Machinery's Handbook, 31st Edition
1630 Nondestructive Testing Thermal/Infrared Testing (IR): The laws of thermodynamics can be applied to the detection of many anomalies in metals, cements, and composites, as well as in the chemical industries. When there is a difference of temperature between two objects, or when an object changes temperature, the net flow energy is transferred from warmer areas to cooler areas or vice versa, until thermal equilibrium is reached. For example, IR can detect moisture that has intruded into the insulation of buildings and liquid CO 2 storage tanks. Acoustic Emission Testing (AE): This NDT method is commonly used to detect and locate faults in mechanically loaded components or structures. When a material is subjected to mechanical loading or stress, such discontinuities release energy in the form of stress waves that can be detected by strategically placing sensors, which pick up these signals and convert the energy into voltage. The voltage is then electronically amplified and timing circuits are used to process the AE signal data and identify the fault. Leak Testing (LT): Leak testing detects a flow of gas or liquid through the wall of a vessel, via a discontinuity (such as a hole, crack, or bad seal). Leaks require a pressure difference and travel from higher to lower pressure, whether from inside an object with greater pressure than the surrounding atmospheric pressure, or from the atmosphere to inside a vessel under a vacuum. Techniques for LT include mass spectrometer, halogen diode, pressure change, and bubble testing. Neutron Radiography (NRT): This technique is often used with conventional radiographic testing. But neutrons interact with the nucleus of the atom, rather than with the electrons, and are absorbed by matter very differently than X-rays and gamma rays. NRT is ideal for imaging light elements with low atomic numbers (such as hydrogen, water, carbon, and plastics) and penetrating heavy elements with high atomic numbers (such as aluminum, lead, steel, and titanium). Thus, neutron radiographs can show organic materials or water, with many structural materials being nearly transparent. Magnetic Flux Leakage (MFL): This method uses a direct current (DC) magnetizing field to create flux density to bring a material to near saturation. Surface or internal discontinuities, such as cracks, pits, and other defects, interrupt the flux field, causing it to leak beyond the test surface. The longitudinal and transverse magnetizing fields enable identification of defects in both directions. Sensors and configured inspection devices gather and transmit data for processing and analysis. Nondestructive Testing Symbol Application.— The application of NDT symbols is cov- ered in American National Standard ANSI/AWS A2.4-2012. This standard also includes the letters used to indicate each basic testing method: Acoustic Emission (AET), Eddy Current (ET), Leak (LT), Magnetic Particle (MT), Neutron Radiographic (NRT), Pen- etrant (PT), Proof (PRT), Radiographic (RT), Ultrasonic (UT), and Visual (VT). Testing Symbol Elements: The testing symbol consists of the following elements: num- ber of tests (N), length of section to be tested (L), reference line, test in field, test-all- around symbol, tail, arrow, and specification or other reference. The standard locations of the testing symbol elements are shown in Fig. 4.
Length of section to be tested
Number of tests
(N)
Reference line
Test in field Test-all-around symbol
{ {
L }
Tail Specifications or other reference
T
}
Basic testing symbol
Arrow
Fig. 4. Locations of Testing Symbol Elements
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