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

Abstract

SUMMARY 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 tension–tension 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 tension–tension 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 © 2011 John Wiley & Sons, Ltd.

Stress intensity factor monitoring under cyclic loading by digital image correlation

“…For example, the change in the material electric resistance or material stiffness, due to crack growth, has led to the development of electrical potential technique and the compliance methods for crack growth measurement, respectively. In the same manner, other methods such as infrared and thermal testing, acoustic emission, eddy current, and ultrasonic have been successfully employed to monitor the defect size. In addition, some efforts were aimed at improving the accuracy of these techniques by combining 2 or more of these techniques.…”

Advanced materials and structures response prediction and monitoring

“…Therefore, this issue of Structural Control and Health Monitoring (SC&HM) journal is focused on this highly active research theme. It contains six extended , peer‐reviewed and revised manuscripts from the initially selected nine ones from those presented orally at the Mini‐Symposium on Composites, Smart Materials, Control and Health Monitoring , co‐organised by present guest editors, that held during the 3rd International Symposium on Air/Craft MAterials‐ACMA at Marrakech (Morocco) from 12 to 14 May 2010.…”

“…Half of the six contributions to this focused issue deals with the advanced structures response prediction , whereas the other half tackles the advanced materials response prediction and monitoring . The same distribution proportion applies for composite versus conventional materials. The same good balance happens also for numerical versus experimental analyses.…”

“…The same distribution proportion applies for composite versus conventional materials. The same good balance happens also for numerical versus experimental analyses. The next six paragraphs provide additional technical information on each of this issue's six contributions.…”

“…Maslouhi applied the acoustic emission and acousto‐ultrasonic non destructive testing techniques, via Lamb waves propagation analysis, for the monitoring of the initiation and propagation of cracks in un‐notched and notched holes in aluminium samples subjected to cyclic fatigue in real time until the complete rupture. The experimental results showed in particular that the severity of the acoustic emission is proportional to the initial cracking state of the samples and the amplitude of the applied stresses.…”