Contemporary Nondestructive Testing Procedures - Term Paper Example

Non Destructive Testing Introduction Nondestructive testing (NDT) may be defined as the test methods used to inspect the reliability of material or structure without damaging its future usefulness. Nondestructive testing is used to examine the material reliability of the test article. The plan of future usefulness is significant to the quality control practice of sampling. It is understood that sampling is nondestructive testing if the same tested sample come back to service. For instance, hardness testing by indentation offers an appealing test case for the clarity of nondestructive testing. Hardness testing machines apply a controlled force to make a small dent in the surface of the part, then the diameter or depth of the dent is measured to calculate the hardness of the material. Due to this test the usefulness of part is not impaired. Hence it can be considered as a non-destructive testing. Nondestructive material classification is a rising field related to material properties together with material identification and micro structural characteristics that have a straight control on the expected service life of the test article. Nondestructive testing has as well explained by listing or classifying the different methods. Contemporary nondestructive testing procedures are presently used by industries to make sure product reliability, to prevent failure of components, in turn, avoid accidents and save individual life, to create a revenue for the user, to guarantee customer satisfaction and sustain the company status, to support superior product design, to manage manufacturing procedures, to lessen manufacturing expenses, to retain consistent quality level, and to ensure preparedness. The important goal of nondestructive testing is to have superior product quality. As a matter of fact the exact purpose of testing is to control and sustain the quality level that quality control managers or design engineers set up for the specific product. It is recognized that perfection may be impossible, but quality managers look for fineness in workmanship and product quality from entry of raw material to finished product delivery. The preferred quality level is the one that is considered most important. After the quality level has been established by the quality management, personnel responsible for production and testing should uphold this level. Nondestructive test can help to ascertain the quality level but quality control management decides the quality standard. Nondestructive testing is thought to be an area of the materials sciences that deals with all features of quality and serviceability of materials and its constitution. Nondestructive methods are by which materials and structures can be inspected without causing damage to structure and serviceability. There are different nondestructive testing methods available for various purposes and requirements. In this thesis two such nondestructive testing methods- Magnetic Particle and Ultrasonic Testing- is explained (ASNT, 2008). Magnetic Particle Inspection (MPI) Magnetic particle inspection (MPI) is considered as a nondestructive testing technique used for flaw detection in ferrous materials and components. Magnetic particle inspection is quick and comparatively simple and part surface preparation is not so crucial like some other NDT methods. These simple requirements make MPI a major extensively utilized nondestructive testing technique. MPI make use a magnetic field and iron filings to spot defects in components. The only condition is that the component supposed to be a ferromagnetic material like iron, nickel, cobalt, and their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will let the inspection to be efficient (NDT Resource Center, N.D). The magnetic particle inspection (MPI) is a simple theory. Think about a bar magnet which has a magnetic field in and around the magnet. The place that a magnetic line of force exits or enters the magnet is called a pole. The pole from which magnetic line of force exits is known a North Pole and a pole where a line of force enters the magnet is considered as South Pole. (See Fig-1) Suppose a bar magnet is broken at the center, two full bar magnets with magnetic poles on each end of the two pieces will be formed. At the same time suppose the magnet is just fractured but not broken totally into two pieces, a north and south pole will form at each edge of the crack. The magnetic line exits the North Pole and enters at the South Pole. The magnetic flux spreads out as it meet the small air gap formed by the crack since the air cannot support so much magnetic field per unit volume as the magnet can. As the field spreads out, it seems to spill out of the material and, hence is described a flux leakage field. In such condition, if iron particles are sprinkled on a cracked magnet, the particles will be attracted to, not only at the poles of the magnet, but as well at the poles at the ends of the crack. This bunch of particles is easier to observe than the real crack and this is the origin for magnetic particle inspection. (See fig.2) The initial stage in a magnetic particle inspection is to magnetize the part that is to be examined. If there is any flaw on the surface present, the defect will create a leakage field and when the iron particles, either in a dry or wet form, are applied to the surface of the magnetized part, the particles will be attracted and form cluster at the flux leakage fields. This gives a visible indication of defect to the inspector (NDT Resource Center, N.D). (See fig.3) Usually, magnetic particles are spread over as a suspension in water or paraffin. This facilitates the particles to cover the surface and form cluster around the flaws if any. Over the hot surfaces, dry powders can be used as a substitute to wet inks. Where the surfaces are dark, a thin coating of white paint is generally applied, to enhance the contrast between the background and the black magnetic particles. However, the most responsive method is to use fluorescent particles observed in UV (black) light. MPI is mainly sensitive to surface-breaking or near-surface cracks, even if the crack gap is very thin. But, if the crack is along the magnetic