11. Nondestructive Testing

This is a reprint of an article from the fifth edition of

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1990

Nondestructive Testing

Vol. B 1

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11. Nondestructive Testing KANJI ONO, Department of Materials Science and Engineering, University of California, Los Angeles, California 90024. United States

1.

Introduction

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2.5.

Penetrant Methods

2.

Methods or Nondestructive Testing

11-2

Magnetics and Electromagnetics

2.1.

Material Identification

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2.2. 2.2.1. 2.2.2. 2.2.3. 2.2.4. 2.2.5. 2.2.6. 2.2.7. 2.2.8. 2.2.9. 2.2.10.

Radiography

2.6. 2.6.1. 2.6.2. 2.6.3.

.2.3. 2.3.1. 2.3.2.

2.3.3. 2.3.4. 2.3.5. 2.3.6. 2.4. 2.4.1. 2.4.2. 2.4.3. 2.4.4.

Radiation Attenuation Imaging Recording Images Screens Radiographic Quality Radiographic Practices Radiographic Interpretation Other Techniques Radiation Safety Ultrasonics Waves Generation and Detection Inspection Methods Display Methods Interpretation Inspection Standards Acoustic Emission

Detection Material Behavior Source Location Acoustic Emission Monitoring

11-4 11-4 11-4 11-6 11-6 11-7 ] 1-8 11-8 11-9 11-10 11-11 H - It H-1 I 11-13

H-tS 11-16 11-17 l1-1!l l1-1!l 11-19 11-19 11-19 11-20

1. Introduction Nondestructive testing of materials has become an indispensable part of modern industry. Beginning with the oil and whiting method, a forerunner of the penetrant testing method used for crack detection, a variety of tests-based on numerous physical principles-have been developed. At the start, the primary task was to find llaws and discontinuities. With the advent of fracture mechanics, the measurement of llaw dimensions became more urgent. Increasing use of fiber-reinforced plastics in the past decade has posed new challenges in material characterization.

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11-22 Magnetic Particle Inspection 11-22 Magnetic Leakage Field Inspection .11-23

Other Magnetic Inspection Methods 2.6.4. Eddy-Current Inspection 2.6.5. Microwave Inspection

11-24 11-24 11-27

2.7. 2.7.1. 2.7.2. 2.7.3.

Thermal and Optical Methods

2.8. 2.8.1. 2.8.2. 2.8.3.

Leak Testing

Pressurized Gas Leaks Pressurized Liquid Leaks Vacuum System Leaks

11-29 11-29 11-30 11-30

3.

Uses

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3.1.

Metallic Pressure Vessels and Piping 11-30

3.2.

Composite Pressure Vessels and Piping

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3.3.

Weldments

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3.4.

Other Uses

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4.

Rererences

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Thermal Inspection Visual and Optical Inspection Optical Holography

11-27 11-27 11-28 11-28

Techniques of nondestructive evaluation have made substantial advances. However, many problems still remain because the current design of components and structures demands higher efficiency and the material capability is exploited to its fullest extent. For example, a bonded joint of composite materials is a desirable design approach, yet no nondestructive testing method is available to assure the strength of the joint. This article provides brief summaries of widely used methods of nondestructive testing, covering basic concepts, techniques in common usc. and examples of industrial applications. Less well developed but promising methods are

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also introduced. Because the field of nondestructive testing is broad, all methods of importance to the chemical industry are included, but those applicable primarily to the aerospace, nuclear, and steel industries are covered only incidentally. Two major problems, corrosion and cracking, confront nondestructive test engineers in the chemical and petroleum industries. Ferrous and corrosion-resistant nonferrous alloys used in vessels and piping are often subjected to chloride- and hydrogen-containing environments. Hydrogen-induced cracking and stress corrosion cracking are found frequently. Cracking of welds from residual stress or poor weld quality is another persistent problem during service. Many vessels and pipes are constructed from carbon steel, and general corrosion from long-term service must be monitored. In addition, fiber-reinforced plastic components require special attention because traditional nondestructive testing methods cannot adequately evaluate their reliability. Nondestructive testing attempts to discover indications of discontinuities and abnonnal conditions in materials and structures. If an indication is found to be beyond an acceptable limit, it is a tlaw and the material (or structure) must be repaired or rejected. A very small gas hole in a weld is innocuous and can be allowed to remain. Beyond a certain size, however, it is no longer tolerated, while cracks of any size are typically rejected. Flaws and discontinuities are treated as interchangeable because the size limits are set by design considerations. Flaws are introduced at three stages. The first is during primary manufacturing. Shrinkage cavities, inclusions. cracks. blowholes, and other faults occur in castings. Flaws in forgings may be traced to ingots that contain segregations, pipes (shrinkage cavities). inclusions and, hydrogeninduced cracks or hydrogen flakes. During forging, forging bursts (internal tears), surface tlaws (such as cold shuts, laps. folds. and seams), burning. and decarburization are often the result of improper setup and control. RoIling and drawing of ingots and billets generate flaws similar to those of the forging process. In addition,lamination and chevron defects may occur. The second stage is fabrication into structures. in which welding is the most important process. Because the fusion of metals is induced, many naws are also introduced. These include gas porosity. slag entrapment, incomplete fusion. incomplete penetration, and cracks. In addition, surface flaws

