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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 Standards tab. To maintain consistency, the techniques described for each test method are those listed in 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), Ground Penetrating Radar (GPR), Guided Wave Testing (GW), Identification of Materials (IM), Laser Testing Methods (LM), Leak Testing (LT), Liquid Penetrant Testing (PT), Magnetic Flux Leakage (MFL), Magnetic Particle Testing (MT), Microwave Testing (MW), Neutron Radiographic Testing (NR), Predictive Maintenance (PdM), 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) 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 the figure below (note the change in magnetic polarity). Since magnetic flux lines do not travel very far well in air, very fine colored ferromagnetic particles are applied to the surface of the part. The outcome of this process is the particles will be drawn into the flux leakage field created by the discontinuity and produces a visible indication on the surface of the part. The magnetic particles may be a dry powder, or suspended in a liquid solution, colored with a visible dye or a fluorescent dye that fluoresces under an ultraviolet wavelength of 365 nm.

MT Techniques

Yokes

Figure 2

Most field inspections are performed using a yoke, as shown at the right. As shown in the figure below, 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. 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 90° so no indications are missed. Because the entire 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

Figure 3
Prod units use direct induction, where the current runs through the part and a circular magnetic field is generated around the legs, shown below. Because the magnetic field between the prods is traveling 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 90° to the first application.

Coils

Figure 4
Electric coils (shown at right) 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." 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 90° 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

Figure 6
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 the figure below. The part is then wetted down with the bath solution and the current is applied, traveling through the central conductor rather than through the part. The inside diameter and outside diameter 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.

Figure 7

The basic principle of liquid penetrant testing (PT) 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. PT 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 ultraviolet excitation source with a wavelength of 365 nanometers. The visible dye penetrant process is shown at right. 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. If fluorescent penetrant is used, then ambient must be minimal and an ultraviolet excitation source is deployed. Most developers are fine-grained, white talcum-like powders that provide an increase in the area of the indication and add a color contrast to the penetrant being used.

The images at right show the process of penetrant entering a surface discontinuity: penetration of liquid penetrant into a surface discontinuity: first the penetrant is applied; next, the surface penetrant is removed; a deep, narrow crack appears after 30 seconds; and finally, a deep, narrow crack appears after 10 minutes.

PT Techniques

Solvent Removable

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

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

Post-emulsifiable penetrants are penetrants that do not have an emulsifier included in its chemical makeup 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 (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).

Figure 8

Industrial radiography, or radiographic testing (RT), 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 2.5 to 3 in., 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 the figure at right.

 

RT Techniques

Computed Radiography

Computed radiography (CR) is a transitional technology between film and direct digital radiography. This technique uses a reusable, flexible, photostimulated phosphor (PSP) plate that 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

Computed tomography (CT) uses a computer to reconstruct an image of a cross-sectional plane of an object as opposed to a conventional radiograph. The CT image is developed from multiple views taken at different viewing angles that are reconstructed using a computer. With traditional radiography, the position of internal discontinuities cannot be accurately determined without making exposures from several angles to locate the item by triangulation. With computed tomography, the computer triangulates using every point in the plane as viewed from many different directions.

Digital Radiography

Digital radiography (DR) digitizes the radiation that passes through an object directly into an image that can be displayed on a computer monitor. The three principal technologies used in direct digital imaging are amorphous silicon, charge coupled devices, and complementary metal oxide semiconductors. These images are available for viewing and analysis in seconds compared to the time needed to scan in computed radiography images. The increased processing speed is a result of the unique construction of the pixels; an arrangement that also allows a superior resolution than is found in computed radiography and most film applications.

Film Radiography

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.

 

Figure 10

Ultrasonic testing (UT) 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 sound 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. The most common sound frequencies used in UT are between 1 and 10 MHz, which are too high to be heard and do not travel through air. The lower frequencies have greater penetrating power but less sensitivity (the ability to "see" small indications), while the higher frequencies do not penetrate as deeply but can detect smaller indications.

The two most commonly used types of sound waves used in industrial inspections are the compression (longitudinal) wave and the shear (transverse) wave, as shown in the figure below. Compression waves cause the atoms in a part to vibrate back and forth parallel to the sound direction and shear waves cause the atoms to vibrate perpendicularly (from side to side) to the direction of the sound. 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 impulses from the UT machine into sound waves, then converts returning sound back into electric impulses that can be displayed as a visual representation on a digital or LCD screen (on older machines, a cathode ray tube screen). If the machine is properly calibrated, the operator can determine the distance from the transducer to the reflector, and in many cases, an experienced operator can determine the type of discontinuity (like slag, porosity, or cracks in a weld) that caused the reflector. Because ultrasound will not travel through air (the atoms in air molecules are too far apart to transmit ultrasound), a liquid or gel called "couplant" is used between the face of the transducer and the surface of the part to allow the sound to be transmitted into the part.

