Introduction to Standards and Nondestructive Testing of Industrial Materials

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1 This sample chapter is for review purposes only. Copyright The Goodheart-Willcox Co., Inc. All rights reserved. KEY CONCEPTS 17 Introduction to Standards and Nondestructive Testing of Industrial Materials Chapter New product and process development. In a competitive world, developing new products and processes is essential to maintain Upon completion of this chapter, you should understand: The purpose and applications of industrial materials testing. The need for materials testing standards and standardizing agencies. Five common nondestructive tests. Principles and applications of common nondestructive tests. The purpose of testing any material is to understand the properties of the material. All areas of industry, especially those dealing with structures and machines, are concerned with material properties. Materials must be tested for several reasons including safety, product performance, product quality, new product and process development, and scientific research. Certain standards have been developed to aid in testing. In addition, many testing methods exists. These include both destructive and nondestructive tests. Destructive testing is discussed in Chapter 18. This chapter introduces testing standards and standardizing agencies. Then, commonly used nondestructive testing methods are discussed. INTRODUCTION TO STANDARDS As stated above, there are several reasons that a material is tested. These reasons are listed and briefly explained in this section. Safety and product performance. All structures and machinery are designed to support specified loads and provide certain services. That translates into certain material property requirements, especially mechanical properties, to ensure the structure and machinery meet the performance and safety requirements. Product quality. The variation in material property and product performance must be known and kept to a minimum. This helps maintain the product quality level and the profitability of manufacturing process. 302 Industrial Materials leadership in the market. Material properties must be tested to understand and develop new products or improve existing products. Scientific research. Modern industry has created the need for new and better materials. To develop new materials and improve existing materials by different processes, material properties must be tested and understood. Standardized tests are followed for almost all industrial material testing applications. It is very important to follow standard tests because different test methods and procedures usually produce different results. In other words, you must make sure you are always comparing apples to apples. Standards provide a common base of comparison for various test results. Standards also offer a common language for communication between different organizations in industry and commerce. Industrial standards regarding material specifications and material testing are generated and maintained by various agencies. These include: standardizing organizations, such as the American Society for Testing and Materials (ASTM); professional or industrial associations, such as the Society of Automotive Engineers (SAE); and governmental agencies, such as the Department of Defense. AMERICAN SOCIETY FOR TESTING AND MATERIALS (ASTM) Founded in 1898, American Society for Testing and Materials (ASTM) is a scientific and technical organization. It was formed for the development of standards on characteristics and performance of materials, products, systems and services and the promotion of related knowledge. ASTM is the world s largest source of voluntary standards. The society operates through 135 main technical committees and 2,080 subcommittees. These committees function in prescribed fields under regulations that ensure balanced representation among producers, users, and general interest participants. ASTM standards cover almost all industrial materials, including metals and alloys. ASTM standards also cover construction products, petroleum products, paints and coatings, textiles, plastics, rubber, electrical insulation and electronics, water technology, nuclear, solar and geothermal energy, and medical devices. Most material test standards are maintained by ASTM. ASTM publishes a collection of its standards called the Annual Book of ASTM Standards. This contains periodically revised, formally approved ASTM standard classifications, guides, practices, specifications, test methods, terminology, and related material. ASTM produces six principal types of standards. Standard test method. A definitive procedure for identification, measurement, and evaluation of one or more qualities, characteristics, or properties of a material, product, system, or service that produces a test result. 301

2 Chapter 17 Introduction to Standards and Nondestructive Testing of Industrial Materials Industrial Materials Standard specification. A precise statement of a set of requirements to be satisfied by a material, product, system, or service. A specification also recommends procedures for determining whether each requirement is satisfied. Standard practice. A definitive procedure for performing one or more specific operations or functions that do not produce a test result. Standard terminology. A document containing terms, definitions, descriptions of terms, explanations of symbols, abbreviations, or acronyms. Standard guide. A series of options or instructions that do not recommend a specific course of action. Standard classification. A systematic arrangement or division of materials, products, systems, or services based on similar characteristics. Similar characteristics may include material origin, composition, properties, or use. AMERICAN NATIONAL STANDARDS INSTITUTE (ANSI) When the American National Standards Institute (ANSI) was founded in 1918, standardization activities were just beginning