Non-Destructive Testing - Magnetic Particle Inspection & Ultrasonic Testing - Essay Example

Guide's NDT- Magnetic Particle Inspection & Ultrasonic testing Non-Destructive Testing, as the name itself suggests, is the product inspection (quality control check) process. Inspection and test typically include measurements of an output and comparison to specific requirements to determine conformity. Inspection is performed for a wide variety of purposes, e.g., distinguishing between good and bad product, determining whether a process is changing, measuring process capability, rating product quality, securing product design information, rating the inspectors accuracy, and determining the precision of measuring instruments (Juran's, 2007, p 467). NDT is a crucial process that is performed after the output process to ensure that Quality standards (which the company commits to its customers) conform to the customer requirements. It is a final "checked ok" type setting. In recent past, several newer methods have evolved under NDT umbrella all having a common thread distinguishing tag of contactless inspection against potential flaws. NDT is popular because of its harmless and unfaltering association with the final product to check its quality. As we acknowledge, all materials are imperfect, but this is only of concern if the imperfections adversely affect intermediate processing or use of the finished product. In order to detect imperfections, some form of testing is necessary that will not have a detrimental effect on the materials/components.NDT encompasses all the test methods that, when applied to a component, do not impair its subsequent utilization (Colangelo, p 44) If the testing does not destroy or damage the material in any way it is known as Non - Destructive Testing (NDT). NDT is crucial in characterizing final products into "zero defect" and "potentially flawed" parts, this characterization is fast and easy. Hence, the quality control is easy itself. For instance, let us take an example of a steel plant which makes "railroads". There is a specific dimensional requirement of every piece and also there is some maximum tolerance level for cracks and porosity holes in those pieces. Using NDT (laser and X-Ray techniques, we can determine the dimensions and flaws inside the rails in a very short period of time). A number of techniques are used in NDT; each is generally dependent on a different energy system. Techniques range from ordinary macroscopic examination with white light to the complex procedure of neutron radiography, each method having an area in which it yields optimum performance (Colangelo, p44) though it can often be used successfully on marginal situations when the need arises. In present case we will discuss TWO vastly used NDT methods: 1. Magnetic Particle Inspection (MPI) 2. Ultrasonic Testing MPI Magnetic Particle Inspection offers a means for the detection of surface and slightly subsurface discontinuities in ferromagnetic materials (Colangelo, p 48). MPI is not applicable to non-ferromagnetic materials thus many structural metals like austenitic stainless steel, aluminum magnesium, copper and titanium are excluded from this inspection. Only ferromagnetic materials are inspected through this method. Magnetic particle inspection (MPI) is a widely used nondestructive inspection method for aerospace applications essentially limited to experiment-based approaches (Betz 1997). The analysis of MPI properties that affect sensitivity and reliability contributes not only reductions in inspection design cost and time but also improvement of analysis of experimental data. Choosing a particle medium, consider the application. For convenience, select dry particles when inspecting large components such as forgings. Wet-particle inspection, often requiring a tank complete with stirring and pumping machinery, works well for production-line Magnetic Testing inspection. This approach is especially useful when the operator must examine large numbers of small, similarly sized components. When portability is important, particularly for field inspections, use the dry-particle method-parts can be magnetized by truck-toted generators. To magnetize large components such as heavy weldments and forgings, generators may need outputs approaching 20,000 A. For Magnetic Testing of large parts by wet testing, such as forgings, inspectors immerse them in tanks with built-in pumps that carry liquid-suspended magnetic particles. Black-light lamps (ultraviolet) are used for inspection with fluorescent magnetic particles. Demagnetizer coils are used to permanently remove residual magnetic fields from Magnetic Testing-examined parts (testing and inspection, MT equipment) 12 Procedure for MPI: The test is conducted by creating a magnetic field in the test specimen. An indicating medium, either liquid or powder, containing high contrast magnetic particles, is applied to the surface which is then examined for the presence of indications. These indications are caused by the concentration of magnetic particles in the area over the defect. Sucface defects appear as sharp indications whereas subsurface defects yield indications that are broader and less defined. In general, the vagueness increases as the depth below the surface increases (Colangelo, p 49). There are some steps to be followed in the experimentation of MPI: 1. Initial Surface preparation of sample: loose rust and scale should be removed, paint should be removed locally for inspection. 2. Initial demagnetization: this is done to avoid false indications. 