The non-destructive testing (NDT) refers to a method of detecting internal flaws in engineering materials, machine components, and structures without breaking them.
These tests are essential to ascertain absolute safety and reliability in engineering applications: particularly in strategic industries such as atomic energy, missiles, defense, aeronautics, space etc. The non-destructive testing techniques can be employed on fabricated components, equipments within a laboratory, or on in-service equipments at the site.
Various Non-destructive Testing Methods
Several non-destructive testing methods are available for evaluation of material’s quality and component’s integrity. Each of these methods is based on a particular physical principle. Generally the following non-destructive testing methods are widely used.
1. Visual Examination (i) with naked eye (ii) with optical aids
2. Liquid penetrant testing
3. Magnetic particle testing
4. Eddy current testing
5. Ultrasonic testing
Modern methods: Besides above, some more modern methods are also getting increasing use for on-line monitoring of plant components: preferably in power plants, petrochemical plants, chemical and fertilizer plants etc. These methods are
- Acoustic emission testing, and
- Thermo-graphic Details
Here, we shall discuss the above six most widely used methods.
It is the simplest, cheapest and most widely used amongst all the non-destructive testing methods. A simple visual test can reveal gross surface defects easily and quickly, however for detection of finer defects, devices/equipments having high degree of precision and illumination are required.
The illumination of test piece is provided by light. An adequate lighting of about 80-100 lux is essential for visual inspection by naked eye.
Equipment used for visual inspection: A visual inspection may be accomplished by naked eye or with the help of an optical aid. These optical aids are equipped with lighting system and magnifying devices. Following devices/instruments/ equipments are generally involved in visual checks.
- Optical microscopes
- Closed circuit television
- Microprocessors and computers
- Image processing and pattern recognition techniques
The optical microscope having magnifications 10 x to 2000 x are generally used for inspection.
The borescope is used to inspect the inside portion of a hollow chamber, narrow tube, or a bore. It is a precision built-in system consisting complex arrangement of prisms, lenses, and light source.
The endoscope is similar to a borescope but incorporates a superior optical system and high-intensity light source. It also has the facility of viewing at various angles.
Flexicope is a flexible fiber-optic borescope that can view the defects around corners and `through passages’ having multi-directional changes. Its field of vision is of about 100°.
Telescope is used to examine those surfaces which are inaccessible or whose vision is concealed. For that purpose, the help is taken of a ‘periscope’ or a ‘closed circuit TV’.
Holography method is used to obtain 3-dimensional image of an object. It involves a photographic plate and a laser beam to prepare a ‘hologram’ (i.e. image of the entire surface) for comparison with a defect free standard surface. Holography technique is used for the non-destructive testing of highly complicated surfaces and precision parts.
The microprocessors and computers assist in a very reliable, quick and easy visual inspection. The techniques of image processing and pattern recognition are used when a large number of components are assessed for automatic acceptance or rejection.
Fields of application: Visual inspection method is mainly suitable for checking in the following applications.
- Leakage in components
- Misalignment of parts
- Cracks and fractures
- Corrosion and erosion
- Minute discontinuities
- Defects in welds
Liquid Penetrant Test
This non-destructive testing method utilizes the ability of a fluorescence liquid that wets the specimen surface, penetrates through the surface cavity, and forms a uniform coating that shines very bright under exposure to a visible or ultraviolet (UV) light.
Since the liquid penetrates into cavities by capillary action, it should be of low viscosity for easy flow-ability. The advantage of using UV light is that the regions holding greater amount of fluorescent penetrant (cavities hold greater amount) appear very bright, whereas the regions clear of the penetrating liquid (non-cavity area) appear black.
Test procedure: The liquid penetrant inspection is accomplished in following sequential operations:
1. Cleaning of the surface by removing scales, grease, oil, paints, dust, dirt, and other chemicals etc. followed by drying.
2. Application of penetrant liquid by spraying, brushing, or dipping the component in penetrant liquid bath. A minimum of about 30 minutes should be allowed after penetrant application.
3. Removal of excess penetrant from the entire surface except from the cracks/defects/cavities. This is essential to get contrast (bright and dark) visibility between defective and non-defective portions of the surface.