in the United States. Many groups were developing standards. Sometimes the interests and activities of these groups overlapped and conflicted. The result was a waste of time and money, and considerable confusion. Five professional and technical societies and three government departments decided a single organization was needed to coordinate everything. As a result, ANSI was created to handle the task. The American National Standard Institute coordinates standard activities on the national level. It is a federation of standards from commerce, industry, government, and professional, trade, consumer, and other organizations. The major goals and activities of ANSI include identifying needs for standards, protecting public interests, protecting consumer rights, promoting standard development and implementation, approving American national standards, and representing interests of the United States in nongovernmental international standards work. PROFESSIONAL ASSOCIATIONS AND GOVERNMENTAL AGENCIES Many professional and trade associations and some governmental agencies are actively involved in standards related to materials used for industry. Examples include the American Iron and Steel Institute, Society of Automotive Engineers, American Concrete Institute, Society of Plastics Industry, National Institute of Standards and Technology, Department of Defense, and the Food and Drug Administration. American Iron and Steel Institute (AISI) The American Iron and Steel Institute (AISI) is a trade association that governs materials specification standards on iron and steel. Its goals are: to provide high-quality, value-added products to a wide array of customers; to lead the world in innovation and technology in the production of steel; to produce steel in a safe and environmentally friendly manner; and to increase the market for North American steel in both traditional and innovative applications. Society of Automotive Engineers (SAE) The Society of Automotive Engineers (SAE) promotes structural material applications in automotive and aerospace industries. Many standards have been established related to structural steels and steel alloys. American Concrete Institute (ACI) The American Concrete Institute (ACI) recommends standards regarding cement, materials made from cement, and applications of cement. It is dedicated to improving the design, construction, manufacturing, and maintenance of concrete structures and facilities. ACI develops and disseminates information and standards on concrete uses. Society of Plastics Industry (SPI) The Society of Plastics Industry (SPI) is an important trade association dealing with polymeric materials. SPI promotes proper uses of plastics by establishing standards, test procedures, and specifications for plastic pipe, bottle, furniture, packaging, and various resins. National Institute of Standards and Technology (NIST) Formerly known as National Bureau of Standards (NBS), the National Institute of Standards and Technology (NIST) has an overall goal of strengthening and advancing the nation s science and technology and facilitating their effective application for public benefit. NIST conducts research to provide a basis for the nation s physical measurement system, and offers scientific and technological services for industry and government. NIST promotes competitiveness for industry, maintains equity in trade and technical services, and promotes public safety. Department of Defense (DOD) The Department of Defense (DOD) is a large governmental agency dealing with military applications of materials. It is made up of various agencies with widely varied functions. However, some of these agencies are charged with maintaining various military standards on materials, processes, and products to ensure the quality and performance of military equipment and supply.

3 Chapter 17 Introduction to Standards and Nondestructive Testing of Industrial Materials Industrial Materials Food and Drug Administration (FDA) NONDESTRUCTIVE TESTING Penetrant The Food and Drug Administration (FDA) makes policies aimed at protecting the health of the nation against impure and unsafe foods, drugs, cosmetics, and other potential hazards. Implications for material scientists and engineers are related to processing equipment, containers, medical devices, and implants. Many material tests result in damage or failure of the test piece. This type of test is called destructive testing and is discussed in Chapter 18. However, it is often desirable to know properties of a material or product without subjecting it to destructive testing. A material test that does not result in damage or failure of the test piece is called a nondestructive test. Advantages of using nondestructive tests are very straight forward since the part is not damaged or destroyed. Usually, nondestructive testing is faster than destructive testing because it does not need extensive specimen preparation. Therefore, it can be used for production online control or construction site inspection. A nondestructive test is also used to detect any flaws existing in raw materials or semifinished products before any further machining or fabrication is performed. Therefore, nondestructive tests can be used for quality control and production cost savings. There are many methods of nondestructive testing and many commercial instruments available. The next section briefly describes five commonly used nondestructive material tests. PENETRANT TEST Penetrant tests are a simple method to detect defects with surface openings. Figure 17-1 illustrates the principle of penetrant method. In this method, the surface is first cleaned and dried. A penetrating dye fluid is then sprayed or swabbed onto the surface. The part is allowed to stand for Figure A The penetrant test starts by coating the cleaned surface with penetrant, which seeps into any cracks. B Excess penetrant is removed from the surface before the developer is applied. C Surface cracks are indicated by a small