field, there is little interruption to the magnetic field and the crack may not be noticed. Because of this it is suggested that the inspection surface is magnetized in two directions at 90° to each other. As other option, methods using swinging or rotating magnetic fields can be used to make sure that all orientations of crack are detectable. The technique of magnetization depends on the surface shape of the component. Permanent magnets are, normally, used for on-site inspection, as they do not need a power. Yet, they are likely only to be used to examine fairly small areas and have to be pulled from the test surface. Even though there is requirement of power supply, electromagnets (yokes) are finding extensive use. The main advantage is that they are easy to remove from the testing part once the supply is switched off and that the strength of the magnetic field can be changed as per the requirement. The disadvantages are in case of arc at the prod contact points, can damage the specimen surface, and since the particles must be applied when the current is on, the inspection requires minimum two people (TWI, 2004). MPI is used to examine a variety of product structures including castings, forgings, and weldments. Various industries use magnetic particle inspection for deciding a part’s acceptability for use. Few examples, manufacturing companies that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace. Underwater inspection is yet another area where magnetic particle inspection is used to test articles like offshore structures and underwater pipelines (NDT Resource Center, N.D). Ultrasonic Testing Ultrasonic Testing (UT) is used to examine the soundness of raw materials as well as finished products. It uses high frequency sound energy to do evaluations and make measurements. Ultrasonic inspection is used for defect finding/evaluation, dimensional measurements, material description, and many other aspects. The normal UT inspection system comprises of some practical units, such as the pulser/receiver, transducer, and indicating devices. The pulser and the receiver is an electronic machine that can generate high voltage pulses. These high voltage pulses cause the transducer to generate high frequency ultrasonic energy. The high frequency sound energy is introduced and propagates through the materials in the form of waves. If there is a discontinuity in material (such as a crack) along the wave path, a portion of the energy will get reflected back to the surface from the defective plane. The returned wave signal is converted to an electrical signal with the help of the transducer and is displayed on a display device. The display system can calculate the time from signal generation to when the reflected signal was received. The travel time of signal can be straight linked to the distance if the velocity of the wave is known. From this information the defect location, size, orientation and other features can be calculated. Ultrasonic Inspection is an NDT method in analyzing material defects and it is an extensively used system. The advantages of ultrasonic inspection may be: It is responsive to both surface and subsurface defects; the depth of penetration of ultrasonic wave for flaw detection/measurement is superior to other NDT systems; one side access is needed while the pulse-echo technique is used; it is very accurate in deciding defect location and estimating size and shape; minimal part preparation is needed; electronic equipment presents immediate results; thorough images can be formed with automatic systems; and it gives other details, like thickness measurement, as well as flaw detection. Like other NDT methods, ultrasonic inspection has its disadvantages that consist: surface has to be reachable to introduce the ultrasonic; knowledge and skill is needed to operate the system; it usually needs a coupling medium to support conveying of sound energy into the test specimen; materials that are rough, uneven in shape, very small, extremely thin or not uniform are hard to inspect; cast iron and other coarse grained materials are not easy to inspect because of low sound transmission and high signal noise; linear defects oriented parallel to the sound ray may go unnoticed and reference standards are necessary for the device calibration and the description of defects (NDT Resource Center, N.D). Modern ultrasonic flaw detectors have a trigonometric role that permits for fast and accurate location determination of flaws while carrying out shear wave inspections. Presently the cathode ray tubes have been substituted with LED or LCD displays. These displays are extremely easy to observe in a wide range of ambient lighting.  Transducers can be programmed with specific instrument settings. The technician needs to connect the transducer and the instrument will automatically set variables such as frequency and probe drive (NDT Resource Center, N.D). Conclusion Normally the consumer of a fabricated product purchases it with high hope that it will give satisfactory service for a reasonable time. Present products are expected to give years of service. Superior product quality must be the goal of nondestructive testing. But this is not always the case, for there is such a thing as too high a quality level. As a matter of fact the real purpose of testing is to manage and sustain the quality level that design engineers set up for the particular product and situations. In the above investigation, one can have a general idea about NDT as a whole and Magnetic Particle Inspection and Ultrasonic Testing in particular. References ASNT (2008) Introduction To Nondestructive Testing [Online] NDT Handbook Available from: [10 March 2008]. NDT Resource Center, (N.D). Introduction to Magnetic Particle Inspection [Online] Available from: < http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Introduction/introduction.htm> [10 March 2008]. NDT Resource Center, (N.D). Basic Principles [Online] Available from: < http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Introduction/basicprinciples.htm> [10 March 2008] NDT Resource Center, (N.D). Basic Principles of Ultrasonic Testing [Online] Available from: NDT Resource Center, (N.D). Present State of Ultrasonic [Online] Available from: Fig-1 Fig-2 Fig-3 Fig-4 Read More