{"id":30728,"date":"2021-07-27T11:16:47","date_gmt":"2021-07-27T11:16:47","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=30728"},"modified":"2023-09-22T11:16:56","modified_gmt":"2023-09-22T11:16:56","slug":"subsurface-ndt-methods","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/materials-science\/non-destructive-testing-ndt\/subsurface-ndt-methods\/","title":{"rendered":"Subsurface NDT Methods"},"content":{"rendered":"
The variety of techniques available can also be divided into two groups: surface methods<\/strong>, which are used to identify surface and near-surface defects such as cracks and surface porosity, and subsurface methods<\/strong>, which can be used to detect defects that lie under thematerial\u2019ss surface.<\/p>\n Ultrasonic testing<\/strong> is a broad group of NDT techniques based on the propagation of ultrasonic waves in the object or material tested. The most commonly used ultrasonic testing technique is pulse echo, wherein high-frequency sound wave beams (normally ranging from 1-5 MHz) are introduced into a test object, and reflections (echoes) are returned to a receiver from internal imperfections or the papart\u2019搜索引擎优化metrical surfaces.<\/p>\n The basic ultrasonic testing method is transforming a voltage pulse into an ultrasonic pulse using a transducer. Transducers used for traditional UT consist of a piezoelectric crystal enclosed within a plastic or stainless steel housing. The piezoelectric crystals expand when electrically charged, thus generating an acoustic wave. The signal travels through the object concerning its geometry and existing defects and then is either transmitted to another transducer or reflected to the original transducer. Defects are detected if they produce a change in the acoustic impedance in the path of the ultrasonic beam. An open crack filled with air has very low acoustic impedance, reflecting virtually all the acoustic energy incident on it. Hence, the sound waves travel through the material and are reflected from cracks or flaws. Defects and flaws affect its way, and a small portion of the pulse will be sent back to the transducer\/receiver before it hits the end of the object.<\/p>\n Since the speed of sound in the parent material is known, the reflected sound energy is then displayed versus time and analyzed to define the presence and location of flaws or discontinuities.<\/p>\n Advantages and Disadvantages of Ultrasonic Testing<\/strong><\/p>\n The advantages and disadvantages of the ultrasonic testing method are as follows:<\/p>\n Advantages:<\/p>\n Disadvantages:<\/p>\n Radiographic testing (RT) involves penetrating gamma or X-radiation to examine parts and products for imperfections. It is one of the conventional NDT methods that has been in use over decades and is still being used by companies worldwide.<\/p>\n In general, RT is a method of inspecting materials for hidden subsurface defects by using the ability of X-rays or gamma rays to penetrate various materials of various thicknesses. The intensity of the radiation that penetrates and passes through the material is either captured by:<\/p>\n Principle of Operation<\/strong><\/p>\n The radiation source can either be an X-ray machine or a radioactive source (Ir-192, Co-60, or in rare cases, Cs-137). The choice between X-rays and gamma radiation depends on factors such as thickness, contrast level, etc. For example, X-rays typically work with a lower amount of energy than gamma rays. The thickness is another parameter that influences the results. For example, at thicknesses more than 50 mm, the use of gamma rays increases significantly.<\/p>\n Radiation is directed through a part and onto film or other imaging media, and the resulting radiograph shows the dimensional features of the part. Both in X-rays and gamma radiation, as the radiation passes more through the material, the darker the film becomes on the image produced,. On the contrary, the more the ray is absorbed by the material,, the lighter the image is in those spots. Therefore, possible imperfections are indicated as density changes on the film in the same manner as a medical X-ray shows broken bones.<\/p>\n Radiographic testing is commonly used for weld verification in various industrial applications. In manufacturing, welds are commonly used to join two or more metal parts. The effects of welding on the material surrounding the weld can be detrimental \u2014depending on the materials used and the heat input of the welding process used. The HAZ can be of varying size and strength. For example, the base metal must reach a certain temperature during the welding process, must cool at a specific rate, and must be welded with compatible materials, or the joint may not be strong enough to hold the parts together, or cracks may form in the weld causing it to fail. Defects usually encountered include incomplete penetration, incomplete fusion, undercutting, porosity, and longitudinal cracking. These defects could cause a structure to break or a pipeline to rupture. Welds may be tested using NDT techniques such as industrial radiography or industrial CT scanning using X-rays or gamma rays, ultrasonic testing, liquid penetrant testing, magnetic particle inspection, or via eddy current.<\/p>\n Advantages and Disadvantages<\/strong><\/p>\n Advantages:<\/strong><\/p>\n Disadvantages:<\/strong><\/p>\n\n
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Ultrasonic Testing<\/h2>\n
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Radiographic Testing<\/h2>\n
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