 

UT Techniques

Straight Beam

Figure 11
Straight beam inspection uses longitudinal waves to interrogate the test piece as shown at the right. If the sound hits an internal reflector, the sound from that reflector will reflect to the transducer faster than the sound coming back from the back-wall of the part due to the shorter distance from the transducer. This results in a screen display like that shown in the figure at right. Digital thickness testers use the same process, but the output is shown as a digital numeric readout rather than a screen presentation.

Angle Beam

Figure 12

Angle beam inspection, shown above, uses the same type of transducer but it is mounted on an angled wedge (also called a "probe") that is designed to transmit the sound beam into the part at a known angle. The most commonly used inspection angles are 45°, 60°, and 70°, with the angle being calculated up from a line drawn through the thickness of the part (not the part surface). If the frequency and wedge angle is not specified by the governing code or specification, it is up to the operator to select a combination that will adequately inspect the part being tested.

In angle beam inspections, the transducer and wedge combination (also referred to as a "probe") is moved back and forth towards the weld so that the sound beam passes through the full volume of the weld. As with straight beam inspections, reflectors aligned more or less perpendicular to the sound beam will send sound back to the transducer and are displayed on the screen.

Immersion Testing

Immersion testing is a technique where the part is immersed in a tank of water with the water being used as the coupling medium to allow the sound beam to travel between the transducer and the part. The UT machine is mounted on a movable platform (a "bridge") on the side of the tank so it can travel down the length of the tank. The transducer is swivel-mounted on at the bottom of a waterproof tube that can be raised, lowered, and moved across the tank. The bridge and tube movement permits the transducer to be moved on the X-, Y-, and Z-axes. All directions of travel are gear driven so the transducer can be moved in accurate increments in all directions, and the swivel allows the transducer to be oriented so the sound beam enters the part at the required angle. Round test parts are often mounted on powered rollers so that the part can be rotated as the transducer travels down its length, allowing the full circumference to be tested. Multiple transducers can be used at the same time so that multiple scans can be performed.

Through Transmission

Figure 13
Through transmission inspections are performed using two transducers, one on each side of the part. The transmitting transducer sends sound through the part and the receiving transducer receives the sound. Reflectors in the part will cause a reduction in the amount of sound reaching the receiver so that the screen presentation will show a signal with a lower amplitude (screen height). The figure at right shows examples of through-transmission in an immersion tank (above) and using water columns (below).

Phased Array

Phased array inspections are done using a probe with multiple elements that can be individually activated. By varying the time when each element is activated, the resulting sound beam can be "steered," and the resulting data can be combined to form a visual image representing a slice through the part being inspected.

Time of Flight Diffraction

Time of Flight Diffraction (TOFD) uses two transducers located on opposite sides of a weld with the transducers set at a specified distance from each other. One transducer transmits sound waves and the other transducer acting as a receiver. Unlike other angle beam inspections, the transducers are not manipulated back and forth towards the weld, but travel along the length of the weld with the transducers remaining at the same distance from the weld. Two sound waves are generated, one traveling along the part surface between the transducers, and the other traveling down through the weld at an angle then back up to the receiver. When a crack is encountered, some of the sound is diffracted from the tips of the crack, generating a low strength sound wave that can be picked up by the receiving unit. By amplifying and running these signals through a computer, defect size and location can be determined with much greater accuracy than by conventional UT methods. The figure at right shows a TOFD display of weld indications.

Electromagnetic testing (ET) is a general test category that includes eddy Current testing, alternating current field measurement, and remote field testing. While magnetic particle testing is also an electromagnetic test, due to its widespread use ET is considered a standalone test method. All of these techniques use the induction of an electric current or magnetic field into a conductive part, after which the resulting effects are recorded and evaluated.

ET Techniques

Eddy Current Testing

Figure 14

When an alternating current coil induces an electromagnetic field into a conductive test piece, a small current is created around the magnetic flux field, much like a magnetic field is generated around an electric current. The flow pattern of this secondary current, called an "eddy" current, is affected when it encounters a discontinuity in the test piece. This change in the eddy current density can be detected and used to characterize the discontinuity causing that change. A simplified schematic of eddy currents generated by an alternating current coil ("probe") is shown in Figure 14a. By varying the type of coil, eddy current testing can be applied to flat surfaces or tubular products.  This technique works best on smooth surfaces and has limited penetration, usually less than 0.25 in.

