electromagnetic testing emt chapter 11

Electromagnetic TestingMagnetic flux leakage / Eddy current/ MicrowaveChapter 11- The Reference 17th February 2015My ASNT Level III Pre-Exam Preparatory Self Study Notes

Charlie Chong/ Fion Zhang


2015-2-17

Fion Zhang at Shanghai17th February 2015

Charlie Chong/ Fion Zhang Shanghai 上海


Greek letter


Chapter Eleven:Reference Standards for Electromagnetic Testing


10.1 PART 1. Introduction to Reference Standards forElectromagnetic Testing

10.1.1 Development and Use of Reference StandardsSuccess in electromagnetic testing depends on the proper use of available reference standards. The development and use of eddy current reference standards requires a thorough understanding of the test to be performed.

Considerations include

(1) the materials tested, (2) their size and shape, (3) discontinuities of interest, (4) means of producing artificial discontinuities, (5) indications from artificial discontinuities, (6) noise that might be encountered, (7) instrument limitations and (8) criteria for relevant indications.


In view of the many types of artificial discontinuities that can be produced, an understanding of their relationship to the discontinuities of interest is critical. For example, if transverse discontinuities in a tube are of interest, it might seem obvious to use a transverse notch. However, that notch could be either straight, which is much easier to fabricate, or curved to match the radius of the tube (see Fig. 1).

Each notch will produce different signals that must be understood to properly relate them to the condition to be detected. Another similar case is that of cracks in a fastener hole, which can be simulated by several notches as shown in Fig. 2, all of which will produce different signals.


FIGURE 1. Transverse notches in tube: (a) straight; (b) curved.

(a)

(b)


FIGURE 2. Notches to simulate cracks as might be found in fastener holes.


One of the primary considerations in designing a reference standard is the ability to make more than one identical reference standard according to the same specifications. The reference standard and artificial discontinuities can be faithfully reproduced so that consistent tests can be performed at more than one location, a test can be repeated at another facility for evaluation and analysis and an identical reference standard can be produced if the original is damaged or lost. Therefore, the fabrication method must be carefully considered. For example, a notch made in a typical machine shop will not be as consistent as an electric discharge machined notch made by a vendor who specializes in them and provides certification. The lack of repeatability is a drawback to the use of natural discontinuities as references because no two natural discontinuities are identical. Another important concern is the purpose or function of the reference standard. It could be used to establish or confirm the proper setup, to duplicate types of discontinuities that might be encountered or to help interpretation by discriminating between noise and discontinuities. Finally, if the test is sufficiently controlled, the reference standard can be used to produce the same response as the discontinuity of interest.


The variables found in electromagnetic testing are almost always nonlinear. There are some situations where the variables are not even monotonic. Figure 3, for example, shows that the thickness impedance curve reverses direction even though the changes in thickness continue in the original direction. Actually, there are test points on all impedance plane diagrams where signal reverses occur. Therefore, intermediate variable reference standards will often be required in addition to reference standards that cover the end points of the tested variables. In conductivity tests, minimum reference standards used for calibration are often provided with the equipment. For greater accuracy, however, additional reference standards that cover the specific range and material being tested are recommended.


FIGURE 3. Expanded, relative scale of thickness variations for tests of three metals at 120 kHz.

Figure 3, for example, shows that the thickness impedance curve reverses direction even though the changes in thickness continue in the original direction.


DiscussionSubject:Statement “Actually, there are test points on all impedance plane diagrams where signal reverses occur.”


Alloy sorting, heat treatment verification, hardness determination and thickness measurement must each have reference standards that properly match all the changes in the variables that might exist in the test objects. Standardization and setup are very important for locating discontinuities. To obtain sufficient information, test choices might include different frequencies, different probes, different orientations or different procedures.


10.1.2 Requirements of Codes and SpecificationsAn important requirement for successful electromagnetic testing is the use of an accurate reference standard for equipment calibration. The reference standard is used to adjust the electromagnetic equipment’s sensitivity to various specimen parameters (cracks, surface roughness, conductivity and permeability variations and other material conditions). Consistency of calibration is maintained with procedural documents such as those issued by ASTM International, the American Petroleum Institute, government military offices and other organizations.


