Identifying transition of fatigue cracks from tensile to shear mode based on acoustic emission signals

摘要

Acoustic emission (AE) based structural health monitoring relies on detection and analysis of stress waves released bydamage growth. The features of AE waveforms such as amplitude, frequency, energy, and rise time could be usedcharacterize the relationship between acoustic emission signals received at a particular instant and the nature of damagegrowth, and several studies have examined such relationships. Acoustic emission generated by fatigue cracks propagateas a combination of a number of Lamb wave modes in plates, and hence by identifying the modal components it ispotentially possible to learn about the conditions under which the crack is propagating. This paper examines the modalfeatures of the acoustic emission signals generated at different stages of fatigue crack growth. A key requirement for thisanalysis is high fidelity required from the sensor recording AE signals, so that the different modes can be identified.Fatigue crack in a 6061 aluminum plate was monitored using a wideband sensor. The fatigue crack grew over a length ofabout 3 inches during 74,000 cycles and resulted in over 100,000 waveforms. The waveforms were examined in detailand the features were evaluated. As expected, the waveform’s peak amplitude and energy content were indicative of therate of fatigue crack growth. More importantly, the modal features of the waveform were found to be indicative of thenature of crack growth. Mode 1 crack growth that resulted in signals that contained almost exclusively the fundamentalsymmetric mode S0. When the crack is propagating in shear mode, the waveforms contained dominant A0 mode. Hence,acoustic emission waveform can potentially identify the transition of a Mode 1 crack to shear crack.