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Non Destructive Testing
Non-destructive Testing (NDT) plays an important role in assuring that structural and mechanical components perform their function in a safe, reliable, and cost-effective manner. NDT technicians perform the necessary tests to locate the indicators and discontinuities that may cause failures or shutdowns in such systems. These tests are performed in a manner that does not affect the future usefulness of the object or material – hence, the name “non-destructive.” NDT allows for careful and thorough materials evaluation without the need for deconstruction or damage.
NDT is typically used at various points in a part’s life cycle. NDT can be used prior to the use of a component for the sake of quality control. NDT is also employed while components are in use to detect service-related conditions caused by wear, fatigue, corrosion, stress, or other factors that affect reliability.
NDT Technologies Include:
Visual and Optical Testing (VT)
Visual testing is the most commonly used testing method in the industry. Because most test methods require that the operator looks at the surface of the part being inspected, visual inspection is inherent in most of the other test methods. As the name implies, VT involves the visual observation of the surface of a test object to evaluate the presence of surface discontinuities. VT inspections may be by Direct Viewing, using line-of-sight vision, or may be enhanced with the use of optical instruments such as magnifying glasses, mirrors, boroscopes, charge-coupled devices (CCDs), and computer-assisted viewing systems (Remote Viewing). Corrosion, misalignment of parts, physical damage, and cracks are just some of the discontinuities that may be detected by visual examinations.
Radiography (RT)
Industrial radiography involves exposing a test object to penetrating radiation so that the radiation passes through the object being inspected and a recording medium placed against the opposite side of that object. Radiographic Examination involves using radioactive isotopes (gamma rays) or X-rays on materials to peer qualitatively for indications the same way a doctor looks for fractures or other conditions within the body. Radiation is directed through a part and projected onto film or a digital detection device leaving an image that can be examined by the qualified Radiographer.
Ultrasonic Testing (UT)
Ultrasonic testing uses the same principle as is used in naval SONAR and fish finders. Ultra-high frequency sound is introduced into the part being inspected and if the sound hits a material with a different acoustic impedance (density and acoustic velocity), some of the sounds will reflect back to the sending unit and can be presented on a visual display. By knowing the speed of the sound through the part (the acoustic velocity) and the time required for the sound to return to the sending unit, the distance to the reflector (the indication with the different acoustic impedance) can be determined.
Magnetic Particle Testing (MT)
Magnetic Particle Testing uses one or more magnetic fields to locate surface and near-surface discontinuities in ferromagnetic materials. The magnetic field can be applied with a permanent magnet or an electromagnet. When using an electromagnet, the field is present only when the current is being applied. When the magnetic field encounters a discontinuity transverse to the direction of the magnetic field, the flux lines produce a magnetic flux leakage field of their own.
Penetrant Testing (PT)
Penetrant Examination is performed with a dye solution. Once applied to the surface, the dye will effectively penetrate any surface-breaking cavity. Excess solution is removed from the object. A developer is then applied to draw out any penetrant that remains unseen. With fluorescent dyes, ultraviolet light is used to make the “bleed-out” fluoresce brightly, allowing imperfections to be readily seen. With visible dyes, color contrast between the penetrant and developer makes the “bleed-out” easy to see.
Electromagnetic Testing (ET)
Electromagnetic testing is a general test category that includes Eddy Current testing, Alternating Current Field Measurement (ACFM), and Remote Field testing. While magnetic particle testing is also an electromagnetic test, due to its widespread use it is considered a stand-alone test method rather than an electromagnetic testing technique. All of these techniques use the induction of an electric current or magnetic field into a conductive part, then the resulting effects are recorded and evaluated.
Phased Array Ultrasonic Testing (PAUT)
PAUT Inspection (Phased Array Ultrasonic Testing) is one of the advanced Ultrasonic testing methods which are famous for its higher probability of detection and precise measurement capability. PAUT Inspection can be used as a direct replacement for radiography testing for piping. This is an advanced method of ultrasonic testing that has applications in medical imaging and industrial non-destructive testing. It can also be used for weld overlays and the heat-affected zones of many components as well such as piping, pipeline, pressure vessels, clad material, storage tanks, and structural steel.
Phased arrays are used for a wide variety of inspection and measurement applications, and they can be used for any job done by conventional ultrasonic. PAUT Inspection / Phased Array Ultrasonic Testing reduces inspection time by simultaneously collecting multiple angle ultrasonic data in a single pass scan from either side of the weld. By ensuring accurate scan plans and specifically designed techniques, construction defects are readily detected, sized, and sentenced accordingly.
Time of Flight Diffraction (TOFD)
Time of Flight Diffraction (TOFD) is a reliable method of non-destructive ultrasonic testing (UT) used to detect flaws in welds. TOFD uses the time of flight of an ultrasonic pulse to find the location and sizing of a reflector. It can also be used for weld overlays and the heat-affected zones of other components as well such as piping, pressure vessels, clad material, storage tanks, and structural steel. TOFD is one of the fastest methods of non-destructive testing because, unlike other types of UT methods, generally only one scan is required to find any defect information within the weld. It can locate and measure the size of many different types of defects with incredible precision. It also has a high degree of repeatability.