NONDESTRUCTIVE TESTING METHODS An NDT method is classified according to its underlying physical principle. For example, the common methods are:
• Visual and optical testing (VT)
• Radiographic testing (RT)
• Ultrasonic testing (UT)
• Liquid penetrant testing (PT)
• Magnetic particle testing (MT)
By far, the most common NDT method is visual and optical testing. In many instances, a trained inspector armed with simple tools, such as a flashlight and magnifying glass, can perform a very effective inspection. In quality control, as well as in maintenance operations, visual testing is the first
line of defense. When deciding upon whether to use visual testing, it is important to understand its potential as well as its limitations. If the visual method is not sufficient for the problem at hand, more complex methods must be considered. Using the visual inspection method for enclosed systems can
be challenging and possibly ineffective. To enable a technician or engineer to inspect these difficult-to-see areas, a device known as a borescope is often used. Borescopes are essentially miniaturized cameras that can be placed on the end of a fiber optic cable. The camera can then be inserted into regions that are obstructed from direct visual inspection, and the resulting images are viewed in real-time on a video screen by the inspector.
Historically, radiography is the next most common NDT method. Significant activity in the field occurred almost immediately after Roentgen’s discovery of X-rays in 1895 .
Early literature notes the ability of radiographs to detect discontinuities in castings, forgings, and welds in metals. Discontinuities such as pores or inclusions in metals are readily detected in many cases. Cracks may also be detected using radiographic techniques, but attention must be paid to
orientation and residual stress issues. Radiography continues to be widely used despite the expense and safety implications of the equipment. Recent advances in digital radiography have helped reduce the cost of employing this method by eliminating the use of film.
Ultrasonic testing employs an extremely diverse set of methods based upon the generation and detection of mechanical vibrations or waves within test objects. The test objects are not restricted to metals, or even to solids. The term ultrasonic refers to sound waves of frequency above the limit of human hearing. Most ultrasonic techniques employ frequencies in the range of 1 to 10 MHz. The velocity of ultrasonic waves traveling through a material is a simple function of the material’s modulus and density, and thus ultrasonic methods are uniquely suited to materials characterization studies. In addition, ultrasonic waves are strongly reflected at boundaries where material properties change, and thus are often used for thickness measurements and crack detection. Recent advances in ultrasonic techniques have largely been in the field of phased array ultrasonics, now available in portable instruments. The timed or phased firing of arrays of ultrasonic elements in a single transducer allows for precise tailoring of the resulting ultrasonic waves introduced into the test object.
Liquid penetrant methods are simple, and are commonly used for the detection of surface breaking discontinuities, especially cracks. These methods involve the application of a penetrant liquid to the test object, subsequent removal of excess penetrant, and application of a developer to enhance the visibility of remaining penetrant. Surface breaking cracks may trap penetrant, and thus provide a visual indication of the crack. Liquid penetrant methods are popular due to their simplicity and visual nature of the results. The process parameters of penetrant and developer dwell time and cleaning are extremely important, and significant efforts continue to be expended to understand and optimize these parameters. Liquid penetrant methods can be applied to virtually any material, but residual stress fields may close cracks and reduce the effectiveness of these methods.
Magnetic particle methods are based on the collection of loose magnetic particles at locations of magnetic flux leakage on an object. This phenomenon is familiar to almost everyone from childhood experiments with magnets and iron filings. Magnetic particle methods are based on surface or near surface discontinuities that influence the electromagnetic properties of the object under test. For these methods to be employed, the object under test must be electrically conductive and ferromagnetic. Magnetic particle techniques thus allow the detection of surface-breaking cracks in steel
objects of complex geometry, which typically is a challenge for RT methods.
Post time: Nov-18-2019