Non-destructive testings basic introduction

Editor's Notes

• #6: There are NDE application at almost any stage in the production or life cycle of a component
• #15: Surface Preparation: One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear the surface of the sample, thus closing the defects. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the parts in a penetrant bath. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected. Minimum dwell times typically range from 5 to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and is often very specific to a particular application. Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treated with an emulsifier and then rinsing with water. Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers). Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes and significantly longer times may be necessary for tight cracks. Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present. Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.
• #28: Diamagnetic materials are solids with all paired electron and, therefore, no permanent net magnetic moment per atom. Diamagnetic properties arise from the realignment of the electron orbits under the influence of an external magnetic field. Most elements in the periodic table, including copper, silver, and gold, are diamagnetic. Paramagnetic properties are due to the presence of some unpaired electrons and from the realignment of the electron orbits caused by the external magnetic field. Paramagnetic materials include Magnesium, molybdenum, lithium, and tantalum. Ferromagnetic materials have some unpaired electrons so their atoms have a net magnetic moment. They get their strong magnetic properties due to the presence of magnetic domains. In these domains, large numbers of atoms moments (10^12 to 10^15) are aligned parallel so that the magnetic force within the domain is strong. When a ferromagnetic material is in the unmagnitized state, the domains are nearly randomly organized and the net magnetic field for the part as a whole is zero. When a magnetizing force is applied, the domains become aligned to produce a strong magnetic field within the part. Iron, Nickel, and cobalt are examples of ferromagnetic materials. Components with these materials are commonly inspected using the magnetic particle method.
• #30: As discussed previously a magnetic field is a change in energy within a volume of space. The magnetic field surrounding a bar magnet can be seen in the magnetograph below. A magnetograph can be created by placing a piece of paper over a magnet and sprinkling the paper with iron filings. The particles align themselves with the lines of magnetic force produced by the magnet. The magnetic lines of force show where the magnetic field exits the material at one pole and reenters the material at another pole along the length of the magnet. It should be noted that the magnetic lines of force exist in three-dimensions but are only seen in two dimensions in the image.
• #34: There are several ways that direct magnetization is commonly accomplished. One way involves clamping the component between two electrical contacts in a special piece of equipment. Current is passed through the component and a circular magnetic field is established in and around the component. When the magnetizing current is stopped, a residual magnetic field will remain within the component. The strength of the induced magnetic field is proportional to the amount of current passed through the component. A second technique involves using clams or prods, which are attached or placed in contact with the component. Current is injected into the component as it flows from the contacts. The current sets up a circular magnetic fields around the path of the current.
• #35: The use of permanent magnets is a low cost method of establishing a magnetic field. However, their use is limited due to lack of control of the field strength and the difficulty of placing and removing strong permanent magnets from the component. Electromagnets in the form of an adjustable horseshoe magnet (called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry. Electromagnets only exhibit a magnetic flux when electric current is flowing around the soft iron core. When the magnet is placed on the component, a magnetic field is established between the north and south poles of the magnet. Another way of indirectly inducting a magnetic field in a material is by using the magnetic field of a current carrying conductor. A circular magnetic field can be established in cylindrical components by using a central conductors. Typically, one or more cylindrical components are hung from a solid copper bar running through the inside diameter. Current is passed through the copper bar and the resulting circular magnetic field established a magnetic field with the test components. The use of coils and solenoids is a third method of indirect magnetization. When the length of a component is several time larger than its diameter, a longitudinal magnetic field can be established in the component. The component is placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a "coil shot."
• #36: When the length of a component is several time larger than its diameter, a longitudinal magnetic field can be established in the component. The component is often placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a "coil shot." The magnetic field travels through the component from end to end with some flux loss along its length as shown in the image to the right. Keep in mind that the magnetic lines of flux occur in three dimensions and are only shown in 2D in the image. The magnetic lines of flux are much denser inside the ferromagnetic material than in air because ferromagnetic materials have much higher permeability than does air. When the concentrated flux within the material comes to the air at the end of the component, it must spread out since the air can not support as many lines of flux per unit volume. To keep from crossing as they spread out, some of the magnetic lines of flux are forced out the side of the component. When a component is magnetized along its complete length, the flux loss is small along its length. Therefore, when a component is uniform in cross section and magnetic permeability, the flux density will be relatively uniform throughout the component. Flaws that run normal to the magnetic lines of flux will disturb the flux lines and often cause a leakage field at the surface of the component. When a component with considerable length is magnetized using a solenoid, it is possible to magnetize only a portion of the component. Only the material within the solenoid and about the same width on each side of the solenoid will be strongly magnetized. At some distance from the solenoid, the magnetic lines of force will abandon their longitudinal direction, leave the part at a pole on one side of the solenoid and return to the part at a opposite pole on the other side of the solenoid. This occurs because the magnetizing force diminishes with increasing distance from the solenoid, and, therefore, the magnetizing force may only be strong enough to align the magnetic domains within and very near the solenoid. The unmagnetized portion of the component will not support as much magnetic flux as the magnetized portion and some of the flux will be forced out of the part as illustrated in the image below. Therefore, a long component must be magnetized and inspected at several locations along its length for complete inspection coverage. Circular Magnetic FieldsDistribution and Intensity As discussed previously, when current is passed through a solid conductor, a magnetic field forms in and around the conductor. The following statements can be made about the distribution and intensity of the magnetic field. The field strength varies from zero at the center of the component to a maximum at the surface. The field strength at the surface of the conductor decreases as the radius of the conductor increases when the current strength is held constant. (However, a larger conductor is capable of carrying more current.) The field strength outside the conductor is directly proportional to the current strength. Inside the conductor the field strength is dependent on the current strength, magnetic permeability of the material, and if magnetic, the location on the B-H curve. The field strength outside the conductor decreases with distance from the conductor.