4. Thin coating of developer over the surface to draw the penetrant out of crack so as to increase its visibility. The developer also covers the surface with a color that increases the visual contrast to the penetrant.
5. Scanning under ultraviolet or visible light to recognize the defect. It may be done with UV or laser incident light and evaluation of defect may be accomplished with human eye or automated optical scanners.
Testing materials: Following materials are generally used to conduct the above test.
- Cleaners: Water, oil or chlorine based solvent.
- Penetrating liquid: Petroleum-based or water-based carrier fluid + fluorescent red color dye for visible light or yellow-green color dye for UV light.
- Developer: Dry powder, aqueous powder suspension, plastic film.
Fields of application: The liquid penetrant method is very much suitable for detecting the porosity, cracks, seams, folds, laps etc. on materials surfaces. Detection of these defects is of utmost important during testing of the following applications:
- Pressure vessels in chemical, petrochemical, fertilizer, and nuclear plants.
- Thin and thick pipes in above industries, power plants, nuclear industry.
- Penstocks in hydro power plants.
- Welded joints in strategic and hazardous equipments
- Fatigue cracks detection during service period of aircraft parts etc.
Magnetic Particle Testing
This technique is more suitable for detecting the flaws in highly magnetic materials (ferromagnetic materials) e.g. Fe, Ni, Co and their alloys which can be easily magnetized. This method is capable of detecting flaws/discontinuities that are open to surface and/or just below the surface.
The inspection method is based on the principle that “a flaw distorts the magnetic field that has been impressed upon the sample to be tested”. Wherever a flaw interrupts the flow of magnetic lines of force, some of these lines exit and re-enter the sample at certain points.
These points form the opposite magnetic poles and attract the magnetic particles sprinkled over the sample surface. The quantity of magnetic particles thus accumulated, indicates the approximate shape and size of the flaw.
The magnetic particles are applied in the form of powder or as liquid suspension. The liquid suspension form is known as ‘magnetic ink’ and is more common in use.
The color of magnetic particles is chosen to be in contrast with the color of the sample surface for easy distinguishing and detection.
For maximum sensitivity, the magnetic flux is oriented at 90° to the direction of discontinuity; but the flux is induced in different directions when the probable flaw orientations are not known.
Methods to magnetize a sample: An essential requirement of this test is the application of magnetic field (i.e. flux flow) of adequate intensity along a known direction of the sample. To produce magnetic field, various techniques such as given below, are employed.
- Magnetization using a horse-shoe type permanent magnet.
- Magnetization using an electromagnet.
- Magnetization by passing current through a bar.
- Magnetization by threading bar (central cable) in a tubular product.
- Magnetization using a coil: carrying the current.
- Magnetization by induced current flow.
Magnetization may be done using a direct current source or an alternating current source, or by half-wave rectification. For this purpose portable type, stationary type, heavy duty type, DC and AC type equipments are available.
The simplest way of magnetizing a sample (test piece) is to use a permanent magnet. The direction of flux flow lines are shown therein.
The intensity of flux can be varied by selecting either (i) a strong or weak magnet, or (ii) by introducing a gap in the flux path with the help of a non- magnetic material.
Test procedure: Testing of the sample is conducted in the following sequence.
1. The sample is cleaned of the grease, paint, scales etc. from its surface.
2. An appropriate magnetic field is applied to the sample.
3. The magnetic particles are sprinkled on the sample either in dry form or in liquid-suspension form. In liquid form, the concentration of particles is about 2% by volume. These particles are red or black in color and are fluorescent. Spherical and columnar particles of about 6 µm mean size yield the best results.
4. The distribution of powder (dense or loose, oriented or distorted) is viewed under proper illumination of daylight or black-light source. The level of illumination is generally kept up to 500 lux at the sample surface.
5. All the relevant indications are marked after draining the magnetic ink. For maintaining a permanent record, the sample area under inspection is photographed. Alternatively, it may be covered with a transparent adhesive film which when taken off, presents the indication of the sample area based on magnetic particles adhered to it.
6. The ferromagnetic sample is demagnetized by either heating above its curie temperature or by applying the coercive force in opposite direction to the originally induced magnetic field.