As an example, the ASME Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers (ASME) is widely used for testing of tubing or pipe in mills (as required by various code sections) and for in-place testing in nuclear power plant steam generators (Section XI). The reference standards described in Section V, Article 8, Appendices I and II, are used for testing of installed non-ferromagnetic heat exchanger tubing and are required when specified by a governing party or referenced by another code section. Article 8 is also voluntarily used for installed tube testing in many industries because there are few governing standards for it. The reference standards described there are used to establish and verify system response and can also provide a means of establishing depth-versus-phase curves for evaluation of signals from discontinuities. Depth-versus-phase calibration is made using a series of flat bottom holes in increments of 20 percent of wall thickness. A plot of the measured phase angle versus the depth of each flat bottom hole is made and used as a reference to estimate the depth of signals from the inspected tube. Other calibration or reference standards should be included to represent specific types of discontinuities to be detected.


As a means for controlling the accuracy of discontinuities in a standard, ASTM E 215 calls for a tolerance of ±0.025 mm (±0.001 in.) for flat bottom hole depths. Several holes are drilled around the circumference of the tubular reference at depths of one-third to two-thirds of the tube wall thickness. To be considered as an acceptable primary reference standard, the responses of any of the one-third thickness holes must be within ±20 percent of the mean of the three one-third thickness holes. Also, the two-thirds thickness response must be within ±10 percent of the mean indication for the two-thirds thickness holes.3 The procedural document often specifies the type of reference standard to be used for a given test, depending on the material under test and its geometry. Figure 4 shows several types of reference standards. These reference standards and the procedural documents that govern their use are often the key to successful electromagnetic testing.


ASTM E215 - 11Standard Practice for Standardizing Equipment for Electromagnetic Testing of Seamless Aluminum-Alloy Tube


FIGURE 4. Types of reference standards; (a) notched tubes; (b) calibrationblock; (c) block with graduated holes.


FIGURE 4. Types of reference standards; (a) notched tubes; (b) calibrationblock; (c) block with graduated holes.

LegendA. Made of same material as part

to be inspected, 6.4 mm (0.25 in.) thick, 32 root mean square finish.

B. Holes reamed to ±0.05 mm (±0.002 in.) tolerance and 32 root mean square finish.

C. Vibration etched hole sizes and material.


Work Reference


Reference Standard - block with graduated holes


Reference Standard - block with graduated holes / slots


10.2 PART 2. Types of Reference Standards10.2.1 Conductivity Reference StandardsEddy current testing is widely used to determine electrical conductivity. Both primary and secondary reference standards can be used to determine conductivity. 1. Primary reference standards have values assigned through direct comparison with

a standard calibrated by the National Institute of Standards and Technology or have been calibrated by an agency that has access to reference standards calibrated in fundamental units. For example, some primary reference standards have values assigned through direct comparison with standards calibrated according to test technique ASTM B193.6 Primary reference standards are usually kept in a laboratory environment and are used only to calibrate secondary reference standards.

1. Secondary reference standards are those reference standards supplied with the instrumentation or reference standards constructed by the user for a specific test. These reference standards are used to calibrate the test instrument.


Conductivity reference standards should be tested with a relatively small coil to determine the uniformity of electrical conductivity over the surface of the standard; both the front and back surface should be tested for conductivity differences. If possible, scanning the surfaces at several input signal frequencies is recommended. Each time the reference standards are used, the probe coil is placed at the same position within 6.4 mm (0.25 in.) of the center of the standard. Conductivity reference standards are precise electrical materials and should be treated as such. Any scratching of the standard surface could introduce error in measurement. Avoid dropping and other rough handling and keep the surface of the standard as clean as possible with a nonreactive liquid and a soft cloth or tissue. Reference standards are stored where the temperature is relatively constant; placing the reference standards where large temperature variations occur may cause thermal shock and should be avoided.


10.2.2 Coating Thickness Reference StandardsEach instrument should be calibrated in accordance with the manufacturer’s instructions before use. Calibration should be checked at frequent intervals during use. Calibration reference standards of uniform thickness are available in two types:

(1) foils or shims of known thicknesses laid on an appropriate substrate and (2) actual coatings affixed to prepared substrates as supplied or recommended by the instrument manufacturer or standardizing organization (Table 1).


