NON-DESTRUCTIVE TESTINGS

NDT is a testing and analysis technique used by industry to evaluate the properties of a material, component, structure or system for characteristic differences or welding defects and discontinuities without causing damage to the original part. NDT also known as non-destructive examination (NDE), non-destructive inspection (NDI) and non-destructive evaluation.

Non-Destructive Testing Methods

Radiographic Testing (RT)

Radiography Testing the test-part is placed between the radiation source and film (or detector). The material density and thickness differences of the test-part will attenuate (i.e. reduce) the penetrating radiation through interaction processes involving scattering and/or absorption. The differences in absorption are then recorded on film(s) or through an electronic means. In industrial radiography there are several imaging methods available, techniques to display the final image, i.e. Film Radiography, Real Time Radiography (RTR), Computed Tomography (CT), Digital Radiography (DR), and Computed Radiography (CR).

There are two different radioactive sources available for industrial use; X-ray and Gamma-ray. These radiation sources use higher energy level, i.e., shorter wavelength, versions of the electromagnetic waves. Because of the radioactivity involved in radiography testing, it is of paramount importance to ensure that the Local Rules is strictly adhered during operation.

 Ultrasonic Testing (UT)

UT inspection system consists of an ultrasonic transducer, pulser/receiver, and display unit. A pulser/receiver is an electronic device that can produce high voltage electrical pulses to the transducer. When driven by the pulser, the transducer generates high frequency ultrasonic sound energy into the material in the form of sound waves.

When there are discontinuities such as inclusions, porosity, cracks, etc. in the sound path, part of the mechanical energy will be reflected from the discontinuities' (reflectors') surface.

The reflected sound waves signal received by the transducer is then transformed back into an electrical signal and its intensity is shown on the display unit.

The sound waves travel time can be directly related to the distance that the signal has travelled. From the signal, information about reflector location, size, orientation and other features can be determined.

Liquid Penetrant Testing (PT)

Liquid penetrant testing involves the application of a fluid with low viscosity to the material to be tested. This fluid seeps into any defects such as cracks or porosity before a developer is applied which allows the penetrant liquid to seep upwards and create a visible indication of the flaw. Liquid penetrant tests can be conducted using solvent removable penetrants, water washable penetrants or post-emulsifiable penetrants.

Magnetic Particle Testing (MT)

Magnetic fields to find discontinuities at or near the surface of ferromagnetic materials. The magnetic field can be created with a permanent magnet or an electromagnet, which requires a current to be applied.

The magnetic field will highlight any discontinuities as the magnetic flux lines produce leakage, which can be seen by using magnetic particles that are drawn into the discontinuity. 

Acoustic Emission Testing (AE)

NDT technique, which relies on detecting the short bursts of ultrasound emitted by active cracks under a load. Sensors dispersed over the surface the structure detect the AE. It is even possible to detect AE from plasticization in highly stressed areas before a crack forms. Frequently a method for use during proof tests of a pressure vessel.

Electromagnetic Testing (ET)

This testing method uses an electric current or magnetic field which is passed through a conductive part. There are three types of electromagnetic testing, including eddy current testing, alternating current field measurement (ACFM) and remote field testing (RFT).

Eddy current testing uses an alternating current coil to induce an electromagnetic field into the test piece, alternating current field measurement and remote field testing both use a probe to introduce a magnetic field, with RFT generally used to test pipes.

Ground Penetrating Radar (GPR)

This geophysical NDT method sends radar pulses through the surface of a material or subsurface structure, such as rock, ice, water or soil. The waves are reflected or refracted when they encounter a buried object or material boundary with different electromagnetic properties.

Laser Testing Methods (LM)

Laser testing falls into three categories including holographic testing, laser profilometry and laser stereography.

Holographic testing uses a laser to detect changes in the surface of the material which has been subjected to stress such as heat, pressure or vibration. The results are then compared to an undamaged reference sample to show defects.

Laser profilometry uses a high-speed rotating laser light source and miniature optics to detect corrosion, pitting, erosion and cracks by detecting changes in the surface via a 3D image generated from the surface topography.

Laser stereography uses laser light to create an image before the surface is stressed and a new image is created. These images are compared to one another to determine if any defects are present.

Leak Testing (LT)

Leak testing can be broken down into four different methods - bubble leak testing, pressure change testing, halogen diode testing and mass spectrometer testing.

Bubble leak testing uses a tank of liquid, or a soap solution for larger parts, to detect gas (usually air) leaking from the test piece in the form of bubbles.

Only used on closed systems, pressure change testing uses either pressure or a vacuum to monitor the test piece. A loss of pressure or vacuum over a set time span will show that there is a leak in the system. 

Halogen diode testing also uses pressure to find leaks, except in this case air and a halogen-based tracer gas are mixed together and a halogen diode detection unit (or 'sniffer') is used to locate any leaks.

Mass spectrometer testing uses helium or a helium and air mix inside a test chamber with a 'sniffer' to detect any changes in the air sample, which would indicate a leak. Alternatively, a vacuum can be used, in which case the mass spectrometer will sample the vacuum chamber to detect ionized helium, which will show that there has been a leak.

Magnetic Flux Leakage (MFL)

This method uses a powerful magnet to create magnetic fields which saturate steel structures such as pipelines and storage tanks. A sensor is then used to detect changes in magnetic flux density which show any reduction in material due to pitting, erosion or corrosion.

Microwave Testing

This method is restricted to use on dielectric materials and uses microwave frequencies transmitted and received by a test probe. The test probe detects changes in dielectric properties, such as shrinkage cavities, pores, foreign materials or cracks and displays the results as B or C scans.

Neutron Radiographic Testing (NR)

Neutron radiography uses a beam of low energy neutrons to penetrate into the workpiece. While the beam is transparent in metallic materials most organic materials allow the beam to be seen, allowing the structural and internal components to be viewed and examined to detect flaws.

Thermal/Infrared Testing (IRT)

Thermography, including infrared thermography (IRT), thermal imaging, and thermal video, is a thermal imaging science used for multiple applications. The technique uses thermographic cameras to detect radiation in the long-infrared range of the electromagnetic spectrum (around 9,000–14,000 nanometers or 9–14 µm) and produces images of this thermal radiation, known as thermograms.

The blackbody radiation law states that all objects with a temperature above absolute zero emit infrared radiation, meaning that it is possible to ‘see’ with thermography without visible illumination by reading temperature variations. As the temperature of an object increases, so does the amount of radiation it emits, which means that thermography can read variations in temperature range.

When using a thermal imaging camera, warm objects show up against cooler backgrounds regardless of time of day, making thermography useful to the military and for surveillance cameras. Thermography also has lots of uses in the medical industry, including items such as infrared thermometers.

Vibration Analysis (VA)

This process uses sensors to measure the vibration signatures from rotating machinery in order to assess the condition of the equipment. The types of sensors used include displacement sensors, velocity sensors, and accelerometers.

Visual Testing (VT)

Visual testing also known as visual inspection is one of the most common techniques which involves the operator looking at the test piece. This can be aided by the use of optical instruments such as magnifying glasses or computer-assisted systems (known as 'Remote Viewing').

This method allows for the detection of corrosion, misalignment, damage, cracks, and more. Visual testing is inherent in most other types of NDT as they will generally require an operator to look for defects.