Non-Destructive Testing or Nondestructive Testing, also known as NDT; is a group of techniques and methods used to evaluate the properties of a material or component without causing damage to that material or component. NDT is also referred to as Nondestructive Examination/Evaluation (NDE) and Nondestructive Inspection (NDI). All are different terms for the same sets of techniques and methods. Because NDT does not alter or damage the component or material being inspected, it is a highly valuable technique that can save both money and time in product evaluation, troubleshooting, and research.
The six most frequently used NDT methods and the six methods The Ocean Corporation trains in are eddy-current (ET), magnetic-particle (MT), liquid penetrant (PT), radiographic (RT), ultrasonic (UT), and visual testing (VT). NDT is commonly used in forensic engineering, mechanical engineering, petroleum engineering, electrical engineering, civil engineering, systems engineering, aeronautical engineering, medicine, and even in art.
NDT relies upon a multitude of different principles from electromagnetism (MT & ET) to sound (UT), and even capillary action (PT) to examine materials ranging from metals like steel, aluminum, and copper to plastics, concrete, and laminate like fiberglass and carbon fiber. NDT Technicians are looking for a multitude of indications in different materials such as; porosity and lack of fusion in welds, thickness variations and corrosion in piping/tubing and pressure vessels, stress fractures from material fatigue, and other types of manufacturing defects and equipment/materials degradation.
Visual Testing is the most prevalent method of NDT. VT is performed before virtually all other methods of NDT. The primary technique of VT is merely to look the component over for defects and indications that are visible to the naked eye. It can also include the use of remote viewing systems like drones/UAVs, and ROVs, as well as instruments like calipers, boroscopes, and microscopes.
Penetrant testing, also called dye penetrant inspection (DPI), or liquid penetrate inspection (LPI), is a widely applied and low-cost inspection method used to check surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). The penetrant may be applied to all non-ferrous(cannot be magnetized) materials and ferrous(can be magnetized) materials, although for ferrous components magnetic-particle inspection is often used instead for its subsurface detection capability. PT is used to detect casting, forging and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.
PT is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed, and a developer is applied. The developer helps to draw penetrant out of the flaw so that an invisible indication becomes visible to the inspector. Inspection is performed under an ultraviolet or white light, depending on the type of dye used – fluorescent or nonfluorescent.
The main advantages of PT are the speed of the test and the low cost. Disadvantages include the detection of only surface flaws, skin irritation, and the inspection should be on a smooth, clean surface where excessive penetrant can be removed before being developed. Conducting the test on rough surfaces, such as “as-welded” welds, will make it difficult to remove any excessive penetrant and could result in false indications. Water-washable penetrant should be considered here if no other option is available. Also, on certain surfaces, a significant enough color contrast cannot be achieved, or the dye will stain the workpiece.
Magnetic particle testing or magnetic particle Inspection (MPI) is an NDT process for detecting surface and shallow subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object, and a magnetic field is formed in the material. Indirect magnetization occurs when no electrical current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current, which may be either alternating current (AC) or some form of direct current (DC).
The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak since air cannot support as much magnetic field per unit volume as metals.
To identify a leak, ferrous(typically iron) particles, either dry or in a wet suspension, are applied to a part. These are attracted to an area of flux leakage and form what is known as an indication, which is evaluated to determine its nature, cause, and course of action if any.
Limited training is required for the operator; although experience is quite valuable. Proper cleaning is necessary to assure that surface contaminants have been removed, and any defects present are clean and dry. Some cleaning methods have been shown to be detrimental to test sensitivity, so acid etching to remove metal smearing and re-open the defect may be necessary.
Ultrasonic testing (UT) is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz, and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws. A typical example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion.
Ultrasonic testing is often performed on steel and other metals and alloys, though it can also be used on concrete, wood, and composites, albeit with less resolution. It is used in many industries including steel and aluminum construction, metallurgy, manufacturing, aerospace, automotive and other transportation sectors.
In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer is typically separated from the test object by a couplant (such as oil) or by water, as in immersion testing. However, when ultrasonic testing is conducted with an Electromagnetic Acoustic Transducer (EMAT) the use of couplant is not required.
There are two methods of receiving the ultrasound waveform: pulse-echo (or reflection) and Through-Transmission (or attenuation). In pulse-echo mode (reflection), the transducer performs both the sending and the receiving of the pulsed waves as the “sound” is reflected back to the device. Reflected ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object. The diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance, depicting the arrival time of the reflection. In through-transmission (attenuation) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after traveling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing their presence.
Radiographic testing (RT) is a method of non-destructive testing where many types of manufactured components can be examined to verify the internal structure and integrity of the specimen. RT can be performed utilizing either X-rays or gamma rays. Both are forms of electromagnetic radiation. The difference between various types of electromagnetic energy is related to the wavelength. X and gamma rays have the shortest wavelength, and this property leads to the ability to penetrate, travel through, and exit various materials such as carbon steel and other metals.
Gamma radiation sources, most commonly iridium-192 and cobalt-60, are used to inspect a variety of materials. The vast majority of radiography concerns the testing and grading of welds on pressurized piping, pressure vessels, high-capacity storage containers, pipelines, and some structural welds. Other tested materials include concrete (locating rebar or conduit), welder’s test coupons, machined parts, plate metal, or pipe wall (locating anomalies due to corrosion or mechanical damage). Non-metal components such as ceramics used in the aerospace industries are also regularly tested. Theoretically, industrial radiographers could radiograph any solid, flat material (walls, ceilings, floors, square or rectangular containers) or any hollow cylindrical or spherical object.
Eddy current testing (ET), also commonly seen as eddy-current testing and ECT is one of many electromagnetic testing methods used in nondestructive testing (NDT) making use of electromagnetic induction to detect and characterize surface and sub-surface flaws in conductive materials.
In its most basic form, the single-element ET probe, a coil of conductive wire is excited with an alternating electrical current. This wire coil produces an alternating magnetic field around itself. The magnetic field oscillates at the same frequency as the current running through the coil. When the coil approaches a conductive material, currents opposed to the ones in the coil are induced in the material, i.e. eddy currents.
Variations in the electrical conductivity and magnetic permeability of the test object, and the presence of defects cause a change in eddy current and a corresponding change in phase and amplitude that can be detected by measuring the impedance changes in the coil, which is a telltale sign of the presence of defects. This is the basis of standard (pancake coil) ET.
ET has an extensive range of applications. ET can be used on any conductive material such as copper, aluminum, and stainless steel. It is useable on ferrous(can be magnetized) metals but is particularly suited for non-ferrous(cannot be magnetized) metals like copper and aluminum. ET is used extensively is the energy and chemical industries in the inspection of heat exchangers as well as in the aerospace industry for the inspection of aircraft skins. There are though, physical limits to generating eddy currents and depth of penetration with the ideal depth being 1/8th of an inch.
Becoming an NDT Technician
If you are interested in becoming an NDT technician an earning an average entry-level income of $65,000*, then consider applying for The Ocean Corporations comprehensive 30 week NDT education program or give us a call and/or request more information to find out if this career is right for you.
*According to 2015 PQNDT Salary Survey