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

Abstract

Thermosonics (also known as ultrasound stimulated thermography or sonic infrared imaging) is a potentially attractive non-destructive 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 gauges across the crack and at the back face for the measurement of both the 'breathing' behaviour 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 cracks were modelled analytically as non-uniform heat sources. Hence the temperature rise expected on the monitoring surface of the specimens could be predicted and compared with the infrared camera measurements. The results show good linear correlation between predictions and measurements, thus validating the prediction algorithm. The relationship between the vibration input and the thermal output will allow, as a longer-term goal, the prediction of the general threshold level of vibration needed for reliable crack detection.