amount of penetrant drawn out by the developer. Developer Figure In the radiographic method, a film is placed behind the parts which are exposed to X rays. Any discontinuity in the part appears on the film due to differences in X ray absorption. sufficient time for the penetrant to fill in any surface cracks. The excess penetrant is flushed from the surface with warm water or cleaner. After that, the surface is carefully dried and coated with developer. The dye left in surface or subsurface cracks is drawn to the developer, which shows the crack on the surface. After suitable developing time, the parts can be inspected for defects exposed on the surface. When visible dyes, such as red dyes, are used, the parts are inspected under normal white light. The operator looks for red dye contrasting against a white developer background. When fluorescent dyes are used, parts are inspected under a black light. The operator looks for a bright yellow-green color against a deep blue-violet background. In both cases, the operators are looking for small amounts of penetrating dye that show the actual flaws. RADIOGRAPHIC TEST Radiographic tests are similar to dental X ray inspection. It uses short wavelengths, such as X rays, beta rays, and gamma rays. Figure 17-2 shows the principle of a radiographic test using X rays. X rays can penetrate a material to certain level, according to its internal structure of the material. A film is placed behind the part and exposed to the radiation source penetrating the material. The film is developed and shows an image of the internal structure of the material. This test method is useful for detecting flaws in welded joints, especially for critical structures. In operation of any radiographic instrument, safety precautions must be taken to avoid accidents. All radiographers and radiographic assistants must wear safety shielding. They must also carry an individual direct reading pocket dosimeter and either a film badge or a thermoluminescent dosimeter. Overexposure to X rays, beta rays, and gamma rays can lead to injuries or health problems. X rays Developed radiograph Film X ray tube A Part B C Part

4 Chapter 17 Introduction to Standards and Nondestructive Testing of Industrial Materials Industrial Materials Figure A flaw in the component is shown by a distortion in the magnetic flux, depicted by the elongated magnetic particles. S Aligned particles N Figure A Ultrasonic testing can be used to detect internal flaws. B The ultrasonic pulse takes a measurable amount of time to travel through a part without flaws. C When the ultrasonic pulse hits a defect inside the material, the travel time for the pulse is reduced. Therefore, the wave moves closer to the initial pulse. Sensor Initial pulse Reflected beam Part MAGNETIC PARTICLE TEST Magnetic particle tests can be used only for ferrous alloys. As shown in Figure 17-3, it starts with laying the steel component across the arms of a magnetizing machine. A special magnetic powder is sprinkled on the magnetized part surface. Lines of magnetic flux are intensified across a crack. Thus, the magnetic powder is drawn to the crack making it visible. Magnetic particle tests use both wet and dry magnetic particles to inspect the surface. Wet particles are primarily used for the detection of hairline surface cracks. These particles are typically used in stationary machines where they can be continuously agitated, pumped, and recycled through a spray nozzle. Dry particles are designed for use with portable equipment. They are especially effective on large parts with rough surfaces. The particles themselves are for one-time use and are seldom recovered. ULTRASONIC TEST Ultrasonic tests use a sonar or sound wave system to detect flaws. Ultrasonic waves are usually produced by the piezoelectric effect using a quartz crystal transducer. The waves are outside the audible range, normally 20 khz to 50 MHz. The sound wave traveling inside a material is reflected back by any boundary, such as outer surface or an internal flaw. A transducer sends a wave into the material being tested. The time the sound wave travels in the material depends on the characteristics and thickness of the material. If there is no defect in the material, the sound wave is reflected by the other side of the material. In this case, the total travel time corresponds to the thickness of the material. However, when the sound wave hits any flaw inside the material, the travel time is reduced accordingly. The reflected wave is shown closer to the transmitted beam or initial pulse. Therefore, a flaw is indicated on a screen, as in Figure Figure 17-5 shows a typical commercial ultrasound flaw detector. It can be used to detect flaws in all types of materials, including composites. A Figure Ultrasonic testing can be used for all materials, including composites. Part Waves reflected from the defect (c) Waves reflected from the other side of the material (b) B C Time

5 Chapter 17 Introduction to Standards and Nondestructive Testing of Industrial Materials Industrial Materials EDDY CURRENT TEST Eddy current tests involve the use of alternating magnetic fields. These tests can be used on any conducting material. When an alternating current is used to excite a coil, an alternating magnetic field is produced. The magnetic lines of flux are concentrated at the center of the coil. Then, as the coil is brought near an electrically conductive material, the alternating magnetic field penetrates the material and generates continuous, circular eddy currents as shown in Figure Larger eddy currents are produced near the test surface. Weaker eddy currents are produced inside the material. Eddy currents induce additional magnetic fields, which interact with the original magnetic field. Any change in the eddy current can be detected by change in the interaction between the original and secondary magnetic fields. A crack in the test material obstructs the eddy current flow, lengthens the eddy current path, and