Encircling coils (Figure 14b) are used to test tubular and bar-shaped products. The tube or bar can be fed through the coil at a relatively high speed, allowing the full cross section of the test object to be interrogated. However, due to the direction of the flux lines, circumferentially oriented discontinuities may not be detected with this application.

Alternating Current Field Measurement

Alternating current field measurement (ACFM) uses a specialized probe that introduces an alternating current into the surface of the test piece, creating a magnetic field. In parts with no discontinuities this field will be uniform, but if there is a discontinuity open to the surface, the magnetic field will flow around and under the discontinuity, causing a disruption of the field that can be detected by sensors within the probe. The resulting feedback can then be fed to software that can determine the length and depth of the discontinuity. ACFM provides better results on rough surfaces than eddy current testing and can be used through many surface coatings.

Remote Field Testing

Remote field testing (RFT) is most commonly used to inspect ferromagnetic tubing due to the presence of a strong skin effect found in such tubes. Compared to standard eddy current techniques, RFT provides better results throughout the thickness of the tube, having approximately equal sensitivity at both the inside diameter and outside diameter surfaces of the tube. For non-ferromagnetic tubes, eddy current testing tends to provide more sensitivity.

As the name implies, visual testing (VT) involves the visual observation of the surface of a test object to evaluate the presence of discontinuities or possibly the absence of one step during manufacturing. VT is the most used test method in industry, as most test methods require the operator look visually inspect surface of a part prior to inspection. Ultrasonic and radiographic inspections typically require visual inspections prior to performing the method. Liquid penetrant and magnetic particle inspections become a visual inspection once the penetrant or magnetic particles have been applied. A film or digital image that has been taken of a part will be evaluated using visual inspection. 

VT Techniques

Direct Visual Testing

When addressing VT as a standalone inspection method rather than supporting other methods it is performed in one of two techniques. First, VT may by performed by direct viewing, normally at a distance no more than 24 in. at an angle less than 30° from the part using line-of-sight vision. Items evaluated may be weld, surface coatings, and dimensional measurements. Testing may be enhanced with the use of optical instruments such as magnifying glasses, optical comparators, mirrors, scales, and supplementary lighting at the inspection surface. 

Remote Visual Testing

The second technique in VT is remote visual testing, which addresses visual inspection with the use of borescopes and cameras, as well as computer-assisted viewing systems allowing remote viewing. Remote visual testing is used to identify corrosion, misalignment of parts, internal physical damage, and cracks. The use of drones is becoming more widely used in petrochemical and structural inspections. Some specialized drones are designed to be submersed in large tanks to evaluate product, valves, and corrosion. Visual inspectors in the field today may one day performing visual inspection of welds and the next controlling a drone evaluating an area that in the past was not assessable to visual inspection without special access.

Acoustic emission testing (AE) is performed by applying a localized external force such as an abrupt mechanical load or rapid temperature or pressure change to the part being tested. The resulting stress waves in turn generate short-lived, high frequency elastic waves in the form of small material displacements, or plastic deformation, on the part surface that are detected by sensors that have been attached to the part surface. When multiple sensors are used, the resulting data can be evaluated to locate discontinuities in the part.

One common application of guided wave testing on piping uses controlled excitation of one or more ultrasonic waveforms that travel along the length of the pipe, reflecting from changes in the pipe stiffness or cross-sectional area. A transducer ring or exciter coil assembly is used to introduce the guided wave into the pipe and receive reflected signals. The control and analysis software can be installed on a laptop computer to drive the transducer ring/exciter, collect data, and to analyze the results. Many times, the transducer ring/exciter setup is designed specifically for the diameter of the pipe being tested, and the system has the advantage of being able to inspect the pipe wall volume over long distances without having to remove coatings or insulation. GW can locate both inside diameter and outside diameter discontinuities but cannot differentiate between them. The figure at right shows the difference between the principles of ultrasonic testing (top) and GW (bottom).

Laser methods (LM) refers to holography and shearography, related NDT techniques that use the wave nature of laser light, called interferometry, to create images of both surface and subsurface defects in ductile materials ranging from foam insulation to steel. In both methods, the laser light does not penetrate the surface, but a small stress change is applied which does pass through the volume of the test part. Defects subjected to the right stress change produce a small out-of-plane deformation on the surface which appears in holography and shearography images. Stressing methods include ultrasonic and sonic vibration, heat, pressure, and partial vacuum. In composites, defects such as disbonds, delamination, fiber wrinkles, foreign object debris, and impact damage are detected. LM techniques offer noncontact inspection, very high throughput, and do not require contour following as required with UT C-scan. 

