Conventional NDT

1. Dye Penetrant Testing

Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI) or penetrant testing (PT), is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). The penetrant may be applied to all non-ferrous materials and ferrous materials, although for ferrous components magnetic-particle inspection is often used instead for its subsurface detection capability. LPI 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.

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2. Magnetic Particle Testing

Magnetic particle Inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such as iron,nickelcobalt, 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 electric 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) (rectified AC).

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. Ferrous iron particles are then applied to the part. The particles may be dry or in a wet suspension. If an area of flux leakage is present, the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication. The indication can then be evaluated to determine what it is, what may have caused it, and what action should be taken, if any.

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3. Radiographic Testing

Radiographic Testing (RT), or industrial radiography, is a nondestructive testing (NDT) method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials.

Either an X-ray machine or a radioactive source, like Ir-192Co-60, or in rarer cases Cs-137 are used in a X-ray computed tomographymachine as a source of photons. Neutron radiographic testing (NR) is a variant of radiographic testing which uses neutrons instead of photons to penetrate materials. This can see very different things from X-rays, because neutrons can pass with ease through lead and steel but are stopped by plastics, water and oils.

Since the amount of radiation emerging from the opposite side of the material can be detected and measured, variations in this amount (or intensity) of radiation are used to determine thickness or composition of material. Penetrating radiations are those restricted to that part of the electromagnetic spectrum of wavelength less than about 10 nanometres.

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4. Ultrasonic Testing

Ultrasonic testing (UT) is a family of non-destructive testing techniques based in the propagation of ultrasonicwaves 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 or to characterize materials. A common 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 onconcrete, wood and composites, albeit with less resolution. It is used in many industries including steel and aluminium construction, metallurgy, manufacturing, aerospaceautomotive 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: reflection andultrasonic-testing attenuation. In reflection (or pulse-echo) mode, 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, representing the arrival time of the reflection. In attenuation (or through-transmission) 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. Using the couplant increases the efficiency of the process by reducing the losses in the ultrasonic wave energy due to separation between the surfaces.

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5. Visual Testing

All methods of Nondestructive Examination (NDE) at some point fall back on the most basic of inspection techniques, Visual Inspection. Whether performing a Liquid PenetrantMagnetic Particle, reviewing radiographs of welds, observing an ultrasonicinstrument’s scope, or evaluating eddy current signals, visual examination at some point comes into play in all methods of nondestructive testing. In stark contrast to this very premise, it was not until 1988 that the American Society for Nondestructive Testing (ASNT) recognized Visual Testing (VT) as a formal Nondestructive Testing (NDT) discipline, including qualification and certification parameters within the 2011 Edition of the SNT-TC-1A Recommended Practice document. Since that time VT has now become included in other Codes and Standards that are considered mainstays of the inspection industry, such as the American Society of Mechanical Engineers (ASME), the American Society for Testing Materials (ASTM), the American Welding Society (AWS) & National Aerospace Standard (NAS-410).

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The term Visual Examination may lead to an over simplification of the method in itself. This is not to say that visual examination methods or techniques are overVTly complicated, as in most cases they are not. But it is important to also understand that numerous visual examination techniques may incorporate remote viewing equipment, machine vision, fiber optics, video presentation systems, optical prisms, magnetic film and the list goes on and on. Furthermore, it is of possibly even greater importance to understand that timely placed Visual Examinations will detect discontinuities that can only be resolved through other more costly methods once welds have been completed.

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