The Application of Nondestructive Testing Methods - Term Paper Example

Nondestructive Testing Name: Course: Institution: Tutor: The application of Nondestructive Testing Methods Introduction Non-destructive testing methods refer to a wide range of group analysis techniques that are used to examine and evaluate the properties of materials without causing any damage on the materials. Because these methods do not alter the material being examined, they are valuable techniques that can be relied on to save time and money as regards product evaluation. Primarily, non-destructive testing methods rely on the use of sound or electro magnetic properties to study materials. Different non-destructive testing methods are available, and these include: Visual Inspection Method; Radiography Method Testing; Liquid Penetrant Method Testing; Ultrasonic Method Testing and Magnetic Particle Testing. The following section discusses each of these methods and their relative advantages over other methods. i. Radiographic Method of Testing The radiographic method relies on x-rays or gamma rays to examine certain properties of materials. The basic principle in this method is that a penetrating radiation is passed through an object to be studied into a photographic film. This results in the object’s internal structures being deposited on the film. The total amount of energy absorbed by the object depends on its density and thickness. Any energy not absorbed by the object results in the exposure of the radiographic film. The areas on the object which do not absorb energy appear dark when the film is developed while those areas exposed to less radiation energy appear lighter. This means that any area on the object where the object’s thickness has been altered due to discontinuities such as cracks or porosity will appear as darkened outlines on the film. Essentially, low-density areas appear as dark areas while high density areas appear as light areas. All discontinuities on the object can be detected by viewing the shape and variations in the density of the processed photographic film (Mix, 2005). As a non-destructive testing method, radiographic testing can provide a permanent photographic record of material quality that is easy to interpret. The method is suited where there is access to both sides of a joint. Although the method is slow and relatively expensive, it is a reliable method for detecting porosity, voids, cracks and inclusions in the interior of welds. It is, however, important that only qualified persons use this method since faulty interpretations of radiographs can be too expensive and interfere with productivity. Moreover, there are certain safety considerations that should be observed when conducting radiographic testing. Both gamma rays and X-rays are visible to the naked eye and can have serious health implications. It is, therefore, recommended that only qualified and suitably trained personnel conduct radiographic testing (Taketo, 2000). ii. Ultrasonic Testing Ultrasonic testing method relies on mechanical vibrations similar to ordinary sound waves but of higher frequency. In this method, a beam of ultrasonic energy is directed on the object or materials to be examined. The beam travels through the object with no significant loss of energy except where it is intercepted or reflected by an apparent discontinuity in the object. To get the exact nature of the discontinuity, ultrasonic contact pulse reflection technique is used. This technique uses a transducer to convert electrical energy into mechanical energy. The transducer gets excited by high-frequency voltage, which causes crystals to start vibrating mechanically. The vibrating crystals become the source of ultrasonic, mechanical vibration. The vibrations are then transmitted into the test piece by use of a coupling fluid, which is called couplant. When the ultrasonic wave pulse strikes a discontinuity in the test object, the wave get reflected back to its point of origin, which is the transducer. The initial wave signal and the returned echoes from the discontinuity are displayed on a cathode-ray oscilloscope. With the display, it becomes easy to detect, locate and evaluate discontinuities in the test object (Charles, 2003). One of the most important qualities of ultrasonic testing is its ability to locate the exact point of a discontinuity in an object such as a weld. However, the method requires a high level of personnel training and experience, besides being dependant on the application of appropriate testing procedures. Sometimes, Electromagnetic Acoustic Transmission Method is used in which case an ultrasonic wave is radiated on the sample. If a defect is present, the wave is reflected and this can be analyzed, and interpreted to evaluate the nature of the sample’s internal surface. Charles, (2003) has noted that ultrasonic testing is used to test both ferrous and non-ferrous materials. In addition, the method is suitable for testing thick sections accessible from only one side, and can be used to detect plainer defects or finer lines, which might not be detected by radiographic testing. Ultrasonic inspection technique can also be used to assess the thickness of a surface and corrosion. Some of the equipment used in this method can be Tank Clawlers and Near Drum Scanners. Both radiographic and ultrasonic testing methods are the most common non-destructive techniques for testing discontinuities in the interior of welds. The main advantage of these two methods is that they are able to establish a weld’s internal integrity without necessarily causing any damage to the welded component. iii. Visual Inspection Visual inspection is the most common and least expensive non-destructive method of testing. Visual inspection is defined as the process of examining and evaluating systems and components by use of human sensory systems. The human sensory systems can be aided by mechanical enhancements to sensory inputs such as dental picks, magnifiers and stethoscopes among others. The inspection process can be done using such behaviors as looking, listening, feeling, smelling, twisting and shaking. Naturally, the visual inspection method includes a cognitive method where observations are correlated with knowledge of structures. Visual method can be applied on most services but is most effective where the test surfaces have been thoroughly cleaned prior to the examination (Cartz, 1995). Visual inspection is another important non-destructive method of testing. When attempting to study the soundness of an object, the first step in the examination is a visual examination. Essentially, almost any material can be visually examined to determine the soundness and accuracy of its fabrication. As an example, visual examination can be used to determine whether an object or part was fabricated to the correct size or whether all parts of a system have been correctly incorporated into a device. Before using the mechanical aids, the test specimen should be illuminated clearly and have a clean surface. Mix (2005) has noted that mechanical aids improve the precision of human vision. As specifications and tolerance levels edge closer, micrometers and calipers become necessary. These devices help determine thread sizes, gap thicknesses, angles between parts, weld features and hole-depths. iv. Magnetic Particle Testing Magnetic particle