3. Degreasing and cleaning: if it is not done, the defects can be covered by grease etc and the flaw detection would be hindered. 4. Magnetization of sample: Electric current sources are chosen to magnetize the sample, which is magnetized according to the current source DC/AC/etc. 5. Application of magnetic particles: two types of application of magnetic particles i.e., wet and dry methods. 6. Viewing/Marking of defects: the magnetic particles get accumulated close to the boundaries of flaws such as cracks and hence are easily detected and marked. 7. Demagnetization: ferromagnetic materials can retain some magnetic character after field has been removed. So it is essential that the magnetic effect should be removed(Raj). Advantages 1. Cheap and robust equipment (Raj, p 17) 2. Easy handling 3. Not much protection needed (such as in Radiography) against any harmul rays. Disadvantages 1. Detects flaws on surface or very close beneath the surface 2. Suitable only for materials that can be magnetized, particularly, ferromagnetic materials. 3. Necessity to complete demagnetization (Raj, p 26) Ultrasonic Testing In ultrasonic testing, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect internal flaws or to characterize materials. It is also commonly used to determine the thickness of the test object - monitoring pipework corrosion being a good example. Ultrasonic Inspection is often performed on steel and other metals and alloys, though it can be used on concrete and other materials such as composites. It is a form of non-destructive testing used in many industries including aerospace, automotive and other transportation sectors. How it works In ultrasonic testing, a transducer connected to a diagnostic machine is passed over the object being inspected. In reflection (or pulse-echo) mode, the transducer sends pulsed waves through a couplant (such as water or oil) on the surface of the object, and receives the "sound" reflected back to the device. Reflected ultrasound comes from an interface - such as the back wall of the object or from an imperfection. The screen on the calibrated diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance taken for the reflection to return to the transducer. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after travelling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted thus indicating their presence.3 4 5Advantages 1. Superior penetrating power, which allows the detection of flaws deep in the part. 2. High sensitivity, permitting the detection of extremely small flaws. 3. Only one surface need to be accessible. 4. Greater accuracy than other nondestructive methods in determining the depth of internal flaws and the thickness of parts with parallel surfaces. 5. Some capability of estimating the size, orientation, shape and nature of defects. 6. Nonhazardous to operations or to nearby personnel and has no effect on equipment and materials in the vicinity. 7. Capable of portable or highly automated operation. Disadvantages 1. Manual operation requires careful attention by experienced technicians 2. Extensive technical knowledge is required for the development of inspection procedures. 3. Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect. 4. Surface must be prepared by cleaning and removing loose scale, paint, etc. (UT can often be used successfully through paint that is properly bonded to a surface.) 5. Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being inspected unless a non-contact technique is used. Non-contact techniques include Laser and Electro Magnetic Acoustic Transducers (EMAT). 6. Inspected items must be water resistant, when using water based couplants that do not contain rust inhibitors.6 Conclusion For MPI, Elongated defects parallel to the magnetic field may not give pattern; for this reason the field should be applied from two directions at or near right angles to each other For Ultrasonic testing, Requires high degree of skill in interpreting pulse-echo patterns. Permanent record is not readily obtained. A good NDE inspection program must recognize the inherent limitations of each process. For example, both radiography and ultrasound have distinct orientation factors that may guide the choice of which process to use for a particular job. Their strengths and weaknesses tend to complement each other. While radiography is unable to reliably detect lamination-like defects, ultrasound is much better at it. On the other hand, ultrasound is poorly suited to detecting scattered porosity, while radiography is very good (Hayes, 1997). Therefore, NDT processes have their particular limitations and far reaching effect when applied to particular spheres of influences in which one of the all methods comes out as a perfect match. References Colangelo V.J., Analysis of metallurgical failures, second edition, john wiley and sons, ISBN 9971-51-097-9 Raj Baldev, Practical Non-Destructive Testing, Narosa Publishing House, ISBN 81-7319-072-0 Gryna Frank M., Jurans Quality Planning and Analysis for Enterprise Quality, TATA McGraw hill Publishing, Fifth Edition, Second Reprint 2007 ISBN 0-07-061848-8 Testing & Inspection. Welding Design & Fabrication, 00432253, Dec99, Vol. 72, Issue 12 Magnetic Particle Inspection, Non Destructive Testing - Surface Examination Techniques, AZOM.com. www.azom.com/details.aspArticleID=522 HAYES CHARLES, The ABC's of Nondestructive Weld Examination, Welding Journal May 1997, published by the The American Welding Society Betz, C. E., Principles of magnetic particle testing, Magnaflux Co., Hardwood Heights, Illinois, 1997. Read More