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NON-DESTRUCTIVE TESTING 1. Introduction to Non-Destructive Testing Many mechani

NON-DESTRUCTIVE TESTING 1. Introduction to Non-Destructive Testing Many mechanical parts are subject to stresses and strains that may eventually lead to part failure. In order to prevent the costly delays associated with equipment down-time, many parts are tested for weaknesses and defects when machinery is dismantled for maintenance. The procedures used are called non-destructive testing, because flaws may be detected without damaging the part. Four types of non-destructive testing are routinely used: (1) magnetic particle inspection, (2) ultrasonic testing, (3) radiographic inspection, and (4) liquid penetrant testing. OBJECTIVES: • Explain the purpose of non-destructive testing. • List four types of non-destructive tests. • Explain the importance of properly preparing parts to be tested. 1.1 Magnetic Particle Inspection Magnetic particle inspection techniques are frequently used to detect racks and flaws in ferrous metal parts (parts of iron or steel, which can be Magnetized). When the part is magnetized electrically, flaws in the metal part cause disturbances in the magnetic field. These disturbances can be detected by the pattern that is created when iron particles are dusted over the part. 1.2 Ultrasonic Testing Ultrasonic testing uses high-frequency sound waves to create a picture of the internal characteristics of parts. The ultrasonic beam is reflected by defects within the part. Indications are displayed on a screen, similar to a sonar scope, or on a digital readout. The locations and sizes of flaws can be determined by comparing the relative sizes and positions of pulses on the screen of an ultrasonic flaw detector. 1.3 Radiographic Testing The technique of radiographic testing is similar to that of a medical x- ray. Radiation passes through the material to be tested and registers an image on a piece of film or a fluorescent screen. A trained examiner can determine the internal state of the material by inspecting the radio-graphic image, just as a medical radiologist can diagnose a broken bone from an x-ray. 1.4 Liquid Penetrant Testing Liquid penetrant testing is a commonly used method of detecting surface flaws in all kinds of materials. A dye is mixed with a thin liquid that readily penetrates surface cracks in the material. The penetrant is applied to the part and allowed time to be drawn into the surface cracks. After the excess dye is removed from the surface of the part, the part is sprayed with a developer that draws the dye out of the cracks and makes it visible on the surface. These visible indications show the sizes and locations of the cracks. 1.5 Preparing Parts for Non-Destructive Tests Non-destructive tests reveal the presence of flaws or foreign matter in parts. Accurate interpretations of test results frequently depend on the cleanliness of the part being tested. All non-destructive tests involve comparative measurements, so it is especially important that test conditions be as uniform as possible. Rust, dirt, or corrosion can lead to false indications. They might disguise a flaw or indicate a defect where none actually exists. To prevent these potential problems, parts must be prepared by thorough cleaning prior to testing. Solvents, detergents, or abrasive blasting may be used to prepare parts for testing. Questions 1.1 What is the purpose of non-destructive testing? 1.2 List four types of non-destructive tests. a. _____________ b. _____________ c. _____________ d. _____________ 1.3 Why is the preparation of parts for non-destructive testing so important? 2. Magnetic Particle Inspection Magnetic particle inspection uses magnetic energy to detect defects on or near the surface of ferrous metal parts. The test is easily administered to large or small parts, and results are generally not difficult to interpret. However, the method has some disadvantages: it is only effective on metals that can be magnetized, such as iron or steel; the equipment is talky and expensive; and flaws deep within the part go undetected. OBJECTIVES: • Explain the uses and limitations of magnetic particle inspection. • List the equipment and materials required for magnetic particle inspection. • Describe the test procedure for magnetic particle inspection. 2.1 Theory Magnetic particle inspection operates on the principle of electromagnetic induction. When a metal part is magnetized, a defect on or near the surface creates a disturbance in the magnetic field. By identifying the location of the magnetic disturbance, the position of the defect can be determined. The magnetic particle technique uses finely divided metal particles to locate defects with different magnetic properties than the surrounding material. Particles are dusted over the part or sprayed on in a liquid suspension. Areas of magnetic disturbance tend to attract the particles, thus giving a visual indication of the flaw. Question: 2.1 Magnetic particle inspection reveals defects that have different ______________________________ than the surrounding material. 