Ultrasonic Examination Instruments
Most UT systems used for the in-service inspection of a pressure vessel employ a “pulse-echo” method of operation, in which the same transducer (described below) that creates the ultrasonic wave (pulse) is also used to detect any ultrasonic waves (echos) reflected by flaws. A typical ultrasonic instrument for the pulse-echo UT is illustrated in Figure 700-12. “Pitch-catch” or “dual-element” transducers have recently greatly improved the reliability of detecting and accurately sizing certain types of planar flaws (or cracks) that can occur during service, such as tight and branched stress-corrosion cracks.
The ultrasonic instrument generates high frequency electrical impulses for activating an ultrasonic transducer and has a control knob for selecting the input frequency to the transducer. Frequencies normally used for UT range from 0.5 to 10 MHz. High frequencies provide greater sensitivity for detecting small flaws, but penetration is reduced due to greater attenuation attributable to scattering by small irregularities in the material. Metals with coarse grained microstructures, such as some austenitic stainless steel weld metals, can present an especially difficult problem for obtaining adequate penetration while retaining sufficient sensitivity for detecting small flaws. Grain sizes that are 1/10 of the ultrasonic wave length and larger tend to have a very serious effect upon attenuation. Low frequencies can be used to increase penetration, but sensitivity for detecting small flaws will be sacrificed.
Controls are also provided for setting the duration of ultrasonic pulses transmitted to the workpiece and the interval between successive pulses (pulse repetition rate). The settings used depend primarily on the thickness of the workpiece. It is desirable for all echos of one ultrasonic pulse to fade out in the workpiece before the next pulse is emitted. “Phantom” reflections can be obtained if the pulse rate is set too high.
An amplifier is incorporated into the instrument to amplify the ultrasonic echos received by the transducer, and a gain (or sensitivity) adjustment is provided to optimize the signal-to-noise ratio.
The ultrasonic echos received by the transducer are visually displayed on an oscilloscope. Both the amplitudes of the echos, and the times at which they are received (normally calibrated to indicate distance) are displayed.
Categories: In-Service Inspection | Tags: Ultrasonic | Leave a comment