TABLE 1. Standard reference materials from National Institute of Standards and Technology for calibration of instruments used in measurement of organic and nonmagnetic inorganic coatings over steel. Each 45 × 45 mm (1.8 × 1.8 in.) block consists of fine grained copper electrodeposited on low carbon steel substrate.


Calibration Shims Calibration foils or shims are placed on the surface of an uncoated basis metal when calibrating electromagnetic testing instruments. Shims are well suited for calibrating on curved surfaces and are often more readily available than a coated standard. To prevent measurement errors due to poor contact between shim and substrate, there must be intimate contact between them. Calibration shims are subject to indentation and should be replaced when damaged. Nonmagnetic shims may be used to calibrate magnetic thickness gages for measurement of nonmagnetic coatings. Plastic shims can be used to calibrate electromagnetic testing instruments for measurement of nonconductive coatings. Shims should be made of materials that will not change shape (strain or bend) when pressed on with the probe and that will not change in thickness with variations in moisture or temperature. Two or more shims on top of each other should be avoided unless flexibility of thin shims is required for a curved surface.


Calibration Shims


Life of colour


Coated Substrate Reference StandardsEach coating specimen used as a reference standard has a coating of a uniform thickness permanently bonded to a substrate material. Calibration reference standards of several known coating thicknesses may be used for a single application. For calibration, the thickness of one reference coating should be as close as possible to the upper limit, one as close as possible the lower limit and another as close as possible to the desired coating thickness. For instruments that measure coatings on magnetic substrates, calibration reference standards should have the same magnetic properties as the coated test specimen. For electromagnetic testing instruments, such as eddy current liftoff gages, the calibration standard should have the same electrical and magnetic properties as the coated test specimens. To determine calibration validity, a reading should be made on a bare specimen identical to the test object in magnetic and electrical properties. If the coating process is changed after previous calibration, the calibration may no longer be valid, especially for magnetic coating gages and eddy current thickness gages, and the initial reading must again be established.


In some cases, calibration of instruments with two-pole probes is checked with the poles rotated 0, 90, 180 and 270 degrees. The substrate thickness for testing and calibration should be the same if their thickness can be sensed by the instrument (that is, if their thickness is less than about four or five standard depths of penetration). Often, it is possible to back up the substrates of standard and test specimens with sufficient thicknesses of the same material (to exceed the critical thickness) and to then make readings independent of substrate thickness. If the curvature of the coating is so extreme as to preclude calibration on a flat surface, then the curvature of the coated standard (or of the substrate on which the calibration foil is placed) should have the same contour.


10.2.3 Magnetic Thickness GagesReference standards are used for the calibration of gages that measure the thickness of nonmagnetic coatings on magnetic materials. In such reference standards, for example, the steel substrates may have the magnetic properties of low carbon steel and the nickel coatings may have the magnetic properties of an annealed watts nickel electro deposit free of cobalt and iron. These reference standards are often used to measure the thickness of paint and other organic coatings on steel, as well as galvanized zinc and other nonmagnetic metallic coatings. The number of different thicknesses required for these calibrations depends on the type of gage and the coating thicknesses to be measured. Magnetic thickness gages may also beused to estimate the magnetic properties of austenitic stainless steel weld etal. Because the magnetic properties of the weld metal are closely related to the ferrite content of the weld, magnetic thickness gages are used to estimate the weld’s ferrite content.


10.2.4 Sorting Reference StandardsWhen sorting with the absolute encircling coil technique, a known acceptable calibration standard and a known unacceptable standard are required. When using the comparative encircling coil technique, usually two known acceptable specimens of the test object and one known unacceptable specimen are required. For a three-way sort, it is best to have three calibration reference standards, including either two for the high and low limits of acceptability for one group or one each for two unacceptable groups. The third reference standard represents the acceptable lot of material.


Keywords: For a three-way sort, it is best to have three calibration reference standards, including either two for the high and low limits of acceptability for one group or one each for two unacceptable groups. The third reference standard represents the acceptable lot of material.


