Non-Destructive Testing (NDT) refers to the process of inspecting, evaluating the quality of the materials while preserving the original form of the material without affecting its usability. NDT is best used in detecting discontinuities and flaws, such as cracks, voids, etc., compared to testing the physical properties of materials such as tensile strength, hardness, impact resistance, etc., which are best investigated using Destructive Tests.
This article takes you through the advantages of NDT, steps to be taken before conducting NDT, and an overview of the fourteen (14) NDT methods used in different industries.
What are the advantages of Non-Destructive Testing?
Aside from the fact that NDT maintains the serviceability of the material even after testing, it provides higher confidence in ensuring that the products do not just have a uniform but also high-quality properties. By using non-destructive testing, the quality of the materials is controlled and maintained to be in accordance with standards; therefore, minimizing process rejections and additional costs while ensuring safe serviceability.
What are the steps to be taken prior to Non-Destructive Testing?
Since the methods to be discussed concern the surface and volumetric levels of inspections, it is essential to prepare the test specimen accordingly to increase the accuracy of the tests.
Here are the critical steps that must be done before the conduct of test methods:
- Identify what defects and discontinuity you are finding to choose the proper test method.
- Gauge the accuracy limit that you are aiming for.
- Clean the surface of the specimen, if applicable. However, this is not a choice for tests used to detect corrosion, erosion, pitting, and any other phenomena that include mass loss.
What are the Non-Destructive Testing (NDT) methods?
Fourteen (14) Non-Destructive Testing methods are used in different industries, with their advantages and disadvantages.
1. Visual Testing (VT)
Visual Testing is usually the first step done in most material evaluations, thus making it inherent with the other test methods. This method is generally done by examining the surfaces for discontinuities and damages using different visual and imaging equipment. Mirrors are used in examining internal facades that are not accessible to direct viewing. Rulers are used in checking on dimensional characteristics of the components. Visual examinations may also be done using cameras to record the macroscopic features. In contrast, optical microscopes, Scanning Electron Microscopy (SEM), or Transmission Electron Microscopy (TEM) are used for the microscopic ones for topography and surface analysis. Magnifying lenses help inspect fracture surfaces and provide greater detail.
Other indicators that may be observed through the naked eyes include changes in color, suggesting the presence of burning or corrosions, and change in shape, indicating the presence of necking and other deformations. Through visual testing, subsequent NDTs may be identified and narrowed down to confirm the assessments further.
By using VT, there can be immediate results with minimum skills required from the viewer. However, it is only suitable for detecting more significant defects on the surface, and sample preparations may cause false-positive results such as misinterpretation of scratches as cracks from the manufacturing or processing.
2. Liquid Penetrant Testing (PT)
The capillary action principle governs Liquid Penetrant Testing. A freely flowing visible liquid dye penetrant is applied on the surface. It then is allowed to penetrate cracks and voids on the surface for a considerably known amount of time (also known as penetrant dwell time). The excess penetrant is rinsed off after ensuring that the liquid has fully gained access to the fissures. A chalk-like developer is then applied and allowed to coat the material within its dwell time thoroughly. This coating then draws the penetrant dye from the defects, giving the location of the cracks or voids, as seen using ultraviolet light.
Some of the advantages of using this method include its fast result, easy execution, and inexpensive means of inspection, regardless of the amount and size of the samples. It is also susceptible to tiny surface defects. It can also be used on a wide range of materials: magnetic (metals) and non-magnetic materials (other metals, glass, plastics, fired ceramics, etc.). However, it is not useable on porous materials. Porosity, cracks, fractures, laps, seams, and other defects on the surface that may be caused by other mechanical processes may be best detected by this method.
3. Leak Testing (LT)
In this method, the escape of liquids or gases from a pressurized system or an entry of these matters into an evacuated component that is intended to hold these media are observed. The location of the leaks, the rate of the leakage, and the tightness of the specimen may be noted in the execution of this test.
Different techniques may be used to perform the leak testing:
- Immersion method is the most used leak testing method since it provides immediate yet accurate results at a relatively low cost. By submerging the test piece in the water contained in a tank, the presence of bubbles on the surface indicates holes in the test piece. This can also be done with the aid of a bubble solution. Once pressurized and immersed in a container with liquid, a soap solution is applied to the surface. If there are leaks, a formation of bubbles may be observed, indicating the leak’s location. This test is suitable for open systems.