Vol. B 1 such as undercut, mismatch. and underfill occur. The third stage of naw generation is during service. Overloading and fatigue loading produce cracks. and corrosion leads to pitting, general reduction of wall thickness. and stress corrosion cracking. Exposure to hydrogen results in hydrogen-induced cracking, and tluid tlow produces erosion damage. High-temperature environments in furnaces and reactors cause creep and oxidation, leading to wall thinning and cracking. The nature and origin of the flaws listed here are discussed in [t)o Detailed infonnation on nondestructive testing can be found in introductory texts (2). training guides [3], handbooks [4]. codes and standards [5]-[7], and a series of monographs and conference proceedings [8], [9]. Several journals and a newsletter are published in English (10). and publications in other languages are also available. More than a dozen national societies for nondestructive testing, such as the American Society for Nondestructive Testing (ASNT), the British Institute for Nondestructive Testing (BINDT), Deutsche Gesellschaft rur Zerstorungsfreie Priifung e. V. (DGZfP). and the Japanese Society for Nondestructive Inspection (JSNDI). exist worldwide, and the International Institute of Welding (IIW) and the International Standards Organization (ISO) are active in establishing standards. The Institut fUr Zerstorungsfreie Priifverfahren (IZfP; Saarbriicken. Federal Republic of Germany) and the Electric Power Research Institute (EPRI) NDT Center are research institutes dedicated to studies of nondestructive testing.

2. Methods of Nondestructive Testing 2.1. Material Identification To verify that an alloy is within specified composition limits, quantitative chemical analysis is the only answer. However. a system of rapid identification of metals and alloys is useful in confinning the alloy type or in sorting mixed lots of alloys. Several techniques are used singly or in combination. The basic technique is to use mass density (specific gravity) for preliminary grouping into light (AI. Mg. Ti). medium (steel, Cu. Ni. Zn). heavy (Pb. Mo, Ag). and very heavy (W. Ta. Pt) alloys. Some alloys have distinct colors, but most have a white or grayish white color.

Vol. 81 X-Ray FluoresceDt Analysis. Each clement generates characteristic X rays at several specific energy levels when it is irradiated with electromagnetic radiation having a higher energy spectrum. Characteristic X-ray energies range from 1 keV for sodium to 118 keY for uranium K radiation, and from I keY for zinc to 17 keY for uranium L radiation. Fluorescent X rays are detected with an energy-discriminating proportional counter or solid-state detector and are electronically proct:sscd to identify the chemical composition of an alloy sample. Because of the high absorption of low-energy X rays, only the surface of the sample can be analyzed. Radioactive isotopes are often used as the source of radiation for exciting fluorescent X rays. An isotope source of 1- 100 millicuries (mCi) can be small and portable. Some sources must be replaced periodically. Regular X-ray sources or small, air-cooled X-ray tubes can also be used. For X-ray energy analysis, a solid-state detector commonly used in a scanning electron microscope (SEM) for elemental detection can be employed, but such a detector requires constant cooling with liquid nitrogen. Another limitation arises from the resolution of the X-ray energy analyzer; elements lighter than aluminum cannot be evaluated. In the electron microprobe analyzer, energetic electrons are used to generate fluorescent X rays; a high-resolution energy spectrum analyzer is employed, which is capable of detecting an element as light as boron. Small samples can be analyzed in an SEM or microprobe. Chemical Spot Testing. Chemical spot testing utilizes chemical color reactions, which are carried out on a sample, on filter paper, or on a spot plate. A test kit consists of about 100 solutions. Carbon steels, for example. can be identified by placing a drop of 50% nitric acid on a fresh surface. After 5 min, the spot is rinsed with water: a brown spot indicates carbon steel. See [1 I] for a listing of solutions and identification tests. Spark Testing. The spark testing technique classifies ferrous alloys aceording to their chemical composition by visual examination of the sparks thrown off when a sample is held against a high-speed grinding wheel. The test is fast and economical. yet an experienced operator can identify a variety of alloys with reasonable accuracy [1), [12).


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Hardness Testing. Hardness testing is not strictly nondestructive because an indenter is forced into the surface of a test piece. The resulting indentation is, however. often sufficiently small to be left on a finished product. The indenter is a ball, cone, or pyramid made of hardened steel or diamond. Constant force is applied on the indenter, and the diameter, length. or depth of the indentation is used as a measure of material hardness. The Brinell, Rockwell, Vickers, and Knoop hardness tests are commonly employed [13) (see also -. Mechanical Properties and Testing of Metallic Materials). Because each alloy has a certain hardness range. knowing the hardness aids in material identification. Hardness measurement is also a nondestructive test method for the yield lind tcnsilc strength of ductile alloys based on empirical correlations. In brittle ceramic materials, cracks emanate from the corners of a Vickers hardness indentation. The Icngth of these cracks has been found to be related to the fracture toughness of ceramics. Thus. nearly nondestructive determination of ceramic fracture toughness can be conducted. Electrical and Magnetic Testing. Electricul resistivity clln be used to classify metals and alloys and the state of heat treatment of many alloys. One technique involves use of a four-point probe with four equally spaced tips. The probe passes a direct current (ld 10 cm). tield inspection becomes difficult because high-energy sources are needed. Ultrasonic methods, especially with shear waves. are widely used for weld inspection. Thickness mellsurement using the normlll beam is commonly practiced for corrosion monitoring. Those areas having .1 high corrosion rate or history of attack arc inspected thoroughly.