Calibration and StandardizationElectromagnetic testing is used to sort objects by comparing them to standard specimens. Empirical data and physical tests on samples representing properties to be separated determine the validity of the sorting. The calibration and standardization procedure is based on the properties of the sample requiring separation. The sorting may require more than one test operation. When using the absolute encircling coil technique, the known acceptable calibration standard is inserted in a fixed position in the coil and the test instrument is adjusted to achieve an on-scale meter reading, an oscilloscope reading or both. The acceptable standard is replaced in the exact position with a known unacceptable standard and the sensitivity of the instrument is adjusted to maximize the indicated difference reading without exceeding 90 percent of the available scale range.


When using the comparative encircling coil technique, a reference standard is selected (usually one that falls within the acceptable limits of the specimens being tested) and placed in the reference coil. This coil and the reference standard are placed in a location where they will not be accidentally disturbed during the sorting operation. When used with a two-way mix, two calibrated reference standards are chosen: one represents the acceptable group and the other represents the unacceptable group. The acceptable calibration standard is placed at a fixed position in the test coil coinciding with the position of the reference standard in the reference coil. Then the operator balances the instrument. This acceptable calibration standard is replaced with one representing the unacceptable group and the test instrument’s phase, sensitivity and coil current are adjusted to maximize the indicator reading without exceeding 90 percent of the available scale range. The acceptable standard is reinserted and the instrument controls are alternately adjusted to retain a null value for the acceptable standard and maximum indication for the unacceptable standard.


A typical case of using reference standards for the high and low limits of acceptability is in the measurement of maximum and minimum acceptable hardness. In this instance, the reference standard representing the acceptable lot is placed in the test coil and the instrument is adjusted for a null or zero reading. The controls are then adjusted to maximize the indications without exceeding ±90 percent of the available scale range from the null for each of the maximum and minimum reference standards. Alternate readjustment of the controls may be necessary to retain the null reading, as well as the maximum and minimum limits for acceptance. For a three-way sort, when three dissimilar grades of material become mixed, the third acceptable reference standard is placed in the test coil and the instrument is nulled.


The two reference standards representing the other two grades are successively inserted into the test coil and the instrument’s controls are adjusted to maximize the indications without exceeding 90 percent of the available scale range from the null for each of the other two reference standards. Alternate readjustment of the controls may be necessary to retain the null reading as well as the indication for the other two reference standards. The procedure with probe coils is similar to that used with encircling coils. Instead of placing reference standards in the coils, however, the probe is positioned in a consistent, suitable location on the reference standard.


10.3 PART 3. Functions of Reference StandardsOne purpose of nondestructive testing is to ensure that all test objects containing critical discontinuities are rejected. Critical discontinuities are those that cause unsatisfactory performance. More importantly, all parts with anomalies that are unsafe and could cause bodily injury or death should be segregated from the acceptable parts.


Tank Farm Fire


10.3.1 Simulation of Acceptable PartsThe favored technique for ensuring that electromagnetic tests will reject anomalous parts is to set up and calibrate the test equipment by using reference standards. Reference standards simulate the parts to be inspected except they contain known discontinuities. The physical, electrical and magnetic characteristics of a reference standard must represent

(1) what is expected of the population of parts and (2) what the test process is sensitive to.

The reference standard should be a stable device with stable characteristics from which the performance of the electromagnetic test can be established and evaluated. Often, a single reference standard can be used to both verify the performance of the electromagnetic test equipment (establish test variables such as frequency, gain, balance and gate threshold), as well as to check the thoroughness of the test coverage (accuracy of the threshold, stability of the electronic equipment, stability of holding fixtures for part and probe and other details). Therefore, the same reference standard can be used both to set up and to calibrate test equipment.


Calibration reference standards are used to verify the accuracy of an electromagnetic test before a group of tested and accepted parts are released. If the total surface of each part is to be tested, then the reference standard must duplicate the total part. If only a portion of a part is to be tested, then the reference standard need only duplicate that portion of the part, providing a reliable means to properly locate the tested portion in a holding fixture. If the dimensions of the parts have tolerances, then the reference standard should be of average size. If there is a possibility that an out-of-tolerance surface may mask a discontinuity, then a preliminary test should ensure that those out-of-tolerance parts are removed before testing or the holding fixture may be made to reject the out-of-tolerance parts. If the surface finish of a part influences the test results, then the reference standard should have an average surface finish of the specified tolerance. If the conductivity of a metal varies, the reference standard should have an average value of electrical conductivity. If the value of magnetic permeability varies, then the reference standard should have an average value of magnetic permeability.


