Introduction to Nondestructive Testing For visitors who are not already familiar with NDT, the general information below is intended to provide a basic description of NDT and the most common test methods and techniques used when performing NDT. As such it is not highly detailed or all encompassing, and for more comprehensive information readers should refer to ASNT publications such as the ASNT NDT Handbooks or the ASNT Personnel Training Publications (PTP) Classroom Training Series, all of which are available from ASNT’s bookstore. Also, standards covering these test methods are listed Ｏn the Codes and Standards Bodies page under the NDT Resources Center tab. To maintain consistency, the techniques described for each test method are those listed in the 2011 edition of ASNT’s Recommended Practice No.
SNT-TC-1A. What Is Nondestructive Testing? Nondestructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components or assemblies for discontinuities, or differences in characteristics without destroying the serviceability of the part or system. In other words, when the inspection or test is completed the part can still be used. In contrast to NDT, other tests are destructive in nature and are therefore done Ｏn a limited number of samples (lot sampling), rather than Ｏn the materials, components or assemblies actually being put into service. These destructive tests are often used to determine the physical properties of materials such as impact resistance, ductility, yield and ultimate tensile strength, fracture toughness and fatigue strength, but discontinuities and differences in material characteristics are more effectively found by NDT. Today modern nondestructive tests are used in manufacturing, fabrication and in-service inspections to ensure product integrity and reliability, to control manufacturing processes, lower production costs and to maintain a uniform quality level. During construction,
NDT is used to ensure the quality of materials and joining processes during the fabrication and erection phases, and in-service NDT inspections are used to ensure that the products in use continue to have the integrity necessary to ensure their usefulness and the safety of the public. It should be noted that while the medical field uses many of the same processes, the term nondestructive testing is generally not used to describe medical applications. NDT Test Methods Test method names often refer to the type of penetrating medium or the equipment used to perform that test. Current NDT methods are: Acoustic Emission Testing (AE), Electromagnetic Testing (ET), Laser Testing Methods (LM), Leak Testing (LT), Magnetic Flux Leakage (MFL), Liquid Penetrant Testing (PT), Magnetic Particle Testing (MT), Neutron Radiographic Testing (NR), Radiographic Testing (RT), Thermal/Infrared Testing (IR), Ultrasonic Testing (UT), Vibration Analysis (VA) and Visual Testing (VT). The six most frequently used test methods are MT, PT, RT, UT, ET and VT. Each of these test methods will be described here, followed by the other, less often used test methods. 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 as shown in Figure 1. Because magnetic flux lines dont travel well in air, when very fine colored ferromagnetic particles (magnetic particles) are applied to the surface of the part the particles will be drawn into the discontinuity, reducing the air gap and producing a visible indication Ｏn the surface of the part. The magnetic particles may be a dry powder or suspended in a liquid solution, and they may be colored with a visible dye or a fluorescent dye that fluoresces under an ultraviolet (black) light. MT Techniques Yokes Most field inspections are performed using a Yoke, as shown at the right. As shown in Figure 2(a), an electric coil is wrapped around a central core, and when the current is applied, a magnetic field is generated that extends from the core down through the articulated legs into the part. This is known as longitudinal magnetization because the magnetic flux lines run from Ｏne leg to the other. When the legs are placed Ｏn a ferromagnetic part and the yoke is energized, a magnetic field is introduced into the part as shown in (b).
Because the flux lines do run from Ｏne leg to the other, discontinuities oriented perpendicular to a line drawn between the legs can be found. To ensure no indications are missed, the yoke is used Ｏnce in the position shown then used again with the yoke turned 90o so no indications are missed. Because all of the electric current is contained in the yoke and Ｏnly the magnetic field penetrates the part, this type of application is known as indirect induction. Prods Prod units use direct induction, where the current runs through the part and a circular magnetic field is generated around the legs as shown in Figure 3. Because the magnetic field between the prods is travelling perpendicular to a line drawn between the prods, indications oriented parallel to a line drawn between the prods can be found. As with the yoke, two inspections are done, the second with the prods oriented 90o to the first application. Coils Electric coils are used to generate a longitudinal magnetic field. When energized, the current creates a magnetic field around the wires making up the coil so that the resulting flux lines are oriented through the coil as shown at the right.
Because of the longitudinal field, indications in parts placed in a coil are oriented transverse to the longitudinal field. Heads Most horizontal wet bath machines (bench units) have both a coil and a set of heads through which electric current can be passed, generating a magnetic field. Most use fluorescent magnetic particles in a liquid solution, hence the name wet bath. A typical bench unit is shown at the right. When testing a part between the heads, the part is placed between the heads, the moveable head is moved up so that the part being tested is held tightly between the heads, the part is wetted down with the bath solution containing the magnetic particles and the current is applied while the particle are flowing over the part. Since the current flow is from head to head and the magnetic field is oriented 90o to the current, indications oriented parallel to a line between the heads will be visible. This type of inspection is commonly called a head shot. Central Conductor When testing hollow parts such as pipes, tubes and fittings, a conductive circular bar can be placed between the heads with the part suspended Ｏn the bar (the central conductor) as shown in Figure 6.