2.2 Equipment and Materials Two things are required for a magnetic particle test: (1) a means of magnetizing the part to be tested, and (2) a means of detecting distur- bances in the magnetic field. Parts are magnetized with high-current, low-voltage electricity. A large step-down transformer, such as the one shown in Figure 2-1, is used to supply the current. A dial on the front of the transformer enables the tester to select the appropriate amperage for a given test. Figure 2-1. Magnetic Particle Machine Two types of magnetic particles may be used to detect magnetic discontinuities: visible particles, or fluorescent particles. Visible parti- cles give an immediate indication of defects; fluorescent particles must be illuminated with an ultraviolet source, such as a black light, before the indication is apparent. Both types of particles are available in liquid suspension and powder form. Figure 2-2 shows some common appli- cators. Figure 2-2. Magnetic Particle Applicators If fluorescent particles are used, an ultraviolet source must also be included in the list of necessary equipment. A hand-held black light (Figure 2-3) is typically used for inspection purposes. Since ultraviolet light is potentially hazardous to vision, the filter must remain in place at all times, and personnel should avoid looking directly into the source. Figure 2-3. Hand-Held Black Light Questions 2.2 A_________supplies the high-current, low-voltage electricity nec- essary to magnetize parts for magnetic particle inspection. 2.3 Fluorescent particles are made visible with__________. 2.4 Since ultraviolet light is potentially hazardous to vision, the black light filter must remain in place at all times, and personnel should avoid_____________________________________________. 2.3 Test Procedure There are three steps in a magnetic particle inspection: (1) magnetizing the part, (2) applying the particles, and (3) examining the part for indications of flaws. Parts may be magnetized in two ways. A contact shot (or head shot) is connected as shown in Figure 2-4. This generates a magnetic field perpendicular to the direction of current flow. Cracks at right angles to the magnetic field (longitudinal cracks) are revealed by this test. Figure 2-4. Typical Contact Shot Lateral and radial cracks are located by using a coil shot (Figure 2-5). This generates a magnetic field running the length of the part, also at right angles to the current flow. In order to locate cracks in all directions, two tests must be performed, each generating a magnetic field at ninety degrees to the other. Figure 2-5. Coil Shot Before beginning a magnetic particle inspection, the part to be tested must be thoroughly cleaned. The transformer cables are then connected in one of the ways described above. When a contact shot is performed, the cable terminals must be tightly attached to the part to prevent arcing. When the connection is complete, the transformer is energized. A remote control (Figure 2-6) is provided on the handle of a probe or at the end of a cable. Figure 2-6. Remote Controls Particles are usually applied while the current is on. The particles are very fine iron filings coloured with a visible or fluorescent dye. With a sweeping, back-and-forth motion, a layer of iron filings is sprayed or dusted onto the surface of the part (Figure 2-7). Visible particles will detect large flaws, but the more sensitive fluorescent particles are used in most applications. After the particles are applied, the current is cut off. Figure 2-7. Applying Magnetic Particles When fluorescent particles are used, examination takes place in a darkened area. The examiner needs to allow a few minutes for his eyes to adjust to the darkness before examining the part. The black light is held close to the port and moved slowly along the surface as each area is inspected Figure 2-8). The indication of a defect glows green when illuminated with the ultraviolet light. Indications are marked on the surface of the part, and the test is repeated, with the magnetic field at right angles to the OK used in the first test. Figure 2-8. Black Light Examination Questions 2.5 Draw a contact shot (head shot) connection, the magnetic field surrounding the part, and a flaw detectable with this test. 2.6 Draw a coil shot hook-up, the magnetic field surrounding the part, and a flaw detectable with this test. 2.7 In order to detect cracks in all directions, ______ tests must be performed. 2.8 When fluorescent particles are used, examination must take place in a uploads/Management/ ndt-students-guide.pdf