Eddy Current Method
Principle: Eddy current refers to such an oscillating (electric) current which is induced in a conductive material by an alternating magnetic field, owing to electromagnetic induction. The method of eddy current testing (ECT) is based on the principles of Faraday’s electromagnetic induction and Oersted’s theory.
According to Faraday’s law, “when a magnetic field cuts a conductor or vice versa, an electric current will flow through the conductor if, a closed path is provided over which the current can circulate”.
And the Oersted’s theory states that “a magnetic flux will exist around a current carrying coil in proportion to the number of turns in the coil and the current”.
Production of eddy current: Based on the above principles, an alternating current of 1 kHz to 2 MHz frequency is allowed to flow in a coil (also called probe) as shown in Figure. It produces an alternating magnetic field around it.
This coil induces an eddy current due to electromagnetic induction, in the metallic material to be inspected, when brought in close proximity of the metallic surface. The eddy current is generally parallel to the coil winding but its flow gets disturbed in presence of any flaw/defect/discontinuity in material.
Then it generates an alternating magnetic field in opposite direction which can be detected in the form of voltage across another coil, or by a change in impedance value of the original coil.
Requirement of depth of penetration and frequency: The depth of penetration and frequency of eddy current is a very important factor for accuracy of the test. If the eddy current does not penetrate through the material’s thickness, it is likely to overlook the internal defects.
The depth of penetration d and the frequency f are related to each other by
d = 500/( σµrf)1/2
Where d is in mm, f is in Hz, σ is conductivity in (ohm-m)-1, and µr, is relative permeability of material to be tested.
Applications: The eddy current method is employed for evaluating the following parameters.
- To measure the conductivity of a metal that may vary with material characteristics.
- To determine the hardness and strength of materials, since the change in these properties also changes the conductivity value.
- To determine the dimensions of thin components and sheets, and thickness of the coatings. Because difference in conductivity exists between coated and non-coated parts.
- In detecting the discontinuities such as inclusions, cracks etc.
- For online testing of wires, bars, tubes etc., and their automated analysis with the help of microprocessors.
Ultrasonic testing utilizes the high frequency ultrasonic waves for testing of surfaces and internal defects/flaws in metals and non- metals. These waves are generated by Piezoelectric transducers in the frequency range of 1 to 10 MHz.
Velocity of these waves depends on the nature of transmitting medium. It is 300 m/s in air at sea level but is lower in solid medium. Therefore in common test materials, the acoustic wavelengths λ (velocity = frequency x wavelength) are of the order of 1 to 10 mm. During the test,
- a highly directional sound beam is transmitted to the workpiece, that propagates through the material but is dissipated or reflected by flaws/defects.
- These flaws are recorded by a set of instruments and display systems using ‘pulse-echo technique’.
- The ultrasonic testing provides information regarding the size, depth and location of flaws/defect in materials.
Flaw detection set-up: A typical flaw detection system consists of the following main units.
- Ultrasonic transducer (or crystal probe)
- Pulse transmitter
- Receiver amplifier
- Cathode ray oscilloscope along with time base generator and a timer (or clock)
In this method, the ultrasonic waves are produced by piezoelectric effect within the ultrasonic transducer. The piezoelectric action is reversible interaction between elastic strain and electric field.
If the piezoelectric crystal is slightly compressed, it produces an electric signal. Similarly, when this crystal is excited with an alternating current of ultrasonic frequency, the ultrasonic waves are produced. Thus the ultrasonic transducer acts as receiver as well as transmitter.
Crystal probe: The transducer (piezoelectric crystal) may be made of Quartz, Lithium sulfate (LiSO4), Lead niobate (PbNbO3), Barium titanate (BaTiOO ), Lead zirconate titanate (PZT).
Amongst these the quartz is most commonly used material due to its excellent piezoelectric properties, mechanical and dielectric strength, and thermal stability. It can operate at high temperatures up to about 500°C. This crystal is mounted suitably in a probe before use. The probe protects the transducer from mechanical damage and also the operator from electric shock.
