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Improved Reliability of Sonic Infrared Testing

Sonic infrared (thermosonics) is a very attractive nondestructive testing technique, particularly for the quick testing of complex components. It uses a pulse of high power ultrasound in the 20 to 100 kHz range applied at one point on the test structure to generate a high frequency vibration field in the structure. These vibrations cause the surfaces of any cracks to rub together, so dissipating energy that causes a surface temperature rise local to the cracks. This transient temperature rise can be imaged by an infrared camera. The vibration produced is typically chaotic and is therefore inherently nonreproducible. Some concern has been shown about the test’s reliability and, in particular, the confidence that can be placed in a null (crack-free) test. This paper proposes the use of a heating index based on a measurement of the component vibration to give a measure of the potential of the vibration generated in a particular test to generate sufficient heat for any cracks of interest to be detected. The procedure is evaluated on a set of turbine blades with known cracks, with the vibration being measured by a simple high frequency microphone. The correlation between the heating index and temperature rise at the crack location is presented and a calibration procedure for practical testing is proposed. It is shown that the heating index provides a real-time measure of whether sufficient excitation has been applied to detect the cracks of interest if they are present in the test component. The technique promises to significantly improve the reliability of sonic infrared testing.

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