Notice that specific metal alloys may not need to be duplicated; only some of their physical, electrical and magnetic characteristics need to be. In some applications, there is no need for acceptance reference standards. The integrity of the electromagnetic test equipment is depended on to accept good parts. In these cases, the purpose of the reference standard is to establish and maintain sensitivity to discontinuities. Therefore, only rejection reference standards for both setup and calibration functions are needed. It has been observed that contractors who use nondestructive testing equipment often have different criteria for establishing rejection thresholds. Some contractors set the threshold so that the reference standards are just barely rejected (the discontinuity signal from the reference standard just penetrates the threshold). Other contractors set the threshold so that the discontinuity signal from the reference standard is twice as high as the threshold. Still others set the discontinuity signal to go 10 percent, 25 percent and even 50 percent beyond the threshold. These differing criteria are another reason why acceptance reference standards are not used.


10.3.2 Simulation of DiscontinuitiesThere are many varieties of natural discontinuities, even of a particular type such as cracks. No two are identical and each will therefore produce a different eddy current signature. If a reference standard is to be used to represent actual discontinuities, then extreme care must be taken to ensure that, however a discontinuity is simulated, it must represent the entire range of discontinuities to be encountered. An extreme example of this is the manufacture of steel bars containing cracks, inclusions, internal bursts, laps, scratches, seams and tears. Manufacture of reference standards representing these types in all their natural variations would be prohibitive. This dilemma can be addressed during technique development by studying many natural discontinuities, by identifying discontinuity mechanisms and by developing procedures to detect the discontinuities of interest.


A reference standard can then be developed that uses artificial discontinuities, not merely to represent rejectable objects, but rather to verify that the equipment is oriented properly and that the test setup adequately reproduces the parameters established in the laboratory to find discontinuities of interest. If some form of cracking is the discontinuity of interest, then the next consideration is the dimensions of a rejectable crack. Fracture mechanics can provide these limits in the form of critical crack size.

The most significant parameter in critical crack size is crack depth. Assuming the length of a crack will be at least ten times the depth (a very conservative assumption), the critical crack depth is the minimum value, under given load and environmental conditions, where catastrophic failure (brittle fracture) can occur. Once the critical crack depth is known, a safety factor is added to determine a smaller crack depth that is acceptable. Product safety is achieved by rejecting the deepest acceptable crack depth.


Plant Fire


Natural Cracks.Natural cracks display a great deal of variation that can be visible to various eddy current instruments. Natural cracks are not flat or parallel, their ends may or may not be tapered and they may contain corrosion products or other foreign material. All these variables can affect the signal produced and make interpretation difficult. Some natural cracks can be grown in reference standards. Artificial fatigue cracks have been available in primary reference standards from the National Institute of Standards and Technology. When detected by electromagnetic testing, a grown fatigue crack normally will not respond the same way as a quench crack. The reason is conductivity.

A tension fatigue crack has shiny faces that rub together as the crack grows; when at rest, these faces often touch one another. Thus, fatigue cracks are usually more conductive than oxide coated or carbon coated quench cracks.

A fatigue crack is an inefficient simulation for a quench crack.


Natural Crack


Natural Crack

http://www.sv.vt.edu/classes/MSE2094_NoteBook/97ClassProj/exper/meyer/www/meyer.html


Natural Crack

http://failure-analysis.info/2010/05/analyzing-material-fatigue/


Artificial tension fatigue crack

http://www.twi-global.com/technical-knowledge/published-papers/validation-of-methods-to-determine-ctod-from-sent-specimens/


Machined Cracks.It is also possible to machine very narrow slots to simulate cracks. Electromagnetic test equipment is sensitive to the depth, length and width of cracks. In eddy current tests, the size of a crack is revealed by the magnitude of the eddy current signal. If the detection equipment uses an analog meter to reveal the presence of discontinuities, then simulated cracks may be used (typical meters respond only to the magnitude of a signal and thus the size of a crack). These detection meters ignore phase angles related to crack depth. Electric discharge machined slots can be cut with faces 0.15 mm (0.006 in.) apart. If possible, the phase angle of separation between natural cracks and simulated cracks should be oriented on a cathode ray tube so that the points of intersection of simulated and typical natural cracks are equidistant from a line that is normal to the threshold and that passes through the origin of both signals. With this arrangement, minimum detection error occurs.