reduces the secondary magnetic field. This change in secondary magnetic field can be sensed by the primary coil and indicated by appropriate instruments. If a test coil is moved over a crack or defect in a metal part, at a constant clearance and constant speed, a momentary change occurs in coil reactance and coil current. An eddy current instrument can be used to detect cracks, seams, laps, pits, inclusions, and other defects. It can also be used to measure thickness, electric conductivity, magnetic permeability, hardness, or physical dimensions because these properties are all related to the eddy current on the surface. ACOUSTIC EMISSION Acoustic emission is an elastic wave generated by the rapid release of energy from a defect within a material. The wave propagates through the solid to the surface, which is detected by one or more sensors. The sensor, which is a transducer, converts the mechanical wave into an electrical signal. This signal is analyzed to determine the location of possible material fatigue or failure. See Figure Figure Principle of the acoustic emission process. Where ultrasonic testing actively probes a structure, an acoustic emission system listens for internal emissions from active defects. Acoustic emission testing is very sensitive to defect propagation created by structure load. INFRARED THERMOGRAPHY Thermal or infrared energy is radiation emitted by an object, based on its temperature. The greater the object s temperature, the greater the radiation emitted. But infrared energy is not visible to the human eye. In order to see images of this energy, infrared thermography cameras are used. These cameras are capable of capturing the infrared spectrum. Infrared thermography has many applications in a variety of industries. It can be used in commercial and residential construction to detect heat loss around roofs, windows, and doors. Other applications include inspecting equipment, especially electrical installations to prevent failure due to poor contacts and shorts, as well as examining pipes and steam jackets for defects. Stimulus (external load) Wave Propagation Signal Sensors Source (crack) Electronics (instrument and display) Stimulus (external load) Figure In an eddy current test, the primary field of the test coil enters test part and generates eddy currents which, in turn, generate a secondary field. Any flaw in the material changes the eddy current and, therefore, can be detected. Eddy currents

6 Chapter 17 Introduction to Standards and Nondestructive Testing of Industrial Materials Industrial Materials SUMMARY IMPORTANT TERMS Testing of industrial materials is necessary for understanding material behavior. Testing ensures product performance, safety, and quality, and aids in developing new materials. There are two major classes of tests for industrial materials destructive and nondestructive. Nondestructive tests do not damage or destroy the test part. They are normally used for inspection or detection of defects in materials and structures. Some nondestructive tests are also used for measuring a material s physical and mechanical properties. Most of the nondestructive tests can be conducted quickly and are suitable for automation, which is extremely useful in production line for quality control. For all tests of industrial materials, standard practices are followed. Most of these practices are established by the American Society for Testing and Materials (ASTM), as well as other organizations. Acoustic Emission American Society for Testing and Materials (ASTM) Eddy Current Tests Infrared Thermography Magnetic Particle Tests Nondestructive Test Penetrant Tests Radiographic Tests Ultrasonic Tests QUESTIONS FOR REVIEW AND DISCUSSION 1. What are the main purposes of testing industrial materials? 2. What is ASTM and what is its main function? 3. Of the six types of standards produced by ASTM, which type would cover tensile strength testing of an aluminum alloy? 4. Of the six types of standards produced by ASTM, which type would cover checking the quality of incoming AISI 4140 steel bars? 5. Which organization governs the standards for concrete materials? 6. What is ANSI and what are the roles of ANSI? 7. What are five common nondestructive tests? FURTHER READINGS INTERNET RESOURCES 8. Why is a magnetic testing method inappropriate to detect flaws in an aluminum casting? 9. An operator uses an ultrasonic flaw detector to detect flaws inside a steel block. The block is one inch thick. The flaw detector is calibrated so the distance between the initial pulse and the reflected beam is 25. When a flaw is sensed by the detector, the distance is 15. How deep is the flaw from the top surface of the steel block? 10. An ultrasound detector is best used to detect cracks the part. 11. After a cast iron part is machined, small pinholes are noticed on the machined surface. In an effort to find these defects before the part is machined, a member of the design team suggests using a penetrant test. Is this an appropriate method to detect these flaws? 12. For the application in Question 11, what other methods may be appropriate? 13. Can an ultrasound detector be used to find flaws in polymeric composites? 14. What is the purpose of using ultraviolet light in certain penetrant test methods, such as the inspection of automotive engine blocks? 1. Davis, H.E., Troxell, G.E., and Hauck, G.F.W. The Testing of Engineering Materials (4th edition). New York: McGraw-Hill Inc. (1982). 2. Shah, V. Handbook of Plastics Testing Technology (2nd edition). New York: John Wiley & Sons (1998). 3. ASTM Annual Book of ASTM Standards. Philadelphia, PA (2000). 4. Mix, P.E. Introduction to Nondestructive Testing: A Training Guide. New York: John Wiley & Sons (1987). 5. Askeland, D.R. The Science and Engineering of Materials (3rd edition). Boston: PWS Publishing Co. (1994). American Society of Nondestructive Testing General Electric American Society for Testing and Materials ZETEC Materials Testing Internet Resources