testing is used to test surface discontinuities in materials that are ferroelectric such as iron, nickel and cobber. The test method relies on putting a magnetic field in the test part. This produces magnetic lines of force that are perpendicular to the direction of electric current used. Any surface discontinuity on the material will allow the magnetic flux to leak out. Ferrous particles are applied to the part, and if flux leakage is present, the ferrous particles get attracted to the area. The ferrous particles build up at the leakage area and form an indication of discontinuity. The indication is then examined to give an idea of what it is. To induce a magnetic field on the test part, an electric current is sued. Different kinds of electric current are used depending on geometric properties of the part to be tested, the type of discontinuity being investigated, the type of material and the extent of magnetic field penetration into the material. If only surface discontinuities are being investigated, alternating current is preferred. This type of current cannot be used to test sub-surface discontinuities due to the skin effect, in which the current runs across the part surface. This happens because the alternating nature of the current prevents it from penetrating into the interior of the test object. As such, magnetic fields are only formed relative to the distance of the current’s penetration into the test part. To test subsurface discontinuities, direct current is used. This current has the capacity to penetrate deep into the study object and magnetize the part at the desired depth (Charles, 2003). Different devices are used to supply the electric current required to magnetize the inspection part. These devices include the automated wet horizontal magnetic particle inspection machine and mobile power packs. The wet horizontal magnetic particle inspection machine has an external power supply and demagnetizing system. It is frequently used to inspect engine cracks. When the part being inspected has been magnetized, it has to be demagnetized. Demagnetization requires use of special equipment which works opposite to the magnetizing equipment. Normally, magnetization is done by use of high current pulse, which reaches the peak current quickly and then turns off instantaneously leaving the inspected part magnetized. To demagnetize the part, an electric current has to be used, and this should be greater than the current used to magnetize the part. This current is then reduced to zero gradually, leaving the part demagnetized (Baldev & Jayakumar, 2002). v. Liquid Penetrant Method Liquid penetrant is another non-destructive method that is widely used to detect superficial defects in materials. With this method, a liquid of desired qualities is applied on the surface of the test part and gets drawn into the defects by the capillary action. After a preset dwell time elapses, excessive penetrant is removed from the test surface and a developer applied to remove the penetrant from the defects after which visual inspection is performed. Liquid penetrant inspection is principally based on capillary action in which a low surface tension liquid penetrates freely into a clean and dry surface discontinuity. The penetrant liquid can be applied into the test part spraying, brushing or dipping, depending on the test material and the liquid being used. Application of the developer helps draw the penetrant out of the flaw, which makes an invisible indication to be visible to the inspector. Inspection of the surface-breaking defect can be done white or ultraviolet light, but this depends on the type of dye used (nonfluorescent or fluorescent) (Taketo, 2000). To be effective, liquid penetrant method requires the test surfaces to be cleaned adequately and the contact time between the penetrant and the surface to be sufficient. Excessive penetrant should be removed carefully from both the flaws and the surface. This requires that the persons carrying out the inspection be specially trained. If used correctly, liquid penetrant inspection offers a cheap, fast and relatively simple means of detecting surface defects. In the aerospace industry, liquid penetrant inspection is used to detect fatigue cracking in turbine blades. In the construction industry, the technique is used in checking that weld areas and other vulnerable areas are fee from all forms of surface-breaking flaws (Cartz, 1995). The various types of penetrants used in this inspection technique are classified according to their sensitivity levels. Basically, visible penetrant are red in color and present the lowest level of sensitivity. Fluorescent penetrants contain several dyes, which fluoresce when exposed to ultra-violet radiation. Fluorescent penetration is performed in darkened environments in which excited dyes emit brilliant dyes, which contrast strongly with the darkened background. As such, this technique is more sensitive to small surface-breaking defects which might not be easily detected by other inspection techniques. Selection of the penetrant and its sensitivity level just take into consideration several factors including the surface finish of the material and the size of the defects. In addition, the test chemical should be fairly compatible with the examination sample so as to ensure that permanent staining is not caused. Liquid penetrant technique can be applied to inspect the following categories of surface defects; i. Micro shrinkage ii. Gas porosity iii. Grinding cracks iv. Poor weld penetration v. Hot tears vi. Cold shuts vii. Stress corrosion cracks viii. Heat treatment cracks ix. Inter-granular corrosion x. Heat effect zone cracks xi. Fatigue cracks xii. Hydrogen cracks xiii. Inclusions xiv. Laminations The main advantage of using this study method is that it is fast and reasonably cheap. However, the technique can only be used to study only surface defects and skin irritations. It also requires that the inspection be carried out on a clean and smooth surface, in which excessive penetrant can be removed with ease. This means that the inspection cannot be carried out on rough surfaces such as weld areas since these areas make it difficult to drain excessive penetrant. Another advantage is that only limited training is required for the inspector. Proper cleaning of the surface is necessary to ensure that any form of surface contamination has been removed and that any defects present are dry and clean for optimum results. It has, however, been shown that certain cleaning methods are detrimental to the test sensitivity of the penetrant. In that case, acid etching is done to reopen the defects or remove metal smearing. Generally, liquid penetrant inspection is related to the visual inspection method and is used to inspect cracks and flaws on non-absorbent materials (Baldev & Jayakumar, 2002). References Baldev, T & Jayakumar, M 2002, Practical non-destructive testing, Delhi, Woodhead Publishing. Cartz, L 1995, Nondestructive testing: radiography, ultrasonics, liquid penetrant, magnetic particle, eddy current, Boston, ASM International. Charles, H 2003, Handbook of Nondestructive Evaluation, New York, McGraw-Hill. Mix, E 2005, Introduction to nondestructive testing: a training guide, New York: John Wiley and Sons. Taketo, U 2000, Non-destructive Testing in Civil Engineering, Tokyo: Elsevier. Read More