Figure 5 illustrates the proper angular arrangement of signals from the reference standard (with a simulated crack) and an anomalous part (with a natural crack). Simulated cracks should be placed in predetermined locations where they can cause the greatest weakness or where there is a high probability of crack occurrence. Orientations most difficult to detect should be included to ensure that natural cracks in those orientations are not overlooked. Several techniques have been developed for fabricating electromagnetic reference standards. The advantages and limitations of these fabrication techniques may vary with the application but excellent dimensional tolerances and repeatabilities are possible.


FIGURE 5. Angle of display for horizontal alarm threshold to minimize rejection error: (a) improper angle; (b) proper angle.

(a)

(b)


10.4 PART 4. Techniques of Reference Standard FabricationReference standards are made to represent the conditions evaluated in the test part. Variables to be considered and controlled during reference standard fabrication include geometry, conductivity, permeability, surface finish and coating. How best to simulate a discontinuity, inclusion or crack has been a topic for discussion and debate since the standardization of reference test objects in the 1950s. The ideal case might be to locate an actual part that contains a natural discontinuity of known size and then to use it as a reference standard. Reference standards have been obtained this way but it is difficult to find several identical parts containing identical discontinuities for use as natural and known reference standards.


10.4.1 Drilled HolesOne of the earliest techniques for making electromagnetic reference standards was the drilling of holes. The reference standard was fabricated from material of the same conductivity and permeability as that of the object to be tested. The reference standard’s geometry matched that of the component and small drilled holes were located at sites that would simulate natural discontinuities. These drilled holes were typically much smaller than the probe coil and the depth of the hole was much greater than the penetration depth of the eddy currents. In this way, equipment response to the reference standard was controlled by the drilled hole diameter alone. The following example of a drilled hole reference standard illustrates this reference standard type, along with its advantages and limitations.


The test incorporated the following specifications for fatigue crack detection. Holes of 0.8 mm (0.03 in.) diameter were drilled into aircraft aluminum alloy having a conductivity of about 32 percent of the International Annealed Copper Standard. Test frequency was about 200 kHz. The test was conducted with a metered probe having a single coil with a diameter of about 2.5 mm (0.1 in.). The response from the hole was a minimum of 4 percent of full scale deflection. Background noise (due primarily to test surface roughness) was typically 2 percent of full scale. Eddy current indications found equal to or exceeding the reference standard hole response were further evaluated as probable fatigue cracks. This test system resulted in the detection of confirmed cracks with measured surface lengths as small as 0.33 mm (0.013 in.).


Advantages of Drilled Hole Reference StandardsThe primary advantage of the drilled hole reference standard is its simplicity. Such a reference standard can be produced and reproduced at minimal cost without expensive tools or machinery. Uniformity of response from one reference standard to the next is quite high, typically within a few percent. When used in the manner described above, this reference standard provides an indication of signal sensitivity, background noise level and, from these, signal-to-noise ratio. In summary, this reference standard is economical to produce with repeatable results for a sensitive discontinuity detection setup.


Disadvantages of Drilled Hole Reference StandardsThe main drawback of a drilled hole is that it does not always behave like a crack. This difference is present in the test response produced by the reference standard. Although the impedance plane response for the reference standard is similar to that of a small crack, that similarity is lost when larger holes are used. Large drilled holes (compared to the probe size) result in modified and larger reference standard responses. These responses resemble the impedance plane response of a specimen edge (edge effect) and not the response of a crack. Effective use of the drilled hole reference standard is restricted to a narrow range of drill sizes that are closely related to the size of the probe coil being used.