Testing procedure: While inspecting a work-piece, the transducer is placed on the workpiece surface as shown in Figure (a). The electric pulse is fed from the transmitter to the probe through a cable.
1. The piezoelectric transducer in the probe is shock-excited by short but high voltage pulse so that it vibrates at its own resonant frequency for a few oscillations.
2. It thus radiates an ultrasonic pulse into the work-piece.
3. The ultrasonic echo pulse is picked-up by the same probe which reconverts it into electric signal.
4. Voltage of this electric signal is much smaller than the transmitted pulse, therefore, it needs to be amplified.
5. The amplification of echo signal is accomplished in receiver- amplifier. It also filters the signal to some extent. After amplification, the signals are rectified and fed to deflection plates of cathode ray tube (CRT).
6. Any flaw/defect in the workpiece, reflects the waves. And this reflection is displayed on the oscilloscope screen as shown in Figure (b).
7. Since the deflection plates (and hence the screen) are calibrated along x and y axes, therefore depth of flaws and their location in the workpiece can be readily seen on screen.
Applications: The pulse-echo technique, described above is the most versatile method among various methods of ultrasonic inspections. It is widely used in the following applications.
- Weld testing in pressure vessels, structures, bridges, aircrafts, marines etc.
- Testing of ferrous and non-ferrous pipes, rods, bars, sheets etc.
- Testing of rails, rolled steel sections, castings (such as machine bodies) and forgings (such as crankshafts, connecting rods) etc.
- Detection of fatigue cracks in boilers and welds.
- Finding the dangerous defects caused due to corrosion in critical areas of petroleum industry components.
- In determining the thickness of components such as pipings, tubings, heat exchanger fins etc.
- In detection of slag, porosity, inclusions, abnormally large grain structure etc.
In this test, the X-rays and gamma-rays are used to detect deep seated internal defects. The short wavelengths of X-rays permit it to penetrate through the opaque materials. The depth of penetration depends on the intensity of X-rays used. For industrial radiography, the hard X-rays are used.
These are produced by X-ray generators available in different ranges of voltage and current. Modern X-ray generators are available in specifications up to 450 kV and 15 mA. They are also equipped with dual focal and ultra-small focal spots. Radiograph: During testing, the X-ray source is applied on one side of the component to be inspected and a photosensitive film is placed on the other side.
When X-ray beam is passed through the component, the film is exposed and displays the flaws as light and dark images. Voids appear as dark image while the non-defective areas appear as non-dark (light) image. The exposed film is called radiograph.
The appearance of a distinguishable image on a radiograph depends on several factors. For getting the minutest flaws (cracks or voids) radiographed sharply, following care should be taken during testing.
- The focal spot size should be as small as possible.
- Distance between the source and object should be as large as possible.
- The film should be in close contact of the object.
- Exposure time should be short.
- Scattering of X-rays should be restricted to small angles.
Radiographs with good contrast, high sensitivity, and finer resolution result in an accurate detection of flaws.
High Energy X-ray Source: Radiographic examination of much thicker components can be accomplished by using a high-energy X- ray source. The energy value of such sources may be as high as 1 MeV or more. X-ray machines like Van De Graff type electrostatic generator, synchrotron, betatron etc. are used for this purpose. These machines can examine the steel pieces of up to 300 mm thickness.
Gamma ray radiography: Gamma rays are the electromagnetic radiations that are emitted from an unstable nucleus. They emit one or a few discrete wavelengths instead of a broad band of wavelengths as emitted by X-ray machines (sources).
Therefore, the radiography by gamma- rays is independent of external power, and with simple apparatus and compact radiation source. This facilitates the inspection of even those assemblies in which the access to their interior is difficult.
Co-60, Ir-192, Cs-137, Th-170 radio-isotopes are generally used as gamma radiography source. Amongst these the cobalt-60 is most common due to intense radiation obtained from its tiny source.
Applications: The radiography method is widely used in the following applications.
- Detecting the change in material composition.
- Thickness measurement.
- Detecting inclusions, segregations, cavities etc., in castings, forgings, and weldments.
- Detecting micro-cracks in micro-miniature electronic components, in mammoth missiles and huge power plant equipments.