• #38: the particles that are used for magnetic particle inspection are a key ingredient as they form the indications that alert the inspector to defects. Particles start out as tiny milled (a machining process) pieces of iron or iron oxide. A pigment (somewhat like paint) is bonded to their surfaces to give the particles color. The metal used for the particles has high magnetic permeability and low retentivity. High magnetic permeability is important because it makes the particles attract easily to small magnetic leakage fields from discontinuities, such as flaws. Low retentivity is important because the particles themselves never become strongly magnetized so they do not stick to each other or the surface of the part. Particles are available in a dry mix or a wet solution. Dry Magnetic ParticlesDry magnetic particles can typically be purchased in are red, black, gray, yellow and several other colors so that a high level of contrast between the particles and the part being inspected can be achieved.. The size of the magnetic particles is also very important. Dry magnetic particle products are produced to include a range of particle sizes. The fine particles are around 50 m (0.002 inch) in size are about three times smaller in diameter and more than 20 times lighter than the coarse particles (150 m or 0.006 inch), which make them more sensitive to the leakage fields from very small discontinuities. However, dry testing particles cannot be made exclusively of the fine particles. Coarser particles are needed to bridge large discontinuities and to reduce the powder's dusty nature. Additionally, small particles easily adhere to surface contamination, such as remanent dirt or moisture, and get trapped in surface roughness features producing a high level of background. It should also be recognized that finer particles will be more easily blown away by the wind and, therefore, windy conditions can reduce the sensitivity of an inspection. Also, reclaiming the dry particles is not recommended because the small particle are less likely to be recaptured and the "once used" mix will result in less sensitive inspections. Wet Magnetic ParticlesMagnetic particles are also supplied in a wet suspension such as water or oil. The wet magnetic particle testing method is generally more sensitive than the dry because the suspension provides the particles with more mobility and makes it possible for smaller particles to be used since dust and adherence to surface contamination is reduced or eliminated. The wet method also makes it easy to apply the particles uniformly to a relatively large area. Wet method magnetic particles products differ from dry powder products in a number of ways. One way is that both visible and fluorescent particle are available. Most nonfluorescent particles are ferromagnetic iron oxides, which are either black or brown in color. Fluorescent particles are coated with pigments that fluoresce when exposed to ultraviolet light. Particles that fluoresce green-yellow are most common to take advantage of the peak color sensitivity of the eye but other fluorescent colors are also available. (For more information on the color sensitivity of the eye, see the penetrant inspection material.) The particles used the wet method are smaller in size than those used in the dry method for the reasons mentioned above. The particles are typically 10 m (0.0004 inch) and smaller and the synthetic iron oxides have particle diameters around 0.1 m (0.000004 inch). This very small size is a result of the process used to form the particles and is not particularly desirable, as the particles are almost too fine to settle out of suspension. However, due to their slight residual magnetism, the oxide particles are present mostly in clusters that settle out of suspension much faster than the individual particles. This makes it possible to see and measure the concentration of the particles for process control purposes. Wet particles are also a mix of long slender and globular particles. The carrier solutions can be water- or oil-based. Water-based carriers form quicker indications, are generally less expensive, present little or no fire hazard, give off no petrochemical fumes, and are easier to clean from the part. Water-based solutions are usually formulated with a corrosion inhibitor to offer some corrosion protection. However, oil-based carrier solutions offer superior corrosion and hydrogen embrittlement protection to those materials that are prone to attack by these mechanisms.