3.2. Composite Pressure Vessels and Piping The performance of tiber-reinforced plastic vessels lind piping has been poor. and many failures have been recorded. Apllrt from acoustic emission. no satisfactory test exists for determin-


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ing the structural adequacy of tiber-reinforced plastic equipment. Radiography is difficult because resins have low absorption coefficients. Fiber-reinforced plastics are anisotropic. and ultrasonic attenulltion is very high, making pulseecho techniques hard to use. In addition. the various types of life-limiting llaws are quite different and some of them arc not amenable to conventional ultrasonic insJX.'Ction. Fiber fracture and tiber-matrix debonding arc difficult to detect with ultrasonics. The use of acoustic emission for testing tiberreinforced phlstic vessels and piping has been highly successful. and standard test procedures have been established [6. E 1067), [22]. (29), (30). Test vessels and piping are pressurized up to 150 % of thc maximum allowable working pressure. The procedures are designed to locate substantial flaws, which are then evaluated by other techniques such as ultrasonic testing (delamination) or visual (resin loss) and penetrant (matrix cracking) inspection. A vessel (or piping) to be tested can be new. in-service. or repaired, and different steps arc specitied for atmospheric, vacuum. and pressurized vessels. A vessel that has been in service must be preconditioned by reducing the operating pressure; for this. the maximum operating pressure within the previous year must be known. Acoustic emission instrumentation should have a sufficient number of channels to localize sources by using the zone location method; that is. many AE sensors arc installed to completely cover the vessel with the corresponding zone marked around each sensor. Acoustic emission activities of llaws within each zone arc detected, and the zone represents the approximate position of these flaws. High-frequency (100-200 kHz) sensors arc used for zone location. Two or more low-frequency (25 - 75 kHz) sensors are used to evaluate the adequacy of coverage of the high-frequency sensors. If a low-frequency sensor detects acoustic emission whereas none of the high-frequency sensors do, the lattcr must be relocated. Sensors arc positioned to detect structuntl flaws at critical sections of the test vessel. such as high-stress areas. geometrical discontinuities, nozzlcs. manways, repaired regions. support rings. and visible flaws. Pressurization of a vessel during AE testing proceeds in steps. with pressure hold periods. For atmospheric vessels. the pressure is held at 50,75,87.5. and IO()% of the test pressure. Pressure vessels arc stressed with 10% increments. with depressure increments (also 10%) above

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Vol. B 1


30% of the test pressure. A test is terminated whenever a rapid increase in AE activity indiC'dtes an impending failure. Acoustic emission data from the high-frequency sensors are used for evaluation, whereas the low-frequency sensors generally detect acoustic emission from significant flaws. Detected flaws are graded according to using several criteria, including emissions during pressure hold periods, felicity ratio (the ratio of the load at the onset of significant emissions to the maximum prior pressure), total AE counts, high-amplitude events, and long-duration events. Emissions during hold indicate continuing permanent damage and lack of structural integrity. For in-service vessels, the felicity ratio criterion (when it is less than 0.95) is an important measure of previous damage. High-amplitude events indicate structural (fiber) damage. especially in new vessels. Long-duration events are characterized by measured area of the rectified signal envelope (MARSE), which is an indicator of combined signal and amplitude duration. Large MARSE values result from delamination. adhesive bond failure, and crack growth.

3.3. Weldments Nondestructive testing methods used for completed fusion weldments include (I) visual inspection, (2) radiography, (3) ultrasonic pulse echo, (4) magnetic particle and leakage field testing. (5) liquid penetrant testing, (6) leak testing. and (7) acoustic emission testing. For many noncritical welds, integrity is assured mainly by visual inspection to look for cracks. bead thickness, bead contour. undercut. overlap. and spatter. For critical welds, both faces and root surfaces are examined, especially for cracks, undercut, root penetration. and unfilled craters. These are tested further by radiography and ultrasonics for internal flaws. Radiography is commonly used for detecting porosity, slag entrapment, and inclusions. A round or oval dark spot represents the image of a pore. Slag inclusions appC'dr along the weld edge as irregular or continuous dark lines. whereas tungsten inclusions give rise to single or clustered light spots. Crdcks are sometimes visible in radiographs as dark narrow irregular lines, but the lack of any radiographic image of cracks does not assure their absence. Radiography may be unable to detect incomplete fusion and incom-

plete penetration be