- On the other hand, pressure measurement is used only for the closed systems. There are two ways of doing this: relative/absolute pressure methodand differential pressure method. The relative/absolute pressure method measures the pressure difference against the ambient pressure and the absolute vacuum, respectively. A decrease in pressure or vacuum suggests the presence of a leak in the system. On the other hand, the differential pressure method involves pressurizing the test specimen and then comparing the pressure of a reference volume whose tightness is already established. The differences then are measured using a differential pressure transducer. However, these tests are sensitive to temperature changes as temperature is directly proportional to the change with the pressure. Thus, the operating temperature must be closely monitored.
- Mass spectrometer testing is done by pressurizing the material with helium or a helium and air mixture then evaluating the surface using a sniffer or the sampling probe, which sends an air sample back to the spectrometer. The mass spectrometer is used to detect the presence and amount of collected helium, suggesting that there was leakage in the system. This can be done in two ways: vacuum methodand overpressure method. In the vacuum method, the helium is sprayed on the specimen, and if there are leaks, the helium is expected to enter these and therefore be detected by the leak test equipment. On the other hand, the overpressure method involves filling in the object with the search gas under slight overpressure, and if there are gas escapes through leaks that are present on the surface, then it will be detected by a sniffer.
- The same principle is observed in the halogen diode testing, where the pressurizing is done using a mixture of air and a halogen-based search or tracer gas, and a “sniffer” is used to detect the leaks.
4. Magnetic Particle Testing (MT)
Magnetic Particle Testing detects surface and subsurface discontinuities only in ferromagnetic materials by magnetizing the test parts. This is done by applying a magnetic field on the surface. Then, magnetic particles, such as iron filings, are introduced to the magnetic field to reveal the magnetic field fluxes. If there are discontinuities, these fluxes will tend to distort the natural pattern exhibited on the field. To ensure the validity of the test, a repeat test with the magnetic field set at 90 degrees to the original direction of the magnetization must be done.
Magnetization can be done in five (5) ways:
- In the prod technique, portable prod or probe-type electrical contacts are used to induce magnetization. These prods are pressed against the surface of the test area. Prod spacing is usually kept between 75 – 200 millimeters, while the magnetizing current source must be greater than 25 volts. The shorter the prod spacing, the better the sensitivity of the test will be.
- For the longitudinal technique, a current is passed through a multi-turn fixed coil that is wrapped around the test specimen. This produces a magnetic field that is parallel to the long axis of the coil and the component.
- Circular technique is the same as the longitudinal technique; however, the circular magnetic field is perpendicular to the direction of the current flow in the test material. This is best used for cylindrical or ring-shaped parts.
- Yoke technique uses a U-shaped magnetic either alternating or direct current electromagnetic or permanent magnet yokes that are carrying the magnetization current to produce to induce a longitudinal magnetic field in the test specimen.
- In multi-directional technique, power packs with high amperage energize the area of inspection to induce magnetization. This results in overall magnetization of the area, with generated circular or longitudinal magnetic fields. However, this technique requires that the field shall be obtained in at least two nearly normal directions with balanced intensities.
One of the advantages of magnetic particle testing is that it is more sensitive than liquid penetrant testing to detect tightly closed cracks, giving more accurate detections. It can also be used in the fast inspection of large surfaces while providing low equipment costs. However, this method can only be used on ferromagnetic materials, and it requires an electricity supply unless a permanent magnetic yoke is used. The surface must also be well cleaned, and it cannot be used if there is a thick paint coating.
5. Electromagnetic Testing (ET)
This is also known as Eddy Current Testing as it utilizes eddy currents or small circular closed loops induced in metallic materials using a coil excited by an electrical current induced by an alternating magnetic field. Discontinuities can disrupt the eddy currents on the material, as similar to the disruption of magnetic fields in the magnetic particle testing, except electromagnetic testing accommodates a wider variety of discontinuities. This is best used to detect surface and near-surface flaws within six (6) millimeters of the surface.
Electromagnetic Testing can also be used to determine the thickness of coatings and the materials and other dimensional and physical characteristics such as electrical conductivity and magnetic permeability. This method is only applicable to conductive materials and some ferromagnetic materials. As electromagnetic testing sensitive to the presence of the flaws, surface contact is not necessary.
