible.

Magnetic Flux Techniques

When the material under test is ferromagnetic the magnetic properties may be used to provide testing methods based on the localized escape of flux around defects in magnetized material. For instance, when a magnetic flux is present in a material, such as iron, below magnetic saturation, the flux will tend to confine itself within the material surface. This is due to the continuity of the tangential component of the magnetic field strength, H, across the magnetic boundary. Since the permeability of iron is high, the external flux density, B, is small. Around a defect, the presence of a normal component of B incident on the defect will provide continuity of the flux to air, and a localized flux escape will be apparent. If only a tangential component is present, no flux leak occurs, maximum leakage conditions being obtained when B is normal to the defect.

Potential Drop Techniques

The measurement of material resistance can be related to measurements of the depth of surface breaking cracks.

A four-point probe head as illustrated in Figure 1.0 below, is applied to a surface and current passed between the outer probes. The potential drop across the crack is measured by the two inner probes and as the crack depth increases, the greater current path causes an increasing potential drop. By varying probe spacing, maximum sensitivity to changes in crack depth can be achieved. In addition, the application of ac current of varying frequency permits the depth of current penetration beneath the surface to be varied due to the ‘skin effect’.

Eddy-Current Testing Technique

This is a powerful method of assessing both the material properties and the presence of defects. A time changing magnetic field is used to induce weak electrical currents in the test material, these currents being sensitive to changes in both material conductivity and permeability. In turn, the intrinsic value of the conductivity depends mainly on the material composition but is influenced by changes in structure due to crystal imperfections, stress conditions, or work hardening dependent upon the state of dislocations in the material. Additionally, the presence of discontinuities will disturb the eddy-current flow patterns giving detectable changes.

A typical eddy-current testing system comprises of a coil which due to the applied current produces an ac magnetic field within the material. This, in turn, excites the eddy currents which produces their own field, thus altering that of the current. This reflects also in the impedance of the coil, whose resistive component is related to eddy-current losses and whose inductance depends on the magnetic circuit conditions. Thus, conductivity changes will be reflected in changes in coil resistance, while changes in permeability or in the presentation of the coil to the surface will affect the coil inductance.

A simple eddy-current detector is shown in Figure 1.1 below normally the coils are incorporated into a balanced-bridge configuration to provide maximum detection of small changes in coil impedance reflecting the material changes.  

In the eddy-current detector above, the magnitude of bridge imbalance is measured. This can be used for material comparison of known against unknown and for simple crack detection. A more versatile type of detector is one which the magnitude and phase of the coil-impedance change is measured, as changes in inductance will be 90° out of phase with those from changes in conductivity.

Non-Destructive Testing Methods for Sub-Surface Inspection

Ultrasonic Testing

Basically, when ultrasonics is applied to the non-destructive testing of an engineering component, it depends on a probing beam of energy directed into the component interacting in an interpretable way with component’s structural features. If a flaw is present within the metal, the progression of the beam of energy is locally modified and the modification is detected and conveniently displayed to enable the flaw to be diagnosed. The diagnosis largely depends on the knowledge of the nature of the probing energy beam, its interaction with the structural features of the component under test, and the manufacturing history of the component.

The ultrasonic energy is directed into the matter under test in the form of mechanical waves or vibrations or very high frequency.

The equation λ = V/f, where λ is the wavelength, V is the velocity and f, the frequency.

The wavelength determines the defect sensitivity in that any defect dimensionally less than half the wavelength will not be detected. As a result, the ability to detect small defects increases with decreasing wavelength of vibration and, since the velocity of sound is characteristic of a particular material, increasing the frequency of vibration will provide the possibility of increased sensitivity. Frequency selection is therefore a significant variable in the ability to detect small flaws.

Ultrasonic non-destructive testing methods are briefly discussed in the following article:

>> Types of Non-Destructive Ultrasonic Testing Methods

Radiography

Radiography is an essential tool for inspection and development work in foundries. It is also used in the pressure vessel, pipeline, offshore drilling platform and many other industries for checking the integrity of welded components at the in-process, completed or in-service stages. This technique also finds application in the aerospace industry.

This technique relies on the ability of high-energy short-wavelength sources of electromagnetic radiation such as X-rays, gamma rays, and neutron sources to penetrate solid materials. By placing an appropriate recording medium, usually photographic film on the side of the specimen remote from the radiation source and with suitable adjustment of technique, a shadowgraph or two-dimensional image of the surface and internal features of the specimen can be obtained. Thus radiography can be used for the detection of internal flaws, and a permanent record of these features is directly produced, which is an advantage of this technique.

Advanced Non-Destructive Testing Techniques

Some of the advanced non-destructive testing techniques include:

• Computed Tomography – in this technique x-rays are used to produce cross-sectional images of components providing 3D insights into internal structures.
• Remote visual inspection – this may involve the use of cameras and robotic systems to visually inspect inaccessible or hazardous locations.
• Phased Array Ultrasonic Testing – this method employs multiple ultrasonic components to direct and focus beams, allowing for detailed imaging and enhanced defect characterization.