Accuracy and RepeatabilityThe accuracy of the drilled hole reference standard is relative and not absolute. Crack length can be estimated or predicted from a reference standard hole size only by correlations established with experience. New correlations are required when significant changes occur in test condition or omponent geometry. Test system repeatability with the drilled hole reference standard is excellent when test conditions and component geometry are held constant. Care should be taken during drilling on thin materials to avoid distortions of the test object and the hole.


10.4.2 NotchesNotches are the first choice for producing simulated crack reference standards. The shape of the notch can generally be controlled during fabrication and the shape of the expected crack can be accurately simulated in the reference standard. Several techniques for producing simulated crack reference standards are discussed below.

Electric Discharge Machining Electric discharge machining (EDM) is a metal removal technique suitable for a wide range of materials. The technique uses a controlled electrical discharge between the reference standard material and a preshaped electrode. The electrode shape determines the shape of the notch, a simulated crack. Electrodes are generally cut from thin foil, graphite, brass or copper tungsten and notch widths of only a few hundredths of a millimeter (a few thousandths of an inch) are commonly produced. Figure 6 shows an apparatus used to position and move the electrode relative to the reference standard.


FIGURE 6. Feed mechanism for electric discharge machining of reference standards: (a) outside surface cutting; (b) inside surface cutting.

(a)


FIGURE 6. Feed mechanism for electric discharge machining of reference standards: (a) outside surface cutting; (b) inside surface cutting.

(b)


Electric Discharge Machining.


Electric Discharge Machining.

http://edmtechman.com/about.cfm?pg=2&chap=1


Electric Discharge Machining - Electrical Discharge Machining (EDM) is a non-contact thermal machining process capable of machining conductive and semi-conductive materials regardless of their hardness. Conversion of electrical energy to thermal energy through repeated occurrence of sparks between the tool and the workpiece immersed in a dielectric medium and separated by a small distance (spark gap) results in the material removal from workpiece as well as tool by melting , evaporation and spalling in some special cases. The material removed is the major source of debris particles. The common dielectric fluids used are kerosene, paraffin, and light hydrocarbon oils. A necessary condition for producing a discharge is the ionization of the dielectric medium and splitting up its molecules into ions and electrons (i.e., formation of plasma). A schematic sketch of EDM and the material removal mechanism are shown in Figure 1 and 2 respectively.

http://www.min.uc.edu/ucman/research/electrical-discharge-machining


One caution should be noted when using electric discharge machining on alloys containing significant amounts of the elements iron, nickel and cobalt. The permeability of such alloys (including the nonmagnetic stainless steels and some of the high temperature nickel base alloys) can be modified by the electric discharge machining process. When high power, high speed electric discharge machine cutting is used, a layer of recast material is produced at the base and on the walls of the notch and this recast material can have greatly altered magnetic permeability. For small notches intended to simulate small cracks, the eddy current response can be dominated or greatly affected by the magnetic recast material. Slower cutting speeds and fluid flow during electric discharge machining will reduce or eliminate this problem. Electrode feed rates less than 0.013 mm (0.0005 in.) per minute are common. The primary advantage of electric discharge machining is accuracy. It is possible to have electrodes with widths as small as 0.05 mm (0.002 in.) and to produce slots with very short surface lengths, as small as 0.13 mm (0.005 in.).


Errors between the desired dimensions of a notch and the resulting dimensions after the electric discharge machining process are often less than 0.013 mm (0.0005 in.). Because of these small dimensions, electric discharge machining can produce simulated discontinuities whose electromagnetic test indications closely resemble those of actual cracks. By designing the electrode according to the shape of the discontinuity, many different widths and length-to-depth ratios are possible. The electrode may also be placed inside a fastener hole, on a radius or on a flat surface.

Figure 7 shows cross sections of finished electric discharge machined notches in carbon steel (Unified Numbering System G10260). One disadvantage of electric discharge machining is expense. It may cost hundreds of dollars per slot to have accurate reference standards made. In summary, the accuracy of electric discharge machining fabrication can be very good but care must be taken so that recast material does not accumulate in the notch. The operator also must carefully choose the electrode material, voltage, current and the right cutting speed for compatibility with the reference standard material.


FIGURE 7. Notch replicas (magnified 50×) representing the stabilizing effect of rectilinear feed guides on notch profiles when machine is subject to lateral vibration: (a) notch made with radial feed guide; (b) notch made with rectilinear feed.

