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8/88 REMR TECHNICAL NOTE CS-ES-1.4 NONDESTRUCTIVE TESTING METHODS FOR METAL STRUCTURES PURPOSE: To provide information on nondestructive testing methods for metal structures that are readily available from most commercial testing laboratories. APPLICATION: The nondestructive testing methods described in this technical note can be used to inspect metal structures, parts, or components for surface and subsurface defects. APPLICABLE CODES AND STANDARDS: AWS Structural Welding Code Bl.10 Ultrasoni
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  18/88 REMR TECHNICAL NOTE   CS-ES-1.4NONDESTRUCTIVE TESTING METHODS FORMETAL STRUCTURES PURPOSE: To provide information on nondestructive testing methods for metalstructures that are readily available from most commercial testinglaboratories.APPLICATION: The nondestructive testing methods described in this technicalnote can be used to inspect metal structures, parts, or components for surfaceand subsurface defects.APPLICABLE CODES AND STANDARDS:AWS Structural Welding Code Bl.10Ultrasonic, ASTM E 114, 164, and 797Dye Penetrant, ASTM E 165Magnetic Particle, ASTM E 709Radiography, ASTM E 94 and E 1032ADVANTAGE:Nondestructive testing is ideally suited for the inspection ofwelds and can be used to monitor the performance of welders, as well as todetermine the quality of welds and metal components.DESCRIPTION: This technical note covers the five most common nondestructivetesting methods used in the construction industry to detect surface and inter-nal discontinuities in welds and fabricated components: visual, penetrants,magnetic particle, ultrasonic, and radiographic testing.a.Visual: Visual inspection of a weld can be used to determine ifthe weld was made properly. Unacceptable weld profiles, insuffi-cient throat, or excessive concavity can reduce the strength of aweld by producing undesirable stress concentrations. Alignment,distortion, arc strikes, and the general condition of the weld canbe observed in addition to weld profiles. The person performingthis inspection should have a magnifying glass, flashlight, and fillet measuring gage.b.Penetrants: Penetrant inspection methods are used for inspectingalmost any nonporous material for defects that are open to thesurface. Surface defects that can be found are all types ofcracks in connection with welding, grinding, fatigue, etc.Penetrant methods are restricted to the location of surfacedefects; therefore, cleanliness is of the utmost importance.Penetrant inspection consists normally of four steps (Figure 1):application of the penetrant, removal of excessive penetrant fromthe surface, application of a developer, and inspection. The twomain groups of penetrants are visible dyes and fluorescentpenetrants.  2REMR TN CS-ES-1.48/88Figure 1. Major steps of fluorescent penetrant inspection (reprinted fromAF Study Guide ABR 53630-201)1.The advantages of the visible dye penetrant method are that itprovides fast, on-the-spot inspection and its initial cost isrelatively low. A white or blank surface indicates freedomfrom cracks or other defects that are open to the surface.Disadvantages are that it is not practical on rough surfacesand it detects only defects open to the surface.2.The advantages of the fluorescent penetrant method are that itcan be used on rough surfaces and it is much more sensitivethan dye penetrants. Disadvantages are that a black light andhood are required (unless testing is performed at night) andonly defects open to the surface are detected.c.Magnetic particle: Magnetic particle inspection will indicatesurface or near-surface defects in ferromagnetic materials such asiron and steel. A magnetic current is introduced into the area tobe inspected, and iron oxide powder is dusted on the area. Theinduced magnetic field will be distorted if there is a discontinu-ity such as a crack on or near the surface (Figure 2). A leakageof this field creates poles that attract the iron oxide powderdusted on the area. A sharp line indicates a surface disconti-nuity. When the discontinuity is below the surface, the field isweaker and less concentrated; therefore, the powder indication onthe surface will be broad and fuzzy.1.A principal limitation of the magnetic particle method is that itapplies only to magnetic materials and is not suited for smalldeep-seated defects. The deeper the defect is below the surface,the larger it must be to be detected. With magnetic particletesting, the surface to be inspected must be accessible. Thismeans shafts or other equipment cannot be inspected withoutremoving pressed wheels, pulleys, or bearing housing.2.The advantages of magnetic particle inspection are that it is apositive method of finding all cracks at the surface, theequipment is portable, and the method is flexible.  