The part is then wetted down with the bath solution and the current is applied, travelling through the central conductor rather than through the part. The ID and OD of the part can then be inspected. As with a head shot, the magnetic field is perpendicular to the current flow, wrapping around the test piece, so indications running axially down the length of the part can be found using this technique. Liquid Penetrant Testing (PT) The basic principle of liquid penetrant testing is that when a very low viscosity (highly fluid) liquid (the penetrant) is applied to the surface of a part, it will penetrate into fissures and voids open to the surface. Ｏnce the excess penetrant is removed, the penetrant trapped in those voids will flow back out, creating an indication. Penetrant testing can be performed Ｏn magnetic and non-magnetic materials, but does not work well Ｏn porous materials. Penetrants may be visible, meaning they can be seen in ambient light, or fluorescent, requiring the use of a black light. The visible dye penetrant process is shown in Figure 7. When performing a PT inspection, it is imperative that the surface being tested is clean and free of any foreign materials or liquids that might block the penetrant from entering voids or fissures open to the surface of the part. After applying the penetrant, it is permitted to sit Ｏn the surface for a specified period of time (the penetrant dwell time), then the part is carefully cleaned to remove excess penetrant from the surface. When removing the penetrant, the operator must be careful not to remove any penetrant that has flowed into voids. A light coating of developer is then be applied to the surface and given time (developer dwell time) to allow the penetrant from any voids or fissures to seep up into the developer, creating a visible indication. Following the prescribed developer dwell time, the part is inspected visually, with the aid of a black light for fluorescent penetrants. Most developers are fine-grained, white talcum-like powders that provide a color contrast to the penetrant being used. PT Techniques Solvent Removable penetrants are those penetrants that require a solvent other than water to remove the excess penetrant. These penetrants are usually visible in nature, commonly dyed a bright red color that will contrast well against a white developer. The penetrant is usually sprayed or brushed Ｏnto the part, then after the penetrant dwell time has expired, the part is cleaned with a cloth dampened with penetrant cleaner after which the developer is applied.
Following the developer dwell time the part is examined to detect any penetrant bleed-out showing through the developer. Water-washable penetrants have an emulsifier included in the penetrant that allows the penetrant to be removed using a water spray. They are most often applied by dipping the part in a penetrant tank, but the penetrant may be applied to large parts by spraying or brushing. Ｏnce the part is fully covered with penetrant, the part is placed Ｏn a drain board for the penetrant dwell time, then taken to a rinse station where it is washed with a course water spray to remove the excess penetrant. Ｏnce the excess penetrant has been removed, the part may be placed in a warm air dryer or in front of a gentle fan until the water has been removed.
The part can then be placed in a dry developer tank and coated with developer, or allowed to sit for the remaining dwell time then inspected. Post-emulsifiable penetrants are penetrants that do not have an emulsifier included in its chemical make-up like water-washable penetrants. Post-emulsifiable penetrants are applied in a similar manner, but prior to the water-washing step, emulsifier is applied to the surface for a prescribed period of time (emulsifier dwell) to remove the excess penetrant. When the emulsifier dwell time has elapsed, the part is subjected to the same water wash and developing process used for water-washable penetrants. Emulsifiers can be lipophilic (oil-based) or hydrophilic (water-based).
Radiographic Testing (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. For thinner or less dense materials such as aluminum, electrically generated x-radiation (X-rays) are commonly used, and for thicker or denser materials, gamma radiation is generally used. Gamma radiation is given off by decaying radioactive materials, with the two most commonly used sources of gamma radiation being Iridium-192 (Ir-192) and Cobalt-60 (Co-60). IR-192 is generally used for steel up to 2-1/2 - 3 inches, depending Ｏn the Curie strength of the source, and Co-60 is usually used for thicker materials due to its greater penetrating ability. The recording media can be industrial x-ray film or Ｏne of several types of digital radiation detectors. With both, the radiation passing through the test object exposes the media, causing an end effect of having darker areas where more radiation has passed through the part and lighter areas where less radiation has penetrated. If there is a void or defect in the part, more radiation passes through, causing a darker image Ｏn the film or detector, as shown in Figure 8.
RT Techniques Film radiography uses a film made up of a thin transparent plastic coated with a fine layer of silver bromide Ｏn Ｏne or both sides of the plastic. When exposed to radiation these crystals undergo a reaction that allows them, when developed, to convert to black metallic silver. That silver is then fixed to the plastic during the developing process, and when dried, becomes a finished radiographic film. To be a usable film, the area of interest (weld area, etc.) Ｏn the film must be within a certain density (darkness) range and must show enough contrast and sensitivity so that discontinuities of interest can be seen. These items are a function of the strength of the radiation, the distance of the source from the film and the thickness of the part being inspected. If any of these parameters are not met, another exposure (shot) must be made for that area of the part. Computed radiography (CR) is a transitional technology between film and direct digital radiography. This technique uses a reusable, flexible, photo-stimulated phosphor (PSP) plate which is loaded into a cassette and is exposed in a manner similar to traditional film radiography. The cassette is then placed in a laser reader where it is scanned and translated into a digital image, which take from Ｏne to five minutes. The image can then be uploaded to a computer or other electronic media for interpretation and storage. Computed tomography (CT) uses a computer to reconstruct an image of a cross sectional plane of an object as opposed to a conventional radiograph, as shown in Figure 9.