(a)

(b)


Planing FabricationThe planing technique for fabricating simulated crack reference standards is also a metal removal technique. A very small tool is used to precisely gouge the surface, producing a very narrow slot. Typically, a tool maker will hand grind a small tip on a rod of tool steel about 0.2 to 0.25 mm (0.008 to 0.01 in.) wide. The tool is placed in the chuck of a planer and aligned with the reference standard. After one pass, the tool is indexed deeper into the material and another pass is made. Care must be taken not to move the tool too quickly because heat or chatter will develop. Heat may alter the conductivity of the reference standard and chatter will cause surface roughness on the walls of the slot. Slow cutting speeds and adequate fluid flow during the machining process will reduce this problem.


A derivation of the planing technique is a manually filed notch mentioned in ASTM E 243.11 The notch is made using a 6.4 mm (0.25 in.) diameter, number four cut, round file.12 The reference standard material is seamless copper and copper lloy tubing. The required tolerance of the otch depth is ±0.013 mm (±0.0005 in.). The primary advantage of the planing technique is economy. Slots can be machined in a short time with conventional shop equipment. The major disadvantage of the process is its inability to produce narrow slots. It is very difficult to make a strong, sharp cutting tool less than 0.25 mm (0.01 in.) wide. In addition, because of the tool’s size and the large forces acting on it as it travels across the reference standard’s metal surface, the tool often breaks. The accuracy and repeatability of this process is good when the same operator is using the same machine and the same tool. Because the tools are usually ground by hand, no two will be alike.


Jet AbrasivesAnother metal removal technique for fabricating simulated crack reference standards is the jet abrasive technique. A high pressure stream of fluid containing abrasive compounds is forced through a small opening, hitting the metal surface with enough force to cause erosion. Slot width is controlled by the orifice size and slot depth is controlled by the number of passes over the material. The major advantage of the jet abrasive process is its repeatability. After the equipment settings are known for a given type of reference material and slot geometry, they can be accurately reset at another time. There are no moving parts or ablative surfaces (as in the electric discharge machining or planing processes) that contribute to inaccuracies and nonrepeatability. A disadvantage is the lack of control over slot geometry, due to the spreading nature of the fluid. Slot widths tend to be wide because of this spreading. Another disadvantage is that, after the process is complete, the surface of the slot has a texture related to the size of the abrasive compound. This pitting may have an effect on eddy currents at high frequencies.


Jet Abrasives


Natural CracksThe cost of obtaining or producing natural cracks for use as electromagnetic reference standards continues to be prohibitive. It must be noted, however, that the electromagnetic response from an electric discharge machined notch or other simulated discontinuity is not the same as that from a natural crack. Care should be taken when selecting reference standards and test systems to ensure that natural crack sensitivity is maintained. The obvious advantage of the natural crack is that it most closely resembles an actual discontinuity. The gap between faces of the crack will be very small.


The main disadvantage in the natural crack technique is the expense of acquiring specimens. If the cracks are grown from fatigue specimens, the cost could be very high. If the cracks are from an inservice part, the availability of the specimens will be a limitation. Another problem is that of size. There is no control over the size of actual inservice specimens. After the reference standard is obtained, some other form of nondestructive testing will be needed to measure the discontinuity’s size. The accuracy of a natural crack reference standard will always be in question because its exact size is not directly known until it is broken open. Repeatability can be controlled somewhat by closely monitoring the growing of the rack and then machining away portions to make it smaller.


10.4.3 Transverse NotchesThe guidelines for producing simulated crack reference standards with longitudinal notches apply equally well to transverse notches. However, changes in geometry must be compensated for by adapting the electrode in electric discharge machining or by developing sophisticated machining motion for planer techniques. It may be less difficult to fabricate notches that travel along the direction of the tube or radius rather than against it. For that reason, it could be more advantageous to fabricate transverse notches using the electric discharge machining or jet abrasive techniques. In general, the problems inherent in the fabrication of simulated crack reference standards are similar to those of longitudinal notches.


Virginia Class


Good Luck!

electromagnetic testing emt chapter 11

Engineering