3REMR TN CS-ES-1.48/8Figure 2. Disruption of magnetic field by weld-metal defect (reprinted fromTechnical Manual 5-805-7, Welding Design, Procedures and Inspection, Headquarters, Department of the Army, 1968)d. Ultrasonic: Ultrasonic inspection uses a beam of high-frequency sound,in the range of 1 to 5 MHz, to inspect a wide range of thicknesses andmaterials. The ultrasonic unit produces electrical pulses that are fedto a handheld transducer or search unit, where the electrical energy isconverted to mechanical energy. A couplant, which can be oil, grease,water, etc., is placed on the area being inspected; the transducer isthen placed on the couplant in contact with the metal. The mechanicalenergy emitted from the transducer is similar to a beam of light from aflashlight. The soundwave is emitted as bursts or pulses of energyvibrations. These vibrations travel into the area being inspected untilthey strike or are interrupted by a crack, inclusion, or other discon-tinuity or by the far side of the material. When a discontinuity isencountered, some of the sound vibrations are reflected to thetransducer. The larger the discontinuity (crack, porosity, slaginclusion, etc.), the larger the amount of energy that will be reflectedto the transducer. The transducer converts the returning vibrationsinto electrical impulses that are amplified and appear on the screen ofa cathode-ray tube (CRT) as indications. The initial pulse on thescreen represents the contact face or the testing surfaces (A, Figure3). The flaw (B) reflects some sound, and the rear surface (C) reflectsmore sound. The presentation on the CRT displays the location in thethickness and relative size of the flaw. The distance between A and Cis representative of the material thickness; therefore, the distancethat the flaw is below the surface can be fairly accurately measured.The size of the flaw determines the height of the indication B. Acalibration standard is required to accurately measure defect size.  4REMR TN CS-ES-1.48/88Figure 3. A scan presentation on CRT (reprinted from MIL HDBK 333(USAF), Handbook for Standardization of NondestructiveTesting Methods, Department of Defense, 1974)1.A limitation of the ultrasonic inspection method is that apermanent record is difficult to obtain during field inspection.A picture of the CRT and written reports of the inspection resultsare sometimes difficult to correlate. Rough surfaces maketransducer contact difficult and sometimes impossible. A couplantmust always be used to eliminate compressible air and fill thevoids and irregularities on the test surface. Calibrationstandards are usually required to calibrate the instrument andevaluate sizes of defects.2.Ultrasonic inspection allows the inspector to inspect almost anymaterial quickly with minimum restriction to size, shape, orthickness. With a variety of plastic wedge angles that can beused with transducers, sound energy can be transmitted into a partat different angles for complete inspection.e.Radiography: Radiography includes X-ray and gamma ray inspection.X-ray is radiation generated from an X-ray tube, whereas gamma is aradioisotope, usually iridium 192. The radiation from thesesources is of such a short wavelength that it can penetrate mate-rials to disclose the presence of flaws and imperfections in theinterior of metals and weldments. The radiation intensity isaffected by flaws and material differences. The radiation emittedfrom either of these sources is passed through, absorbed, or scat-tered in the metal. For example, if there is a slag inclusion orporosity as the radiation passes through a weld, the void resultsin the reduction of the total thickness of the weld figure. Thisreduction in metal allows more radiation to pass through the sec-tion containing the void than through the surrounding metal. Vari-ations in the radiation beam are recorded as an image on a film. Adark spot, corresponding to the projected position of the void,will appear on the film when it is developed. Thus, a radiographis a kind of shadow picture: the darker regions on the film repre-sent the more penetrable parts of the weld; the lighter regions,the more opaque.
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