6. Magnetic Flux Leakage (MFL)
Magnetic Flux Leakage detects discontinuities by examining changes or disruptions in the normal flux patterns saturated by a magnetic field induced by permanent magnets. The defects influence the path of the magnetic field and cause some of the flux to leak out of the tube wall and get detected by Hall-effect sensors. The pull speed of the probe, its shape, dimension, and the location of the defects determine the size of the leakage. This is all monitored on a computer screen. Magnetic Flux Leakage test can be best used in detecting corrosion, erosion, pitting, and others.
7. Radiographic Testing (RT)
Radiographic Testing uses x-ray or gamma radiation to generate images that show that defects inside the test specimen. In this method, the radiation passes through the test specimen, and the differential absorption of the material due to differences in the density and thickness will result in unabsorbed radiation that will be then recorded by a medium placed on the opposite side of the test specimen.
RT is best used to detect subsurface flaws in welded joints and hidden flaws located at significant depths. It is non-destructive, with the ease of sparing the material from cleaning preparations, and it can be used with the specimen is in operation.
Three (3) techniques may be used in this test:
- In film or paper radiography, a two-dimensional projected image is generated on a sheet of film or paper.
- Radioscopy processes the image that can be manipulated in real-time into a viewing screening or a monitor.
- Computed tomography uses computer programs to digitize the sample in a large number of different directions to calculate the absorption at each position and then display the X-ray images.
Gamma radiation is generally used for thicker or denser materials. For thinner or less dense materials such as aluminum, electrically generated X-rays are commonly used.
8. Neutron Radiographic Testing (NR)
Like the radiographic testing, neutron radiographic testing generates images of the internal structures of the test objects using differential radiation medium, but in the form of low energy neutrons rather than the usual electron-based X- rays and gamma rays.
Neutron radiographic mechanism is based on the similar penetrating characteristics of the low-energy neutrons with the X-rays, but with a key difference in its intensity. Its resulting cross-section or rate of attendant loss of energy (attenuation rate) for light materials is much more opaque and much more transparent than the x-ray cross-sections for dense materials. This gives the advantage of examining energetic devices, producing both the structural and internal scans of the test objects. Furthermore, its enhanced capability to be strongly absorbed by hydrogen, boron, and other elements renders its main advantage of detecting corrosion products, oil, water, and plastic materials inside the test assemblies.
Common sources of neutrons for radiography are atomic reactors, particle accelerators, and radioisotopes. Like RT method, real-time imaging and tomography can be used in its execution.
9. Ultrasonic Testing (UT)
Ultrasonic Testing is a volumetric NDT method that uses high-frequency sound waves that are then recorded using an ultrasonic transducer. Defects on the material then deflect the sound waves, resulting in attendant loss of energy or attenuation upon the traversing of the sound waves back to the transducer. The reflected beam is then recorded and analyzed to identify the presence and location of the discontinuities.
This Testing typically uses frequencies that range from 0.5 to 10,000 MHz that are above the range of human hearing (20 – 20,000 kHz).
There are three types of modes:
- A-Scan mode is a quantitative display of signal intensity received as a function of time is obtained at one point on the surface of the test piece.
- B-scan mode is the quantitative display of time of flight along a line on the surface.
- C-Scan mode is a two-dimensional semi-quantitative display of echo intensity of a test piece and shows the plane section of a test piece but provides no depth or orientation information, and the entire surface is scanned or rostered.
Ultrasonic Testing is best used to detect surface and subsurface flows since it has high sensitivity and penetrating power. It can also give information about the size, orientation, shape, and nature of the defects. However, the accuracy of this method is highly dependent on the sound transmissibility and signal noise capacity of the material. Due to low sound transmission and high signal noise, cast iron and other coarse-grained materials are not suitable to be tested using this method. Additionally, parallel linear flaws in the direction of the sound waves may become undetected.
10. Acoustic Emission Testing (AE)
In Acoustic Emission Testing, a localized external force that would produce waves is applied to the test specimen. These are high-frequency (30 kHz – 5 MHz) waves that radiate out into the structure and are then detected by a piezoelectric transducer. The presence of deformations such as crack growth, plastic deformation, or phase transformation would suggest sudden movement or rapid release of strain energy, causing the following occurrence of stress waves or acoustic emissions. As the length of the cracks increases, the strain energy released increases and the acoustic emissions.