 At top, a thermal stress shearography image reveals Kapton inserts at various depths in a 12 x 36 in. solid carbon fiber laminate panel. The inserts range 0.2 to 1.0 in. in size.
 

LM Techniques

Holographic Testing

Holography NDT is generally used for component-level parts with complex shapes. The holography process is degraded by ambient vibration requiring a vibration isolation table to support the camera, stressing equipment, and the test part. Test part size is limited to a maximum of approximately 3 ft. The benefits of holography NDT include speed and the ability to inspect complex geometries. Ultrasonic holography is highly effective for the NDT of multilayer metallic structures including medical devices and turbine aircraft engine parts such as brazed compressor honeycomb or Feltmetal seals. Using small pressure changes, metallic heat transfer components such as radiator panels and actively cooled missile or aircraft parts.

At right, an ultrasonic holographic image is shown of an aircraft turbine engine 
compressor seal. The dark indications are braze disbonds.
 

Laser Profilometry

Laser Profilometry uses a high-speed rotating laser light source, miniature optics and a computer with high-speed digital signal processing software.  The ID surface of a tube is scanned in two dimensions and the reflected light is passed through a lens that focuses that light onto a photo-detector, generating a signal that is proportional to the spot's position in its image plane.  As the distance from the laser to the ID surface changes, the position of the focal spot on the photo-detector changes due to parallax, generating a high resolution three-dimensional image of the part surface that represents the surface topography of the part.  This technique can be used to detect corrosion, pitting, erosion and cracks in pipes and tubes.

Laser Shearography

Shearography is much less sensitive than holography to ambient vibration and part motion. Shearography cameras are designed for portable applications or for use on tripods, scan gantries, or robots. As a result, large objects are routinely scanned for defects. Composite race cars, rail and monorail train cars, yachts, naval vessels, aircraft, launch vehicles, and spacecraft as scanned with shearography for a wide range of defect types.

The shearography images at right (top to bottom) show Teflon inserts located throughout the volume of a 0.5 in. thick composite laminate, fiber bridging in a composite overwrapped pressure vessel for a space application, wrinkled fibers in a composite aircraft fuselage, disbonds in an aircraft radome, and defects in a large liquid propellant rocket engine.
 

Leak Testing, as the name implies, is used to detect through leaks using one of the four major LT techniques: bubble, pressure change, halogen diode, and mass spectrometer testing. These techniques are described below.

LT Techniques

Bubble Leak Testing

Figure 15
Bubble leak testing, as the name implies, relies on the visual detection of a gas (usually air) leaking from a pressurized system. Small parts can be pressurized and immersed in a tank of liquid and larger vessels can be pressurized and inspected by spraying a soap solution that creates fine bubbles to the area being tested. For flat surfaces, the soap solution can be applied to the surface and a vacuum box (Figure 15) can be used to create a negative pressure from the inspection side. If there are through leaks, bubbles will form, showing the location of the leak.

Pressure Change Testing

Pressure change testing can be performed on closed systems only. Detection of a leak is done by either pressurizing the system or pulling a vacuum then monitoring the pressure. Loss of pressure or vacuum over a set period of time indicates that there is a leak in the system. Changes in temperature within the system can cause changes in pressure, so readings may have to be adjusted accordingly.

Halogen Diode Testing

Halogen diode testing is done by pressurizing a system with a mixture of air and a halogen-based tracer gas. After a set period of time, a halogen diode detection unit, or "sniffer," is used to locate leaks.

Mass Spectrometer Testing

Mass spectrometer testing can be done by pressurizing the test part with helium or a helium/air mixture within a test chamber then surveying the surfaces using a sniffer, which sends an air sample back to the spectrometer. Another technique creates a vacuum within the test chamber so that the gas within the pressurized system is drawn into the chamber through any leaks. The mass spectrometer is then used to sample the vacuum chamber and any helium present will be ionized, making very small amounts of helium readily detectable.

Figure 16

Magnetic flux leakage testing (MFL) is capable of detecting anomalies in normal flux patterns created by discontinuities in ferrous material has an induced magnetic field that is at or near saturation. MFL can be used for piping and tubing inspection, tank floor inspection and other applications. 