• #53: Advantages of gamma ray sources include portability and the ability to penetrate thick materials in a relativity short time. As can be noted above cobalt will produce energies comparable to a 1.25 MeV x-ray system. Iridium will produce energies comparable to a 460 kV x-ray system. Not requiring electrical sources the gamma radiography is well adapted for use in remote locations. Disadvantages include shielding requirements and safety considerations. Depleted uranium is used as a shielding material for sources. The storage container (camera) for iridium sources will contain 45 pounds of shielding materials. Cobalt will require 500 pounds of shielding. Cobalt cameras are often fixed to a trailer and transported to and from inspection sites. Iridium is used whenever possible, and not all companies using source material will have a cobalt source. Source materials are constantly generating very penetrating radiation and in a short time considerable damage can be done to living tissue. Technicians must be trained in potential hazards to themselves and the public associated with use of gamma radiography.
• #54: The most common detector used in industrial radiography is film. The high sensitivity to ionizing radiation provides excellent detail and sensitivity to density changes when producing images of industrial materials.
• #56: In the example to the left, a two-step step tablet with the transition from step to step represented by Line BC is quite sharp or abrupt. Translated into a radiograph, we see that the transition from the high density to the low density is abrupt. The Edge Line BC is still a vertical line quite similar to the step tablet itself. We can say that the detail portrayed in the radiograph is equivalent to physical change present in the step tablet. Hence, we can say that the imaging system produced a faithful visual reproduction of the step table. It produced essentially all of the information present in the step tablet on the radiograph. In the example on the right, the same two-step step tablet has been radiographed. However, here we note that, for some reason, the imaging system did not produce a faithful visual reproduction. The Edge Line BC on the step tablet is not vertical. This is evidenced by the gradual transition between the high and low density areas on the radiograph. The edge definition or detail is not present because of certain factors or conditions which exist in the imaging system.
• #63: Below is an example of shear wave weld inspection. Notice the indication extending to the upper limits of the screen. This indication is produced by sound reflected from a defect within the weld.
• #67: In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle movement responsible for the propagation of longitudinal and shear waves is illustrated below. Different types of waves As the reed vibrates back and forth, the sound waves produced move the same direction (left and right). Waves that move in the same direction, or are parallel to their source are called longitudinal waves. Longitudinal sound waves are the easiest to produce and have the highest speed, however, it is possible to produce other types. Waves which move perpendicular to the way their source does are called shear waves. Shear waves travel at slower speeds than longitudinal waves, and can only be made in solids. Another type of wave is the surface wave. Surface waves travel at the surface of a material and move in elliptical orbits. They are slightly slower than shear waves but difficult to make. A final type of sound wave is the plate wave. These waves also move in elliptical orbits but are much more complex. They can only be created in very thin pieces of material.
• #68: In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in liquids, as well as solids because the energy travels through the atomic structure by a series of comparison and expansion (rarefaction) movements. In the transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation and, therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves.
• #69: Rayleigh waves travel the surface of a relative thick solid material penetrating to a depth of one wavelength. Rayleigh waves are useful because they are very sensitive to surface defects and since they will follow the surface around, curves can also be used to inspect areas that other waves might have difficulty reaching. Lamb waves, also known as plate waves, can be propagated only in very thin metals. Lamb waves are a complex vibrational wave that travels through the entire thickness of a material. Lamb waves provide a means for inspection of very thin materials. Propagation of Lamb waves depends on density, elastic, and material properties of a component, and they are influenced by a great deal by selected frequency and material thickness.
• #70: When piezoelectric ceramics were introduced they soon became the dominant material for transducers due to their good piezoelectric properties and their ease of manufacture into a variety of shapes and sizes. The first piezoceramic in general use was barium titanate, and that was followed during the 1960's by lead zirconate titanate compositions, which are now the most commonly employed ceramic for making transducers. In selecting a transducer the piezoelectric material is always a consideration as some materials are more efficient transmitters and some are more efficient receivers. Understanding the internal structure of the material to be inspected, as well as type, size, and probable location of defects is helpful when selecting a transducer. A transducer that performs well in one application will not always produce similar results when material properties change. For example, sensitivity to small defects is proportional to the product of the efficiency of the transducer as a transmitter and a receiver. Resolution, the ability to locate defects near surface or in close proximity in the material, requires a highly damped transducer. The backing material supporting the crystal has a great influence on damping characteristics of a transducer. Using a backing material with an impedance similar to that of the crystal will produce the most effective damping. Such a transducer will have a narrow bandwidth resulting in higher sensitivity. As the mismatch in impedance between crystal and backing material increases, transducer sensitivity is reduced and material penetration increased. It is of importance to understand the concept of bandwidth, or range of frequencies, associated with a transducer. The frequency noted on a transducer is the central or center frequency and depends primarily on the backing material. Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range, poorer resolving power, but greater penetration. The central frequency will also define capabilities of a transducers. Lower frequencies (0.5Mhz-2.25Mhz) provide greater energy and penetration in a material, while high frequency crystals (15.0Mhz-25.0Mhz) provide reduced penetration but greater sensitivity to small discontinuities.
