Fatigue crack growth monitoring in aluminum using acoustic emission and acousto-ultrasonic methods

Damage monitoring, failure prognostics, and remaining service life prediction represent great technological challenges for reliable maintenance of aeronautical structures. Consequently, there is a whole variety of non-destructive evaluation techniques that are available to ensure the quality of the metallic structures. The majority of these techniques are able to detect defects such as discontinuities of surface or volume and the variations of section and are applied to discrete intervals. The application of these techniques proves too long and generates high maintenance costs. This paper proposes experimental methodologies to monitor fatigue damage growth in real time and also use a physics-based model for fatigue life prediction. The aluminum alloy samples, with inserted pre-cracks in the fastener holes, was tested mechanically in fatigue tensiontension cyclic loading with follow-up of two complementary health monitoring techniques such as acoustic emission (AE) and acousto-ultrasonic (AU). The approach uses AE to detect fatigue crack initiation and crack growth in aluminum samples and also applies AU measurements in order to assess global health conditions and damage accumulation during tensiontension cyclic loading. Nasgro analytical fracture mechanic model was used to predict crack growth and to determine the number of the load cycles Nf required to grow the initial crack to final crack size ac. The results indicate that exploiting health monitoring data such as AE signals coupled with analytical physics-based models provides a convenient methodology to determine the fatigue life and to estimate safety factor on life of the materials tested. Furthermore, this paper demonstrates that AU flexural Lamb wave (A0) offers high potential to track damage before the occurrence of the first crack, such as fatigue damage caused by plastic deformation, and quantitatively to assess fatigue damage stages of the material. Copyright (C) 2011 John Wiley & Sons, Ltd.