This test may be used in detecting surface and subsurface flow. By placing multiple sensors and raising the stress in the material, the resulting data can be evaluated to locate discontinuities in the workpiece and its relative severity. Acoustic emission testing applications include monitoring fatigue and stress corrosion crack growth and delayed cracking associated with hydrogen embrittlement.
11. Vibration Analysis (VA)
This test aims to determine how the material responds to an applied external force that will cause vibrations (free vibration, forced vibration, flow-induced vibration, random vibration, etc.).
This method uses different techniques:
- In vibration surveying and monitoring, potable vibration sensors are installed in multiple locations around the material to gather data about the type and magnitude of the vibrational modes. Data will then help narrow down the next steps for the analysis.
- Experimental modal analysis applies loads that simulate the operating conditions of the material and measuring the vibration signals. This is done when the material is not in service.
- Operational modal analysis, on the other hand, is performed while the material is still in operation and when the background noise signals are tiny to no distinguishable from actual vibration signals.
- Computer simulations include finite element analysis (FEA) and computational fluid dynamics (CFD) to simulate operating conditions involving flow-induced vibrations.
This method supports remote condition monitoring and real-time reaction to loads. However, fault localization is challenging to conduct, and crack propagation is difficult to monitor.
12. Guided Wave (GW)
Guided waves are ultrasonic waves that propagate along a direction that is guided by the geometry of the material being inspected. As the waves propagate along a pipe axis, defects may deflect these waves, causing a loss in the expected waves received by the detector or transducer at the end of the material.
It utilizes a lower frequency range of only 5 – 250 kHz, way lower than the ultrasonic testing, but this helps generate non-dispersive waves that reduce the attenuation for long-range inspections. The nature of the test also makes it produce fast inspection speed and access to areas with limited access and is independent of the fluid type inside the pipe.
13. Laser Testing (LM)
Laser testing utilizes the coherent light amplified by stimulated emission of radiation.
Laser testing can be executed using three (3) techniques:
- Holographic technique uses laser beam scans across the surface of a material that has been induced with stress. The beam will reflect to the sensors, and an image of the topographical map will reveal the deformations on the surface. This map will be compared to an undamaged reference sample to locate and analyze flaws.
- Laser shearography uses an interferometer to detect out-of-plane derivatives of deformation created by applying external stresses. Stressed and non-stressed images picked up by a charge-coupled device are superimposed to reveal any deformities.
- In laser profilometry, a high-speed rotating laser light scans the surface of the test specimen in two-dimension (2D), and the reflected light passes through a photodetector, producing signals that indicate its distance from the 2D image plane. This goes on for more runs, with adjusting the position of the focal spot and the distance of the surface until a 3D image is produced similar to a surface topographic image.
14. Thermal/Infrared Testing (IR)
Thermal or infrared testing, also known as thermography, is used to obtain an image of the distribution of heat over the surface of an object. The visible range of the spectrum of wavelength is increased, allowing the measurability of the radiated energy using infrared cameras.
Different techniques may be used in executing this test:
- Passive thermography is a method done before or after the service operation of the test specimen and does not need an external energy source.
- Active thermography differs from the passive method as it requires an external energy source.
- Flash thermography applies a pulse of light energy then measures the changes in the surface temperature of the material.
- Vibrothermography uses acoustic energy to cause friction on two sheared crack surfaces, resulting in heat production.
- Lock-in thermography periodically applies external energy to produce differential heat onto the surface of interest and is best suitable for thicker-walled components.
IR thermography is best used in detecting corrosion, delamination, voids, inclusions, and other flows that affect heat transfer. It is also used in locating hot spots and trouble spots that usually experience too much friction. However, to ensure the effectiveness of the testing, there must be sufficient differences between the component and its surroundings. It is also dependent on the surface properties such as emissivity, reflectance, and transmittance.
NDT plays a significant role in achieving and maintaining high-quality functionality and serviceability of the material and attaining safer working conditions and operations in the respective industries it is used. Choosing the proper method for testing the materials contributes to decreasing the operational and maintenance costs and resources of the operations, further improving the quality of the ventures.