• #71: Many factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior a transducer. Mechanical construction is the factor that influences performance, with important parameters such as radiation surface area, mechanical damping, housing, and other variables of physical construction. As of this writing, transducer manufactures are hard pressed when constructing two transducers that have identical performance characteristics. Transducer manufacture still has something of a "black art" component. Contact Transducers have elements protected in a rugged casing to withstand direct contact with a variety of materials. These transducers have an ergonomic design so that they are easy to grip and move along a surface. They also often have replaceable wear plates to lengthen their useful life. Immersion Transducers are designed to transmit ultrasound in situations where the test part is immersed in water. Immersion transducers are typically used inside a water tank or as part of a squirter or bubbler system in scanning applications. Immersion transducers usually have a impedance matching layer that helps to get more sound energy into the water and, in turn, into the component being inspected. Immersion transducers can be purchased with in a planner, cylindrically focused or spherically focused lens. A focused transducer can improve sensitivity and axial resolution by concentrating the sound energy to a smaller area.
• #72: Dual Element Transducers contain two independently operating elements in a single housing. One of the elements transmits and the other receives. Dual element transducers are especially well suited for making measurements in applications where reflectors are very near the transducer since this design eliminates the ring down effect that single-element transducers experience. (When single-element transducers are operating in pulse echo mode, the element can not start receiving reflected signals until the element has stopped ringing from it transmit function.) Dual element transducers are very useful when making thickness measurements of thin materials and when inspecting for near surface defects. The two elements are angled towards each other to create a crossed-beam sound path in the test material. Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. Transducers can be purchased in a variety of fixed angles or in adjustable versions where the user determines the angles of incident and refraction. In the fixed angle versions, the angle of refraction that is marked on the transducer is only accurate for a particular material, which is usually steel. The angled sound path allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas. They are also used to generate surface waves for use in detecting defects on the surface of a component. Normal Incidence Shear Wave Transducers are unique because they allow introduction of shear waves directly into a test piece without the use of an angle beam wedge. Careful design has enabled manufacturing of transducers with minimal longitudinal wave contamination. The ratio of the longitudinal to shear wave components is generally below -30dB. Delay Line Transducers provide versatility with a variety of replaceable options. Removable delay line, surface conforming membrane, and protective wear cap options can make a single transducer effective for a wide range of applications. As the name implies, the primary function of a delay line transducer is to introduce a time delay between the generation of the sound wave and the arrival of any reflected waves. This allows the transducer to complete its "sending" function before it starts it "listening" function. Delay line transducers are recommended for applications that require a contact transducer with good near surface resolution. They are designed for use in applications such as high precision thickness gauging of thin materials and delamination checks in composite materials. They are also useful in high-temperature measurement applications since the delay line provides some insulation to the piezoelectric element from the heat. High Frequency Transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically. Broadband transducers with frequencies between 20 MHz and 150 MHz are commercially available.
• #79: In the illustration of the A-scan presentation to the right, the initial pulse generated by the transducer is represented by the signal IP, which is near time zero. As the transducer is scanned along the surface of the part, four other signals are likely to appear at different times on the screen. When the transducer is in its far left position, only the IP signal and signal A, the sound energy reflecting from surface A, will be seen on the trace. As the transducer is scanned to the right, a signal from the backwall BW will appear latter in time showing that the sound has traveled farther to reach this surface. When the transducer is over flaw B, signal B, will appear at a point on the time scale that is approximately halfway between the IP signal and the BW signal. Since the IP signal corresponds to the front surface of the material, this indicates that flaw B is about halfway between the front and back surfaces of the sample. When the transducer is moved over flaw C, signal C will appear earlier in time since the sound travel path is shorter and signal B will disappear since sound will no longer be reflecting from it.
• #80: The B-scan presentations is a profile (cross-sectional) view of the a test specimen. In the B-scan, the time-of-flight (travel time) of the sound energy is displayed along the vertical and the linear position of the transducer is displayed along the horizontal axis. From the B-scan, the depth of the reflector and its approximate linear dimensions in the scan direction can be determined. The B-scan is typically produced by establishing a trigger gate on the A-scan. Whenever the signal intensity is great enough to trigger the gate, a point is produced on the B-scan. The gate is triggered by the sound reflecting from the backwall of the specimen and by smaller reflectors within the material. In the B-scan image above, line A is produced as the transducer is scanned over the reduced thickness portion of the specimen. When the transducer moves to the right of this section, the backwall line BW is produced. When the transducer is over flaws B and C lines that are similar to the length of the flaws and at similar depths within the material are drawn on the B-scan. It should be noted that a limitation to this display technique is that reflectors may be masked by larger reflectors near the surface.