ultrasonic Instruments
A portable,
battery-powered ultrasonic instrument is used for field inspection of airplane
structure. The instrument generates an ultrasonic pulse, detects and amplifies
the returning echo, and displays the detected signal on a CRT or similar
display.
Piezoelectric transducers produce longitudinal or shear waves,
the most commonly used wave forms for aircraft structural inspection. Reference
Standards Reference standards are used to calibrate the ultrasonic instrument.
Reference standards serve two purposes: to provide an
ultrasonic response pattern that is related to the part being inspected and to
establish the required inspection sensitivity.
To obtain a
representative response pattern, the reference standard configuration is the
same as that of the test structure or is a configuration that provides an
ultrasonic response pattern representative of the test structure.
The reference
standard contains a simulated defect (notch) that is positioned to provide a
calibration signal representative of the expected defect.
The notch size is chosen to establish inspection sensitivity
(response to the expected defect size). The inspection procedure gives a
detailed description of the required reference standard.
Couplants Inspection with ultrasonics is limited to the part
in contact with the transducer. A layer of couplant is required to couple the
transducer to the test piece, because ultrasonic energy does not travel through
air. Some typical couplants used are water, glycerin, motor oils, and grease.