Nondestructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components or assemblies for discontinuities or differences in characteristics without destroying the part. NDT is 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. NDT is used to ensure the quality of materials and joining processes during the fabrication and erection phases, it’s also used to ensure that the products in use continue to have the integrity when in-service. Because NDT does not permanently alter the article being inspected, it is a highly valuable technique that can save both money and time in product evaluation, troubleshooting, and research.
High frequency sound is introduced into the part being inspected and if the sound hits a material with a different acoustic impedance (material density and acoustic velocity), some of the sound will reflect back to the sending unit and can be presented on a visual display. By knowing the speed of the sound in the part and the time required for the sound to return to the sending unit, the distance to the reflector can be calculated.
The most common sound frequencies used in UT are between 1.0 and 10.0 MHz, which are too high to be heard and do not travel through air. The lower frequencies have greater penetrating power but less sensitivity, i.e. less ability to detect small indications, while higher frequencies don’t penetrate as deeply but can detect smaller indications.
The two most common types of sound waves used in industrial application are compression (longitudinal) waves and shear (transverse) waves. Shear waves travel at approximately half the speed of longitudinal waves.
Sound is introduced into the part using an ultrasonic transducer (“probe”) that converts electrical pulses into mechanical waves, then converts returning sound back into electric impulses. This signal can be displayed as a visual representation on a digital or LCD screen. The distance from the transducer to the reflector can be determined. The signal can also be interpreted by operators to determine the type of discontinuity (lack of fusions, slag, lack of root penetration, missed edge, porosity, cracks, segregations). Because ultrasound will not travel through air a “couplant” is used between the face of the transducer and the surface of the part to allow the sound to propagate into the part.
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.
X-rays are generally used for less dense material (i.e. aluminium), or for small wall thicknesses. For thicker material or more dense materials, gamma-ray are used.
Gamma radiation is given off by decaying radioactive materials, with the three most commonly used sources of gamma radiation being Selenium-75 (Se-75), Iridium-192 (Ir-192) and Cobalt-60 (Co-60).
The recording media can be industrial x-ray film or a digital radiation detector. 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.
Films are used in conventional film radiography, while detectors are used in computed and digital radiography.
Magnetic Particle Testing is applied to detect surface and near-surface discontinuities in ferromagnetic materials. The sensitivity is higher for surface discontinuities and decreases rapidly with depth. Typical flaws that can be detected with MPI are cracks, lack of fusions, cold laps, laminations.
A magnetic field is induced on the area to be examined with a permanent magnet or an electromagnet. Ferromagnetic particles are applied on the surface. When the magnetic field encounters a discontinuity transverse to the direction of the field, the field is forced out of the part and over the discontinuity causing a leakage field that attracts the ferromagnetic particles.
The magnetic field can be generated with permanent magnets (Direct Current, DC), which penetrates deeper in the part, or electromagnets (Alternating Current, AC).
Either dry or wet magnetic powders may be used for magnetic particle testing and they may be colored with a visible dye (Color Contrast method) or a fluorescent dye (Fluorescent method).
Magnetization techniques are generally one of the following: prods, longitudinal magnetization, circular magnetization, yokes, multidirectional magnetization.
Temperature range limitations are set by the manufacturer.
Liquid penetrant testing is a method used to detect discontinuities open to the surface of nonporous materials. Typical flaws detected with PT are cracks, lack of fusions, cold laps, laminations and porosities.
Principle: a very low viscosity liquid (dye) is applied to the surface to be examined and some time is allowed to the fluid to penetrate into discontinuities open to the surface. The excess penetrant is then removed and the part is dried. At this stage the developer is applied: the developer has the function of both absorbing the penetrant trapped in the discontinuities, and create a background contrast to enhance the visibility of indications.
The dyes can be either visible under white light (Color Contrast) or visible under ultraviolet light (Fluorescent). Either of them can be used with one of the following techniques: water washable, post-emulsifying, solvent removable.
The standard technique is applied between 5°C and 52°C temperature. The method might be used with temperatures outside this range, but the procedure has to be qualified according to the specific reference code.
Visual examination is generally used to determine surface condition of the part under test, shape or dimensions of parts, misalignments, corrosion or evidence of leaking. The visual examination is inherent in most other methods, as a visual inspection is often made prior to, or as part of another non destructive test.
The technique can be direct, remote or translucent. Direct visual examination is performed when there is sufficient access to the surface or object to be examined at the right angle and with the proper light intensity. Aids may be used to perform the test, such as mirrors, artificial lights, magnifying lenses. Remote visual examination may have to be used in cases when direct visual inspection is not possible. Remote inspection can be conducted by means of mirrors, borescopes, cameras, other remote viewing devices, etc.. The light intensity, resolution of the image, angle of view shall be equivalent to a direct visual inspection.