In tubular applications, the inspection head is composed of either magnetic poles pieces or a coil configuration, detectors, and mechanical assemblies. The magnetizing force and the detectors are in different configurations, depending on the orientation of the intended imperfection detection. The detectors are connected to a recording source. These recording devices come in many different designs. The MFL head can either move along the tube or the tube can move through the inspection head. MFL is capable of detecting corrosion as well as rounded and linear imperfections. These imperfections cause a disturbance in the magnetic lines, which the detectors pick up and then send back to the recording device. The detected location is marked on the tube and typically sent for further evaluation. Many different techniques can be utilized, from new manufacturing to in-service applications.

Tank floor inspection applies the same principle but uses a series of magnetic field generators ("bridges") and sensors (as shown in the figure below) located side-by-side across the front of a vacuum sweeper-like machine. The bridges generate a magnetic field that is at or near saturation of the tank floor, and any reduction in thickness or loss of material due to pitting or corrosion will cause the field to "leak" upwards out of the floor material where it can be picked up by the sensors. On very basic machines, each sensor will be connected to an audio and/or visual display that lets the operator know there is an indication; more advanced machines can have both visual displays and recording capability so that the results can be stored, analyzed, and compared to earlier results to monitor discontinuity growth.

 

Neutron radiography, or neutron radiographic testing (NR), differs from conventional radiography in that neutrons are the penetrating radiation rather than X-rays. Neutrons pass easily through nearly all metals but are readily absorbed by materials that contain hydrogen (water and plastics), carbon, and some rare earth elements. Performance of neutron radiography relies on the generation of neutrons from such sources as betatrons, linear accelerators, and the portable isotope Californium-252 (252Cf), as well as the diversion of a beam of neutrons from a live reactor.

A common use of NR is the examination of pyrotechnic devices such as ejection seat cartridges for the aerospace industry. Anomalies such as cracks, separations, or incomplete fill of the explosive material can be readily imaged through the metal case. The figure at right illustrates the difference between neutron radiography (left) and X-ray (right).

The X-radiation reveals only the metal components of the camera. The image on the right is the same camera exposed to neutrons. The nonmetallic and plastic components are readily visible within the metal structure.

 

Thermal/infrared testing, or infrared thermography, is used to measure or map apparent surface temperatures based on the infrared radiation given off by an object as heat flows through, to or from that object. Persons who practice Infrared thermography may be certified one of three specific disciplines: electrical and mechanical Inspections, building diagnostics, and materials testing. Electrical and mechanical inspection encompasses infrared inspections of electrical power distribution systems and mechanical equipment or processes. Building diagnostics uses infrared thermography primarily to diagnose energy loss in buildings, often identifying issues with the insulation materials and air infiltration issues in conjunction with blower door testing. Materials testing utilizes infrared thermography to detect anomalies such as delaminations, disbonds, voids, and cracks in composite materials such as aircraft components. 

The figures shown at right depict the following: (top) hot spots found on indoor exposed electrical connections can be easy for thermographers to locate. The fault in this case was attributed to an unbalanced load; (middle) refractory problems in petrochemical facilities can be detected, mapped, and quantified with infrared, as this thermal image clearly illustrates; (bottom) in addition to the overheating belts in this thermal image, the bearing on the right is 22.3 °C (40.1 °F) warmer than the bearing on the left. 

Vibration analysis (VA) is a method contained within predictive maintenance (PdM). ANSI/ASNT CP-189 ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel discusses PdM as an evaluation of the condition of operating (in-service) machinery through condition monitoring which evaluates key performance indicators. PdM uses the principles of statistical process control to determine at what point in the future corrective maintenance activities will be appropriate. The ultimate goal of PdM is to perform maintenance at a scheduled future time when the maintenance activity is most cost-effective, and before the equipment affects quality, loses optimum performance or fails.
 
VA is performed on virtually any type of machinery that produces or provides a product used in the everyday life of people, including power, water, aluminum cans, tires, and so forth. Its benefits are not limited to just production machinery, however. It can also be effective in structural health monitoring of aircraft, bridges, and buildings.
 
Locations on the machine/object are chosen where the internal vibration signature can be measured on the surface. An analog vibration transducer (for example, proximity probe, velocity transducer or accelerometer) is placed/mounted at these locations to convert the mechanical vibration to a scaled electrical signal. This analog waveform (time domain) is transmitted to the collection system where it will be digitized and converted into a spectrum (frequency domain) using the fast Fourier transform (FFT). The time-waveform and spectrum are often simplified into amplitude-vs-frequency parameters, which are also trended over an extended period of time.
 
Evaluation of the data includes frequency analysis, time-waveform analysis, and trend analysis. Typical problems identified through VA can be basic in nature such as unbalance, misalignment, and bearing wear. More complex problems can also be identified such as machine tool chatter (resonance), degrading structural integrity, or process control problems.

 
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