Prediction of the thermosonic signal from fatigue cracks in metals using vibration damping measurements

Thermosonics (also known as ultrasound stimulated thermography or sonic infrared imaging) is a potentially attractive nondestructive testing method for the detection of contacting interface-type defects such as fatigue cracks in metals and delaminations in composites. A high power acoustic horn is typically used to excite a complex vibration field, which causes the defect interfaces to rub and dissipate energy as heat. The resulting local increase in temperature at one of the specimen surfaces can then be measured by an infrared camera. In this study a set of steel beams with fatigue cracks of different depth and variable partial crack opening was tested. Each beam was instrumented with strain gages across the crack and at the back face for the measurement of both the “breathing” behavior of the cracks and the excited vibration. The heat released at the crack was predicted from the measured vibration and an experimental estimate of the additional damping introduced in the specimens by each crack. The crac...

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